I'm just an electronics hobbyist and your videos have taught me a lot. If you had been doing these videos 30 years ago I'm sure I would have changed my major to electrical engineering rather than finance. When you retire please consider teaching engineering or math at the college level. You would have a great impact on young minds.
I think he's having a much larger impact here, on youtube, teaching at the "hobbyist level", rather than teaching at the college level. Inspirational. Technical details (unfortunately?) are needed at some point to do advanced things, and those require a formal (university level) training, which can be more boring.
Hi Alan, great video and tutorial to measure output impedance. Most of us also wants to know how to measure an unknown network impedance from 100kHz to 1GHz or higher but this one can only be achieved using a network analyzer. In my experience in Electronics, EMC and RF, and before I started my ECErcuit RUclips channel, I been working in understanding how to measure unknown network impedance using spectrum analyser and signal generator. In somehow, I have managed and successfully measured unknown network impedance accurately from 100kHz to at least 100MHz. Measurement was achieved by deriving a formula through a series of derivation and calculations. I automate the system by writing and application to control the spectrum analyser and signal generator to process the measurement against the calibration reference using the formula which gives you the result of impedance plot and data. I'm thinking of sharing this work on my RUclips channel but I'm not sure if someone is interested to know. Cheers, ECErcuit.
For the beginers What Alan does here is : He devide's 2.5V bij 1000 ohms (1K) of R load. That give the current flows in R load which result 0.0025 Amps (2.5 milli amp's) that will also be current flows in Ro (since current is equal everywhere in a close circuit). We also know that when R load is connected and current start passing it creates 1.5 Voltage drop on Ro 4Volts - 2.5 (V drop on R load that is because the sommof all voltage drops must be 4Volts ). So 1.5V voltage drop on Ro devided withthe current flow's in it will give the value of R out which is 600 Ω (ohms).
Another EXCELLENT review!! If you are still considering suggestions for tutorials, Have you considered FET's and in particular their utility in audio. I have been amazed how well FET's sound in preamps and low power amps. Being a voltage device apparently is a plus at audio freq's. My experience has shown them to rival many tubes I have played with such as 45, 2A3, 6L6 tubes. I have played with MANY transistors and opamps at audio freqs but they sound flat and dull compared to FETs...Many of the people I know are OK with transistor theory but don't understand FET's....Another OUTSTANDING audio device is the nuvistor for low level work like preamps...Microphonic as heck but once tamed, they are VERY nice not to mention fast and reasonably cheap.. And a great big OSCILLATOR to you too...dit dit
Just dropped in to see what was happening. LOL! Had just used the resistance substitution method (5:06) to get output impedance of an amplifier. Thanks again, Alan.
Wow, this is great! Have you considered publishing all of your notepad explanations with links to videos (or a companion DVD...)? I think this would make a fantastic workbook for those of us without that elusive EE degree!
OH GOD, finally got that equation. For a very long time, I am just tryin Rload backandforth to get 2Vloaded=Vunloaded, like dumb. Thank you so much w2aew!
+w2aew Thanks, yet another masterful video I've found in your archive that makes interesting topics so easy to understand! You mentioned "designing the output impedance" along the way, which got my attention as it is something I'm trying to research. Sorry if I've missed it, but I don't think you've covered this explicitly before? If I could add my vote, it would be great to see you explain how to go about designing for a specific output impedance (say for audio amp or function generator) and what practical circuit changes you might make depending whether the expected next stage is e.g. a low-impedance speaker or higher impedance mixer or power amp.
+pratalife I'll put that on my list. Most often you'd choose the output impedance based on what the circuit needs to drive. A low impedance load (50 ohm coax/load, loudspeaker, etc.) require a low output impedance. For maximum power transfer (if that is the goal) the output impedance should match the load impedance (complex conjugate actually). If the load is a high impedance, then the output impedance of the circuit driving it is less important.
+w2aew cheers, that would be neat. "If the load is a high impedance, then the output impedance of the circuit driving it is less important." ... I think you've already helped;-) I've been casually thinking about (low impedance) opamp circuits possibly plugged into high-impedance inputs and I've been mentally hung-up on the impedance mismatch. But why artificially make my source high-impedance? Doesn't seem to make much sense .. perhaps I need to go and revise power transfer theory! In the meantime, always look forward to your videos no matter the topic .. more often than not, give me something to experiment with for the week;-)
+pratalife Sometimes other factors figure into why the output impedance is what it is - power dissipation requirements, amplifier design constraints, etc.
I've wondered what output impedance is for ... years. Knowing that it is a design choice explains why downstream elements like antennas must match it. I am still struggling to understand in a physical sense what it is. Alan, is there anything else you can say about it? Many thanks.
Thanks Alan for that video, it will definitely be usefull. Correct me if I'm wrong, but with some manipulations there would be a way to determine an input impedance as well, right ? I was thinking to perform the steps you described to evaluate the output impedance of an opamp circuit with 2 resistors, R1 and R2 as you did. Therefore once the opamp output impedance is known, measurements could be performed using the input of the circuit we want to know the input impedance in place of R2. Having access to the opamp output impedance value, along with the V1, R1 and the newly measured V2, we could deduct the value of R2, which would normally be equivalent to the input impedance of the circuit being tested. I understand that if either the opamp output or the tested circuit input input are significally inductive or capacitive, a close attention will need to be paid to the frequency, but generally speaking, I would be lead to believe that what I described could be achieved too. Let me know if that make sense. Keep up with the good work !
The first formula is simply re-arranging the typical voltage divider equation: V1 = Vopen * (R1 / (R1+R2)), solving for R2. The second is starting with the ratio of two of these equations and solving for the source resistance.
U have a great understanding of how circuits work and how to compare two or more method's of measuring things. Could u show us the sinad and signal to noise methods of measuring receiver sensitivity and any trick or tips u know of how and what to use to make these measurements with equipment on a budget. For example I can remember reading of a method of using an analogue multi tester to monitor carrier level. Thanks Alan
Dear Alan, I have RF magnitude and phase probe circuit with two toroids connected to 4 diode based peak detectors to give output voltage corresponding to magnitude and phase errors detected. I am somehow not sure how that voltage is mapped to magnitude and phase errors and in what ratio. This circuit is inside a 300W auto matching network which is used to do impedance matching. Load is a sputter source which is used to do sputtering. Generator is a 13.56 MHz, 300W unit. The circuit which I was talking about senses this magnitude and phase error and gives a feedback to a microcontroller which in turn rotates ganged series and shunt capacitor to match the impedance. I just want to understand the circuit better and make know how to make which motor move to get to 50 ohms. I am missing some bits and parts due to which my understanding about this thing is not complete. Please help me understand this thing. Let me know if you want any circuit diagram from or anything else. Thanks !
I am aircraft technician and I need clarification on the following impedance subject related to ARINC 429 circuits. The transmitting source output impedance should be 75 W ± 5 W divided equally between Line A and Line B (line A and B are twisted pairs i.e 4 wires two for transmission and two for receive). This balanced output should closely match the impedance of the cable. The receiving sink must have an effective input impedance of 8k W minimums. …How is this physically measure?
Hi Alan. I'm building a CW transmitter. I have a buffer amplifier after the crystal oscillator. I tried measuring the output impedance of that stage using the method you described (with two load resistors). I calculated the output impedance . I then tried another pair of resistors (with different values) to see if I come up with the same output impedance. I did not. It was quite different. Could the output impedance change based on the load?
Well your right. Just because I have a three prong could be misleading. I will have to investigate by removing the panel and looking for myself. Thank you Alan.......
Good video, of course the next question that comes to mind is: How do I measure input impedance of device. I have one of those eBay 2.4GHz frequency meter modules without much documentation of course ;) Knowing my signal generators output impedance of 50 ohms, would I place say a 10 turn 10K pot in parallel with the two devices and adjust for half the signal voltage and calculate from there? This may be an easy answer but sleep has been a precious commodity the past few days lol. As always thanks.
Allen, is there a way to calculate the feed point impedance of an Antenna? Like an EFHW uses a 49:1 impedance transformation. How do one come to a conclusion that for HF bands the feed point impedance of an EFHW is around 2500 Ohms and required a 49:1 impedance transformer? I couldn't find a satisfying reply anywhere.
I have a TC Helicon Harmonizer pedal. MIc in, through the effect then back out to mic in amp etc. It works as expected on many different amps, but does not work well with my Fender Acoustic Jr GO. The amp 100W and with the TC pedal, out put volume is lower than an plain acoustic guitar with no amplification. It is not my amp it is an incompatibility with this model of Fender, tried 4 amps. I am suspecting an impedance mis match. TC is 400 ohm out. I have no input data for the Fender but both channels accept XLR for mic or 1/4 unbalanced for a guitar. Anybody's thoughts on what might being going on, and also ways to test/measure or add something between the pedal and amp mic input greatly appreciated. Cheers
im just a aermodellling hobbyist, right now i use the video transmitter with 5.8ghz freq , i need to know is this method can be used to measure the output impedance for my video trasmitter
Probably not, since most hobbyists don't have a way to measure the amplitude of a 5.8GHz signal. You can safely assume that is is likely 50-75 ohms as most RF outputs are.
Rather than memorizing a formula you can also see that the open circuit voltage equals the internal, Thevenin equivalent voltage since no current flows through the output (Thevenin) resistance. When you add a load resistance, you can divide the new output voltage by the load resistance to find the current flowing through the loop. Also, the change in output voltage MUST be due to Ohm's law applied to the output resistance. Thus, V_loaded = V_open - IR_output or R_output = (V_open - V_loaded)/I. Since I = V_loaded / R_load this yields the formula. ALWAYS the output resistance is R_out = -dV/dI by definition. You need only apply two different loads (resistances), measure the two output voltages, deduce the two output currents, and compute this slope. Be aware that taking the difference between two nearly equal numbers always loses precision so you need to make the two loads as different as practical.
What is the difference between an impedance curve compared to a frequency response curve? For Speakers there is two curve graphs that are completely different from each other, there is a speaker impedance curve and a frequency response curve. What I'm confused on is that the you input an audio signal from the function generator and sweep a frequency range. The graphs will curve out the speakers frequency response curve but the Impedance curve is completely different. I'm confused because I would think the frequency response curve would show you what the speaker frequency response "profile" is but the speaker impedance curve is actually what the speaker will be not the frequency response curve? What I'm saying is that the speakers frequency response is not really the graph to look at but the impedance curve is, plus they are not related to each other it seems. You sweep 20hz to 20K hz and the speakers frequency response will graph a curve profile from 20hz to 20khz. This is what most people think would be the correct profile. But the speakers impedance curve from 20hz to 20khz is the actual graph profile the speaker will be it seems.
Frequency Response = the audible response of the speaker - in other words, how well it reproduces sound over the specified frequency range. It's what you hear. This is what people want to know when buying a speaker - how well it produces sound at various frequencies. Impedance Curve - the electrical impedance at the speaker terminals vs. frequency - which is the electrical load that the amplifier sees when driving the speaker. This is what the amplifier designer needs to know in order to properly design his amplifier circuit.
Question, on Leader LFG 1300S generator requires a 50 ohm cable. can you explain this to me? Does it mean that different generators require different cables for testing? Thanks
The vast majority of coaxial cables used with test and measurement equipment are 50 ohm cables. This means that the coaxial cable has a 50 ohm characteristic impedance. For low frequency signals, this is unimportant. But for high frequency signals (above about 1MHz or so), or for signals with fast rising/falling edges (faster than 40-50ns), the 50 ohm cables and proper termination begins to be important. You may want to review this video on transmission lines and terminations: ruclips.net/video/g_jxh0Qe_FY/видео.html
not really. As mentioned early in the video, we use a low frequency to avoid transmission line, termination, etc. issues that occur at higher frequencies.
w2aew , how do you measure output impedance in an RF circuit? For a small signal I could use a test load and calculate what the value is, but what about a high power amplifier like for ham radio?
This video shows one method to measure complex input impedance: ruclips.net/video/eYN7dhdt1Dw/видео.html ...or you can use an inexpensive analyzer like the NanoVNA: ruclips.net/video/Sb3q8f0NBZc/видео.html ruclips.net/video/xa6dqx9udcg/видео.html
Ok, great explanation! But, I would like to find out the output impedance of my signal gerenator, and not the resistance. Because, the open-circuit output voltage of it changes with frequency. Please, how would I do this?
It is *very* likely that the change in output amplitude is *not* due to the presence of reactance in your output impedance - it likely has more to do with the signal generator circuit itself or the measurement method.
Ok, thanks a lot w2aew! I was thinking that I could get some kind of a model of the signal generator's output. Maybe, with an impedance like in a transmission line. But, I don't know if that is possible. Anyway, thank you so much for your attention!
is this a good response of a signal gen? cause ideally i think it should not be affected when loaded. the feedback circuit should take care of maintaining the amplitude
High frequency signal generators typically have a 50 ohm output impedance. If you use feedback to set the amplitude regardless of load, then the effective output impedance is then lower than 50 ohms, thus giving the generator a poorer output return loss.
Can you make a video of how to bias a one transistor mixer (say BJT - 2N3904) with modulating signal applied at the base and the LO signal applied to the emitter. How to get clean AM signal with no distortion. Say like 12 volts power supply to this simple circuit. How figure out the LO drive and what kind of drive the modulating signal requires. Anyways it is just an idea if you have time. Best regards, Pete
Hi sometimes our signal is very high frequency like ghz and we can't measure amplitude by an oscilloscope or even an spectrum analyzer how we can measure output impedance in this case?
A four port VNA is typically used to measure the small-signal output impedance in this case. If you are talking about large signal or high power outputs, then often a load-pull analyzer ($$$) is used.
When converting high impedance to lower impedance the signal drops down 20dB or more. This means that high impedance audio signals has more current drive, power, energy compared to low impedance audio signals? because what is the difference between an audio signal that is high impedance audio signal compared to a low impedance audio signal? it must have to do with the signals energy and power?
@@w2aew But when concerting from High impedance audio signal to a low impedance audio signal its a 20dB loss from input to output, any reasons why? The high impedance audio signal is 20dB lower when converting it to a low impedance audio signal. If low impedance has more current and more power it should be +20dB not -20dB
@@waynegram8907 Makes perfect sense. It's a simple resistive voltage divider. A high impedance source effectively has a high resistance in series with the output. A low impedance input looks like a low value resistor. So, you have a resistive voltage divider which makes a lower voltage appear at across the low impedance load.
One example is when an unloaded output causes the output to saturate or hit a limiting condition prior to reaching the theoretical open circuit voltage.
A simple example would be a current source output. If open circuited, the output will saturate before it reaches the Thevenin equivalent open circuit voltage.
How do reflections work? Can it be thought of like sound energy echoing off the walls because the brick wall serves like a infinite impedance (a solid object that requires infinite force to move)? Why aren't function generators designed like a constant voltage power supply, so that the output impedance is almost zero, and have a current limit for if there is a short circuit or other overload condition? Is it because this is just was too difficult to design back in the tube and discrete BJT era? I never did understand why would termination be a good thing, since if one is trying to get a ~20V rms signal down a line, and the source has ESR or output impedance, wouldn't terminating the output reduce the voltage and the power dissipated by the effective resistors for termination and inside the function generator? I suppose if signal if the signal gets 'bounced' and distorted, then efficiency isn't priority.
It all comes down to wave propagation. I did a video on reflections and terminations: ruclips.net/video/g_jxh0Qe_FY/видео.html and another on how to use these reflections to measure coax length and impedance: ruclips.net/video/Il_eju4D_TM/видео.html and this is my favorite video that visually illustrates wave propagation and reflections: ruclips.net/video/DovunOxlY1k/видео.html
Let me check it up...am still trying to find out how to calculate input and output impedance for audio systems and then how to design circuitry for impedance matching
But the impedance changes when varying the input frequency. So how can an amplifier designer choose the correct impedance if its changing when varying the input frequency? The impedance is not constant or fixed impedance its not always going to be a constant 8 ohms or 4 ohms load. A tube power amplifier section the output tubes impedance is always changing impedance. A Solid state output amplifier section the output transistors impedance is always changing impedance because the frequency input is varying.
The designer would simply have to ensure that the amplifier can adequately drive the speaker's impedance over the desired frequency range - by either compensating for it, or by making it insensitive to it.
Couldn't you output calculation as " LOAD ohms X (v2/(v1-v2)) = device input impedance " simple instead of oscilloscopes can be used diod bridge made of 4 diodes and a cap of 10uf simply make a switch that passes original signal v1 through switch to volt meter and another position on switch to pass signal including load in known ohms as v2 then voltages us as in my formula above ..in that case formula is spot on and can be used without oscilloscope.. I made such PCB for myself
You use the term impedance but only consider that the output impedance is purely resistive. What about the condition where the output "impedance" has reactive [capcitive or inductive] elements along with the resistive element? Should consider taking measurements using more than one frequency.
The formula at the end doesn't work. I have a 555 oscillator which seems to drop to half output voltage around 10 ohms, and using a 6R8 resistor made some measurements and ran some calculations, but had an impedance of 125 ohms. Turn the figures around and you get a negative resistance! The formula is unreliable.
The problem is likely that when driving such low impedances, that the output impedance of the device under test isn't constant, but is a function of the load current. The formula assumes a constant output impedance.
Thanks for the answer. It was irritating to see the impedance wasn't even close and using lower voltages (DC multimeter measurements but could easily have come from dropping the voltage) gave the negative result. The other ways seem to give best results, getting a feel for the half voltage impedance, but as you say, if that is too low, it could be guesswork after that! All down to operating voltages and frequency bandwidths.
Hey, great video. I was wondering if you'd be willing to post the derivation for the second formula. I can't seem to set up that equation from scratch. Much appreciated if you find the time.
I remember watching this video years ago, and I was really in the very early stages of my electronics hobby. I vaguely understood it, but I thought to myself that hey, nice video, but let's be honest, I'll never need to know how to do this... Well, I was wrong. Today I need to know how to measure output impedance, and I also could use a primer on transmission line termination. w2aew to the rescue! Your drawings and practical examples have taught me more than anything else. Thank you, genuinely!
I wish I could get all the other RUclips "instructors" to watch your videos. You are so polished. You could make a living instructing the instructors and I wish you would.
I am glad you still have this video posted. The equations used are easy to derive once you think about the relations, but when electronics is a hobby and not a field of study it sure speeds up the process to see it laid out so nicely as in this video.
Still valuable and excellent, and it will be even in two decades from now on. A really outstanding quality in terms of clarity, accuracy and style. Can't thank you enough for your work!
Alan, when I watch your vids, it's like learning all over again. I must admit, I feel that same giggly sensation that I felt when I first learned these principles many years ago. I've found that a huge number of beginning electronics students, hobbyist and on occasion, even some tech's I've spoken to, have had a hard time understanding this. You sir, have accomplished the perfect explanation of the very concept of output impedance and how to measure it in a simple straight forward way.... I'm always amazed by your teaching skills which seem so easy but in fact, are difficult in practice. God indeed blessed you with an outstanding teaching ability and a keen mind as well. I'm so glad that you share this talent with others. It will unlock a new world of understanding for some they never knew existed. Again, Kudos!!! Job Well Done!
Lowering load Z (or R) also shifts the HPF created by output DC blocking capacitor (if any) upwards, so, f must be high enough compared to the cut-off freq of that.
When you have a chance, could you do a video that addresses the case where the signal is high-frequency and not sinusoidal -- for example, a high-speed clock (say a few hundred MHz)? Also (or separately), how might one make this measurement on a differential signal (such as LVDS)? Many thanks for your many instructive videos... I've learned a ton both about theory and practice and I'm always excited to see new posts from you. Cheers, Alex
Hello sir, can you make a video about how to measure the output impedance of RF amplifier circuit ? The methods in this video generally apply to relatively low frequency use cases, I want to know that is threre any approach to measure output impedance at RF frequencies , looking forward to your video, thanks a lot
Perfect, thank you! This helps a diy guy a lot :) When measure a heaphone output that can have something between 1 and 30 Ohms, will it fit when using 50 and 100 Ohm resistors and do the third type of measurement explained in the video?
Hi, Thank you for your great education videos! Would you please explain how to measure the input impedance of a receiver ( typically with input sensitivity of -110dBm). I tried to do the job using a nano VNA and attenuators, but it seems the measurement is not precise since the nano VNA cannot work with very low level signals. It would be much appreciated if you share your rich knowledge. Thank you!
The output impedance of the collector of a Darlington transistor is higher or lower compared to a bjt's collector output impedance? But I have heard is that a darlington's output impedance on The collector is much lower compared to a bjt's output and complaints on The collector but what is the advantage of the output impedance of a darlington's collector compared to a bjt's output impedance collector?
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I'm just an electronics hobbyist and your videos have taught me a lot. If you had been doing these videos 30 years ago I'm sure I would have changed my major to electrical engineering rather than finance. When you retire please consider teaching engineering or math at the college level. You would have a great impact on young minds.
I totally agree
I think he's having a much larger impact here, on youtube, teaching at the "hobbyist level", rather than teaching at the college level. Inspirational. Technical details (unfortunately?) are needed at some point to do advanced things, and those require a formal (university level) training, which can be more boring.
Alan, I often find myself going back to your videos to brush up on calculations I don’t use every day. It’s a great resource.
Hi Alan, great video and tutorial to measure output impedance. Most of us also wants to know how to measure an unknown network impedance from 100kHz to 1GHz or higher but this one can only be achieved using a network analyzer. In my experience in Electronics, EMC and RF, and before I started my ECErcuit RUclips channel, I been working in understanding how to measure unknown network impedance using spectrum analyser and signal generator. In somehow, I have managed and successfully measured unknown network impedance accurately from 100kHz to at least 100MHz. Measurement was achieved by deriving a formula through a series of derivation and calculations. I automate the system by writing and application to control the spectrum analyser and signal generator to process the measurement against the calibration reference using the formula which gives you the result of impedance plot and data. I'm thinking of sharing this work on my RUclips channel but I'm not sure if someone is interested to know. Cheers, ECErcuit.
Love that step resistor box. I need to make or buy one. You explained this process much MUCH better than my "engineer" instructor. Thanks Allan
For the beginers What Alan does here is : He devide's 2.5V bij 1000 ohms (1K) of R load. That give the current flows in R load which result 0.0025 Amps (2.5 milli amp's) that will also be current flows in Ro (since current is equal everywhere in a close circuit). We also know that when R load is connected and current start passing it creates 1.5 Voltage drop on Ro 4Volts - 2.5 (V drop on R load that is because the sommof all voltage drops must be 4Volts ). So 1.5V voltage drop on Ro devided withthe current flow's in it will give the value of R out which is 600 Ω (ohms).
I watch these videos many times. Each tim I do I find something more. Thank you.
Another EXCELLENT review!!
If you are still considering suggestions for tutorials, Have you considered FET's and in particular their utility in audio. I have been amazed how well FET's sound in preamps and low power amps. Being a voltage device apparently is a plus at audio freq's. My experience has shown them to rival many tubes I have played with such as 45, 2A3, 6L6 tubes. I have played with MANY transistors and opamps at audio freqs but they sound flat and dull compared to FETs...Many of the people I know are OK with transistor theory but don't understand FET's....Another OUTSTANDING audio device is the nuvistor for low level work like preamps...Microphonic as heck but once tamed, they are VERY nice not to mention fast and reasonably cheap..
And a great big OSCILLATOR to you too...dit dit
Just dropped in to see what was happening. LOL! Had just used the resistance substitution method (5:06) to get output impedance of an amplifier. Thanks again, Alan.
ok I have to say im hooked on your videos. You explain this stuff very clearly. Thank you for your time
Do you have a PDF of all of your notes ? This would be great as either an e-book. Great videos.
Wow, this is great! Have you considered publishing all of your notepad explanations with links to videos (or a companion DVD...)? I think this would make a fantastic workbook for those of us without that elusive EE degree!
Thank you for making this video.I just used it to measure the impedance of my flea market function generator.
OH GOD, finally got that equation. For a very long time, I am just tryin Rload backandforth to get 2Vloaded=Vunloaded, like dumb. Thank you so much w2aew!
+w2aew Thanks, yet another masterful video I've found in your archive that makes interesting topics so easy to understand!
You mentioned "designing the output impedance" along the way, which got my attention as it is something I'm trying to research. Sorry if I've missed it, but I don't think you've covered this explicitly before?
If I could add my vote, it would be great to see you explain how to go about designing for a specific output impedance (say for audio amp or function generator) and what practical circuit changes you might make depending whether the expected next stage is e.g. a low-impedance speaker or higher impedance mixer or power amp.
+pratalife I'll put that on my list. Most often you'd choose the output impedance based on what the circuit needs to drive. A low impedance load (50 ohm coax/load, loudspeaker, etc.) require a low output impedance. For maximum power transfer (if that is the goal) the output impedance should match the load impedance (complex conjugate actually). If the load is a high impedance, then the output impedance of the circuit driving it is less important.
+w2aew cheers, that would be neat. "If the load is a high impedance, then the output impedance of the circuit driving it is less important." ... I think you've already helped;-) I've been casually thinking about (low impedance) opamp circuits possibly plugged into high-impedance inputs and I've been mentally hung-up on the impedance mismatch. But why artificially make my source high-impedance? Doesn't seem to make much sense .. perhaps I need to go and revise power transfer theory!
In the meantime, always look forward to your videos no matter the topic .. more often than not, give me something to experiment with for the week;-)
+pratalife Sometimes other factors figure into why the output impedance is what it is - power dissipation requirements, amplifier design constraints, etc.
I've wondered what output impedance is for ... years. Knowing that it is a design choice explains why downstream elements like antennas must match it. I am still struggling to understand in a physical sense what it is. Alan, is there anything else you can say about it? Many thanks.
Just found your channel. Very interesting and informative. Also, I REALLY like that scope :)
Thanks. This is helpful. Best wishes from New Zealand.
Thanks Alan for that video, it will definitely be usefull.
Correct me if I'm wrong, but with some manipulations there would be a way to determine an input impedance as well, right ?
I was thinking to perform the steps you described to evaluate the output impedance of an opamp circuit with 2 resistors, R1 and R2 as you did. Therefore once the opamp output impedance is known, measurements could be performed using the input of the circuit we want to know the input impedance in place of R2.
Having access to the opamp output impedance value, along with the V1, R1 and the newly measured V2, we could deduct the value of R2, which would normally be equivalent to the input impedance of the circuit being tested.
I understand that if either the opamp output or the tested circuit input input are significally inductive or capacitive, a close attention will need to be paid to the frequency, but generally speaking, I would be lead to believe that what I described could be achieved too.
Let me know if that make sense.
Keep up with the good work !
Very well explained. Is there a derivation for the first and last formula?
The first formula is simply re-arranging the typical voltage divider equation:
V1 = Vopen * (R1 / (R1+R2)), solving for R2. The second is starting with the ratio of two of these equations and solving for the source resistance.
***** That's perfect. Thank you!
Great video, sir. But how to measure the output impedance of an RF signal generator with a Directional Coupler and a Spectrum Analyzer?
Increase the load until you see a 6 dB power drop. That's the output impedance.
U have a great understanding of how circuits work and how to compare two or more method's of measuring things. Could u show us the sinad and signal to noise methods of measuring receiver sensitivity and any trick or tips u know of how and what to use to make these measurements with equipment on a budget. For example I can remember reading of a method of using an analogue multi tester to monitor carrier level. Thanks Alan
Dear Alan,
I have RF magnitude and phase probe circuit with two toroids connected to 4 diode based peak detectors to give output voltage corresponding to magnitude and phase errors detected. I am somehow not sure how that voltage is mapped to magnitude and phase errors and in what ratio. This circuit is inside a 300W auto matching network which is used to do impedance matching. Load is a sputter source which is used to do sputtering. Generator is a 13.56 MHz, 300W unit. The circuit which I was talking about senses this magnitude and phase error and gives a feedback to a microcontroller which in turn rotates ganged series and shunt capacitor to match the impedance. I just want to understand the circuit better and make know how to make which motor move to get to 50 ohms.
I am missing some bits and parts due to which my understanding about this thing is not complete. Please help me understand this thing. Let me know if you want any circuit diagram from or anything else. Thanks !
Excellent! Thanks for posting this !
Occ-il-Later, you be a funny man. It's the imaginary component that trips me up when trying to figure out Impedance for RF
I am aircraft technician and I need clarification on the following impedance subject related to ARINC 429 circuits. The transmitting source output impedance should be 75 W ± 5 W divided equally between Line A and Line B (line A and B are twisted pairs i.e 4 wires two for transmission and two for receive). This balanced output should closely match the impedance of the cable. The receiving sink must have an effective input impedance of 8k W minimums. …How is this physically measure?
None of those measurements are impedances.
Mind doing a video on building V/I Curve Tracer?
Great video. Love the HP-15C!
really awesome tutorial video for me. I have been wondered this issue for long time. Thanks.
"Oscilater"
That... that actually hertz my head.
what happened to amplifier? oscillator. I can think of some more, probably wouldn't want to say them.... jim
Hi Alan. I'm building a CW transmitter. I have a buffer amplifier after the crystal oscillator. I tried measuring the output impedance of that stage using the method you described (with two load resistors). I calculated the output impedance . I then tried another pair of resistors (with different values) to see if I come up with the same output impedance. I did not. It was quite different. Could the output impedance change based on the load?
Thank you. The length was for just bench work mostly.
Well your right. Just because I have a three prong could be misleading. I will have to investigate by removing the panel and looking for myself. Thank you Alan.......
Great video. Thanks for sharing your knowledge.
Good video, of course the next question that comes to mind is: How do I measure input impedance of device. I have one of those eBay 2.4GHz frequency meter modules without much documentation of course ;) Knowing my signal generators output impedance of 50 ohms, would I place say a 10 turn 10K pot in parallel with the two devices and adjust for half the signal voltage and calculate from there? This may be an easy answer but sleep has been a precious commodity the past few days lol. As always thanks.
Another very good video! Thanks very much Alan!
Allen, is there a way to calculate the feed point impedance of an Antenna? Like an EFHW uses a 49:1 impedance transformation. How do one come to a conclusion that for HF bands the feed point impedance of an EFHW is around 2500 Ohms and required a 49:1 impedance transformer? I couldn't find a satisfying reply anywhere.
That's exactly what an antenna analyzer does.
I have a TC Helicon Harmonizer pedal. MIc in, through the effect then back out to mic in amp etc.
It works as expected on many different amps, but does not work well with my Fender Acoustic Jr GO.
The amp 100W and with the TC pedal, out put volume is lower than an plain acoustic guitar with no amplification. It is not my amp it is an incompatibility with this model of Fender, tried 4 amps.
I am suspecting an impedance mis match. TC is 400 ohm out. I have no input data for the Fender but both channels accept XLR for mic or 1/4 unbalanced for a guitar.
Anybody's thoughts on what might being going on, and also ways to test/measure or add something between the pedal and amp mic input greatly appreciated. Cheers
im just a aermodellling hobbyist, right now i use the video transmitter with 5.8ghz freq , i need to know is this method can be used to measure the output impedance for my video trasmitter
Probably not, since most hobbyists don't have a way to measure the amplitude of a 5.8GHz signal. You can safely assume that is is likely 50-75 ohms as most RF outputs are.
Rather than memorizing a formula you can also see that the open circuit voltage equals the internal, Thevenin equivalent voltage since no current flows through the output (Thevenin) resistance. When you add a load resistance, you can divide the new output voltage by the load resistance to find the current flowing through the loop. Also, the change in output voltage MUST be due to Ohm's law applied to the output resistance.
Thus, V_loaded = V_open - IR_output or R_output = (V_open - V_loaded)/I. Since
I = V_loaded / R_load this yields the formula.
ALWAYS the output resistance is R_out = -dV/dI by definition. You need only apply two different loads (resistances), measure the two output voltages, deduce the two output currents, and compute this slope. Be aware that taking the difference between two nearly equal numbers always loses precision so you need to make the two loads as different as practical.
What is the difference between an impedance curve compared to a frequency response curve? For Speakers there is two curve graphs that are completely different from each other, there is a speaker impedance curve and a frequency response curve. What I'm confused on is that the you input an audio signal from the function generator and sweep a frequency range. The graphs will curve out the speakers frequency response curve but the Impedance curve is completely different. I'm confused because I would think the frequency response curve would show you what the speaker frequency response "profile" is but the speaker impedance curve is actually what the speaker will be not the frequency response curve? What I'm saying is that the speakers frequency response is not really the graph to look at but the impedance curve is, plus they are not related to each other it seems. You sweep 20hz to 20K hz and the speakers frequency response will graph a curve profile from 20hz to 20khz. This is what most people think would be the correct profile. But the speakers impedance curve from 20hz to 20khz is the actual graph profile the speaker will be it seems.
Frequency Response = the audible response of the speaker - in other words, how well it reproduces sound over the specified frequency range. It's what you hear. This is what people want to know when buying a speaker - how well it produces sound at various frequencies.
Impedance Curve - the electrical impedance at the speaker terminals vs. frequency - which is the electrical load that the amplifier sees when driving the speaker. This is what the amplifier designer needs to know in order to properly design his amplifier circuit.
Thank you so much. I certainly did get valuable information from this video. A++
can you do a video on impedance matching??
+GOWTHAM KRISH (Gowtham Shanmugaraj) Big topic - it is on my list - hopefully I'll get to it later this year.
***** I'm waiting
Question, on Leader LFG 1300S generator requires a 50 ohm cable. can you explain this to me? Does it mean that different generators require different cables for testing?
Thanks
The vast majority of coaxial cables used with test and measurement equipment are 50 ohm cables. This means that the coaxial cable has a 50 ohm characteristic impedance. For low frequency signals, this is unimportant. But for high frequency signals (above about 1MHz or so), or for signals with fast rising/falling edges (faster than 40-50ns), the 50 ohm cables and proper termination begins to be important. You may want to review this video on transmission lines and terminations:
ruclips.net/video/g_jxh0Qe_FY/видео.html
Hi, If I want measure output impedance in RF circuit example in my "colpitts oscillator". I can measure how you explained?
not really. As mentioned early in the video, we use a low frequency to avoid transmission line, termination, etc. issues that occur at higher frequencies.
w2aew , how do you measure output impedance in an RF circuit? For a small signal I could use a test load and calculate what the value is, but what about a high power amplifier like for ham radio?
👍Thank you sir.
Hi very nice video! But is there a similar way to mesure INPUT impedance of receiver?
This video shows one method to measure complex input impedance:
ruclips.net/video/eYN7dhdt1Dw/видео.html
...or you can use an inexpensive analyzer like the NanoVNA:
ruclips.net/video/Sb3q8f0NBZc/видео.html
ruclips.net/video/xa6dqx9udcg/видео.html
Ok, great explanation! But, I would like to find out the output impedance of my signal gerenator, and not the resistance. Because, the open-circuit output voltage of it changes with frequency. Please, how would I do this?
It is *very* likely that the change in output amplitude is *not* due to the presence of reactance in your output impedance - it likely has more to do with the signal generator circuit itself or the measurement method.
Ok, thanks a lot w2aew! I was thinking that I could get some kind of a model of the signal generator's output. Maybe, with an impedance like in a transmission line. But, I don't know if that is possible. Anyway, thank you so much for your attention!
Great video but what about finding out the input impedance of a device?
is this a good response of a signal gen? cause ideally i think it should not be affected when loaded. the feedback circuit should take care of maintaining the amplitude
unless i could design my output impedance to be relatively small so that it would be negligible in the voltage divider
High frequency signal generators typically have a 50 ohm output impedance. If you use feedback to set the amplitude regardless of load, then the effective output impedance is then lower than 50 ohms, thus giving the generator a poorer output return loss.
Can you make a video of how to bias a one transistor mixer (say BJT - 2N3904)
with modulating signal applied at the base and the LO signal applied to the emitter. How to get clean AM signal with no distortion. Say like 12 volts power supply to this simple circuit. How figure out the LO drive and what kind of drive the modulating signal requires. Anyways it is just an idea if you have time.
Best regards,
Pete
Thanks Alan. Great video.
Hi sometimes our signal is very high frequency like ghz and we can't measure amplitude by an oscilloscope or even an spectrum analyzer how we can measure output impedance in this case?
A four port VNA is typically used to measure the small-signal output impedance in this case. If you are talking about large signal or high power outputs, then often a load-pull analyzer ($$$) is used.
@@w2aew everything can be done with $$$$ 😁
Interesting way to sine off.
When converting high impedance to lower impedance the signal drops down 20dB or more. This means that high impedance audio signals has more current drive, power, energy compared to low impedance audio signals? because what is the difference between an audio signal that is high impedance audio signal compared to a low impedance audio signal? it must have to do with the signals energy and power?
Just the opposite! High impedance sources can't deliver as much current or power as low impedance sources.
@@w2aew But when concerting from High impedance audio signal to a low impedance audio signal its a 20dB loss from input to output, any reasons why? The high impedance audio signal is 20dB lower when converting it to a low impedance audio signal. If low impedance has more current and more power it should be +20dB not -20dB
@@waynegram8907 Makes perfect sense. It's a simple resistive voltage divider. A high impedance source effectively has a high resistance in series with the output. A low impedance input looks like a low value resistor. So, you have a resistive voltage divider which makes a lower voltage appear at across the low impedance load.
Hi Alan, for the second case, please give an example of when not working with no load.
One example is when an unloaded output causes the output to saturate or hit a limiting condition prior to reaching the theoretical open circuit voltage.
***** Could you give an unequivocal example of what you say is very abstract
A simple example would be a current source output. If open circuited, the output will saturate before it reaches the Thevenin equivalent open circuit voltage.
***** Got it thanks !!!
***** If I understand you had in mind such a scheme?
screenshot.su/show.php?img=0e6dc6caf1938140fb7836ffb0b6b617.jpg
Very good video and I like the new camera. OCIL LATER.
Are able to provide the schematic for that wave generator?
I built that generator over 30 yrs ago, the schematic is long lost...
@@w2aew thank you for the reply. Most importantly thank you for sharing all your knowledge. Hope to shake your hand some day. Have a good one.
How do reflections work? Can it be thought of like sound energy echoing off the walls because the brick wall serves like a infinite impedance (a solid object that requires infinite force to move)? Why aren't function generators designed like a constant voltage power supply, so that the output impedance is almost zero, and have a current limit for if there is a short circuit or other overload condition? Is it because this is just was too difficult to design back in the tube and discrete BJT era?
I never did understand why would termination be a good thing, since if one is trying to get a ~20V rms signal down a line, and the source has ESR or output impedance, wouldn't terminating the output reduce the voltage and the power dissipated by the effective resistors for termination and inside the function generator? I suppose if signal if the signal gets 'bounced' and distorted, then efficiency isn't priority.
It all comes down to wave propagation. I did a video on reflections and terminations:
ruclips.net/video/g_jxh0Qe_FY/видео.html
and another on how to use these reflections to measure coax length and impedance:
ruclips.net/video/Il_eju4D_TM/видео.html
and this is my favorite video that visually illustrates wave propagation and reflections:
ruclips.net/video/DovunOxlY1k/видео.html
Very useful. Thank you!
Where do I learn the concepts like reflection
This video might help: ruclips.net/video/g_jxh0Qe_FY/видео.html
Let me check it up...am still trying to find out how to calculate input and output impedance for audio systems and then how to design circuitry for impedance matching
But the impedance changes when varying the input frequency. So how can an amplifier designer choose the correct impedance if its changing when varying the input frequency? The impedance is not constant or fixed impedance its not always going to be a constant 8 ohms or 4 ohms load. A tube power amplifier section the output tubes impedance is always changing impedance. A Solid state output amplifier section the output transistors impedance is always changing impedance because the frequency input is varying.
The designer would simply have to ensure that the amplifier can adequately drive the speaker's impedance over the desired frequency range - by either compensating for it, or by making it insensitive to it.
can i use the same process for measuring input impedances making my test circuit as the load?
that's one way to do it.
Can I just say thank God for the internet... My mind exploded and now I need to go and look up more shit WTF? Thank you
Couldn't you output calculation as " LOAD ohms X (v2/(v1-v2)) = device input impedance " simple instead of oscilloscopes can be used diod bridge made of 4 diodes and a cap of 10uf simply make a switch that passes original signal v1 through switch to volt meter and another position on switch to pass signal including load in known ohms as v2 then voltages us as in my formula above ..in that case formula is spot on and can be used without oscilloscope.. I made such PCB for myself
Great video. Thanks !!!
Power transfer is maximum when impedance is matched.
You use the term impedance but only consider that the output impedance is purely resistive. What about the condition where the output "impedance" has reactive [capcitive or inductive] elements along with the resistive element? Should consider taking measurements using more than one frequency.
Very useful thanks :)
How do you measure the reactance component of the impedance?
That would be a bit more complicated, maybe a future video.
Great video. That R substitution box is a fire hazard! The switch could short out a power supply when turned, and Poof! Smoke got let out! 👎
Outstanding lesson! Thank You!
I think he should have showed how the equations are derived. It's simple but it would make it clear as to what he is doing.
Another Excellent video .. Regards de Gw0wvl 👍
Thanks Alan!
Thank you sir!
Thanks so much!
Many thanks.
Thanks you.
oscil later
Nerd humor...
I'll see you later = oscillator
***** after a while crocodile
a beautiful pun
The formula at the end doesn't work. I have a 555 oscillator which seems to drop to half output voltage around 10 ohms, and using a 6R8 resistor made some measurements and ran some calculations, but had an impedance of 125 ohms. Turn the figures around and you get a negative resistance! The formula is unreliable.
The problem is likely that when driving such low impedances, that the output impedance of the device under test isn't constant, but is a function of the load current. The formula assumes a constant output impedance.
Thanks for the answer. It was irritating to see the impedance wasn't even close and using lower voltages (DC multimeter measurements but could easily have come from dropping the voltage) gave the negative result. The other ways seem to give best results, getting a feel for the half voltage impedance, but as you say, if that is too low, it could be guesswork after that! All down to operating voltages and frequency bandwidths.
Cool beans.
Always
Damn...that osciloscope is a beaaaauty!
Digital osciloscope with a crt...beaaauty!
Hey, great video. I was wondering if you'd be willing to post the derivation for the second formula. I can't seem to set up that equation from scratch. Much appreciated if you find the time.
I remember watching this video years ago, and I was really in the very early stages of my electronics hobby. I vaguely understood it, but I thought to myself that hey, nice video, but let's be honest, I'll never need to know how to do this...
Well, I was wrong. Today I need to know how to measure output impedance, and I also could use a primer on transmission line termination. w2aew to the rescue!
Your drawings and practical examples have taught me more than anything else. Thank you, genuinely!
Glad to hear it! I hope you found the videos on transmission lines and terminations!
I wish I could get all the other RUclips "instructors" to watch your videos. You are so polished. You could make a living instructing the instructors and I wish you would.
I like your videos as they teach the fundamentals with real world, hands-on, models. If you don't already, you would make a great teacher!
Maybe when I retire...
Man you rock! Great at explaining things. Very good teacher. Thanks!
I use the same HP 15, thanks.
I am glad you still have this video posted. The equations used are easy to derive once you think about the relations, but when electronics is a hobby and not a field of study it sure speeds up the process to see it laid out so nicely as in this video.
Still valuable and excellent, and it will be even in two decades from now on. A really outstanding quality in terms of clarity, accuracy and style. Can't thank you enough for your work!
Alan, when I watch your vids, it's like learning all over again. I must admit, I feel that same giggly sensation that I felt when I first learned these principles many years ago. I've found that a huge number of beginning electronics students, hobbyist and on occasion, even some tech's I've spoken to, have had a hard time understanding this. You sir, have accomplished the perfect explanation of the very concept of output impedance and how to measure it in a simple straight forward way.... I'm always amazed by your teaching skills which seem so easy but in fact, are difficult in practice. God indeed blessed you with an outstanding teaching ability and a keen mind as well. I'm so glad that you share this talent with others. It will unlock a new world of understanding for some they never knew existed. Again, Kudos!!! Job Well Done!
Lowering load Z (or R) also shifts the HPF created by output DC blocking capacitor (if any) upwards, so, f must be high enough compared to the cut-off freq of that.
When you have a chance, could you do a video that addresses the case where the signal is high-frequency and not sinusoidal -- for example, a high-speed clock (say a few hundred MHz)?
Also (or separately), how might one make this measurement on a differential signal (such as LVDS)?
Many thanks for your many instructive videos... I've learned a ton both about theory and practice and I'm always excited to see new posts from you.
Cheers,
Alex
ruclips.net/video/g_jxh0Qe_FY/видео.html
Thank goodness for your videos! They help me understand my Measurements and Troubleshooting laboratories.
Hello sir, can you make a video about how to measure the output impedance of RF amplifier circuit ? The methods in this video generally apply to relatively low frequency use cases, I want to know that is threre any approach to measure output impedance at RF frequencies , looking forward to your video, thanks a lot
How about testing the output impedance of a ham radio in the HF range from 160 meters to 10 meters?
Perfect, thank you! This helps a diy guy a lot :)
When measure a heaphone output that can have something between 1 and 30 Ohms, will it fit when using 50 and 100 Ohm resistors and do the third type of measurement explained in the video?
Hi, Thank you for your great education videos! Would you please explain how to measure the input impedance of a receiver ( typically with input sensitivity of -110dBm). I tried to do the job using a nano VNA and attenuators, but it seems the measurement is not precise since the nano VNA cannot work with very low level signals. It would be much appreciated if you share your rich knowledge. Thank you!
I always learn something from your videos. Great work. Thanks!
The output impedance of the collector of a Darlington transistor is higher or lower compared to a bjt's collector output impedance? But I have heard is that a darlington's output impedance on The collector is much lower compared to a bjt's output and complaints on The collector but what is the advantage of the output impedance of a darlington's collector compared to a bjt's output impedance collector?