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- Опубликовано: 26 июн 2024
- This video shows a few methods for measuring the output impedance or resistance of a function generator, amplifier, or other circuits. These methods generally apply to relatively low frequency use cases, and won't necessarily apply to all circuit types. The methods shown are all variations on measuring the response of the circuit to different load impedances, and calculating the device or circuit's output impedance from those readings. A low frequency sinusoid (sinewave) is used as a test signal to prevent problems from reflections on misterminated transmission lines, etc. Notes from the video can be found here:
www.qsl.net/w/w2aew//youtube/H... 
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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!
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.
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.
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.
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 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...
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.
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).
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!
I watch these videos many times. Each tim I do I find something more. Thank you.
ok I have to say im hooked on your videos. You explain this stuff very clearly. Thank you for your time
Thank goodness for your videos! They help me understand my Measurements and Troubleshooting laboratories.
Nice tutorial. You're a good instructor. Thanks for showing us.
I always learn something from your videos. Great work. Thanks!
Thank you for making this video.I just used it to measure the impedance of my flea market function generator.
Fantastic tutorial! I just discovered your channel.... now I gotta watch all your videos. Thanks....
Exactly the tutorial I was looking for.
Explained very well.
Man you rock! Great at explaining things. Very good teacher. Thanks!
Great video! Very well put together, perfectly explained, and super useful information. Thanks for your work!!
your videos are indispensable. thank you.
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.
Thanks. This is helpful. Best wishes from New Zealand.
Great video. Thanks for sharing your knowledge.
really awesome tutorial video for me. I have been wondered this issue for long time. Thanks.
Another very good video! Thanks very much Alan!
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.
Excellent! Thanks for posting this !
Nice and straightforward.
Gotta say it. You have brilliant Forest Mims style graf paper skillz.
Just found your channel. Very interesting and informative. Also, I REALLY like that scope :)
Nicely done, as always.
Great video. Love the HP-15C!
Thanks for all the videos!
"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
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!
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!
Another great video. Thanks!
Thanks Alan. Great video.
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
Thank you so much. I certainly did get valuable information from this video. A++
Very clear explained. thanks
Excellent video. Thank you!
you're just so amaizing to teach us thus way
Do you have a PDF of all of your notes ? This would be great as either an e-book. Great videos.
Thank you. The length was for just bench work mostly.
This is an excellent video! Do you have a similar video on measuring the input impedance?
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.
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.
Great video. Thanks !!!
Loved it!
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
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 !
Yet another excellent video. Can you use the same method to measure the input impedance of lets say a common emitter amplifier or any other type of electronic circuit?
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
Very useful. Thank you!
A wonderful collection of videos.
I could listen to your voice all day...
but can you help me with a problem on home made swr meters?
I have constructed many types using single or double pcb's....
The forward measurement is spot on but the reflected is always higher.
I'm thinking, if the impedance of the stripline is out, that's my basic error
to be added to the real error on the test load....
SO. how do I measure the impedance of any swr meter?
Damn...that osciloscope is a beaaaauty!
Digital osciloscope with a crt...beaaauty!
awesome, very simple, Thanks.
Thanks Alan!
Outstanding lesson! Thank You!
Thank you sir!
Some active devices instead of running in a resistive mode, run in a constant current mode. For example in a high negative feedback amplifier, the output voltage changes very little with a changing load, so the impedance appears to be very low and the inverse of the impedance called damping factor is used. A common high performance amp with a damping factor of 30 would have an apparent impedance of 1/30th of an ohm or 33 miliohms. However protection first limits current to about 10A(varies) and then the output protection relay or other shutdown trips. Now at 60 volts at 10 Amps, the limit is about 600Watts peak into 6 ohms. Protection kicks in before the output can be dropped to 1/2 voltage.
Very good video and I like the new camera. OCIL LATER.
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.......
thank you i will trk
thanks again
rodney
Very nice presentation once again. A couple of observations: I did this on a breadboard with the output impedance of my awg set to 50 ohm. I used a small 100 ohm trim pot and adjusted the pot until Vpp on the oscilloscope measured the set voltage on the awg. I got 47 ohm. Not bad. It just seems to me that using a pot makes it much easier to fine tune the load resistance compared to a decade box. Secondly this method assumes that the circuit under test consists of linear circuit elements which can be modeled as a thevenin voltage and resistance. The failure of the first method (2:47) to work all the time may be an indication of this. Using a second load resistance measurement (6:54) may or may not satisfactorily adjust for this. Best way is to measure open circuit voltage (Vth), calculate voltage at several load resistance levels, calculate current V/R at each point and plot voltage vs current. (You can do a least squares approximation on the data if the line is not perfectly straight) Getting a straight line insures that you are actually dealing with linear circuit elements. As V (measured) = - Rth*I + Vth. The slope of this line is the negative of the output impedance (Rth).
Please note that the 50 ohm setting on your AWG is likely *NOT* setting the output impedance. The output impedance is *always* 50 ohms. This setting is telling the AWG what the *load impedance* is that you're connecting to. It uses this to properly set and display the voltage at the load.
@@w2aew Yes I know. When I say I set the output impedance I should have said the input impedance of the load. My Siglent SDX1032X has two options for load impedance 50 ohm and HiZ. When set at 50 ohm the open circuit voltage is twice the displayed voltage and when set at HiZ the open circuit voltage is equal to the displayed (set) voltage. The question I had is what is considered a high load impedance? If we desire less than a 1% discrepancy between the displayed and measured voltages (across the load) we have (at the Hiz setting) Vm (measured) = R*Vset/( R + 50 ). Where Vs is the set (displayed) voltage. If we require Vm to be >= 99% of Vset then R (load) must be greater than 4950 ohms. If, instead, can live with a 5% discrepancy then R must be greater than 950 ohms. In practice I actually tested this out and found that at a 700 ohm load the output voltage was within 5% of the displayed voltage and that at greater than 1750 ohms the output voltage was identical to the displayed voltage. So the question arises what if our actual load impedance is below 1750 ohm (for 1%) or less than 700 ohm (for 5%)? Well with the the awg set to HiZ we have Vm = Vset*R/( R + 50) we can turn this around (setting Vm to the desired voltage) and Vset = (1 + 50/R)*Vdesired. Eg lets say we have a 100 ohm load and desire 4 volts. We would then set the awg to
Vset = (1 + 50/100)*4 = 6 volts. OTOH if the awg were set to expect a 50 ohm load and we had a load resistance R other than 50 ohm (but less than 700) then the formula would be 1/2 of this Vset = (0.5 + 25/R)*Vdesired.
Hi
I have a question regarding conductance, can you measure conductance of a let say car battery ,this by a scope?
I sort of measure the resistance and this in certain time peiods i think then in days or week or month so see how the resistance increases in time
I think of sending a certain freq on one side collect it on the other side then calc the time this must increase when resistance increases or not?
Hello Alan, thank you for your great explanations. Is it possible to put a link with the schematics of your generator ?
I built that homemade generator over 25 years ago, and the schematic is lost unfortunately.
+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.
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?
Mind doing a video on building V/I Curve Tracer?
Very useful thanks :)
Nice handwriting! Seriously, sure not calligraphy, but I always struggle to make my diagrams look neat like that.
nice video can we use true RMS DMM without peak measure capability instead of oscilloscope
How do you measure the output impedance of high frequency devices, for example a crystal oscillator (if it makes sense) or an amplifier?
As usual, thanks for your videos!
Another Excellent video .. Regards de Gw0wvl 👍
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
Thanks so much!
Hello Alan, my oscilloscope has a ground power cord. I hear other technicians saying to use an isolated transformer 1:1 or simply use a two prong adapter. What is your recommendations?
Keep your scope grounded and use an isolation transformer on whatever you’re working on - but only when necessary. And, always be cognizant if the fact that your scope probe ground lead *is* grounded.
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!
Everytime I watch your video, I learn something :D But can I ask how much you pay for that osci-scope? The one at my school is not that cool
Hi Allen
I just watched your video on How to Measure Output Impedance.
I wanted to measure the source impedance of a SI5351. When I use the 1/2 voltage method I get 49 ohms, very close.
Next I tried the 2 resistor method and it only works out to be 1/2 of what I was expecting, 24 ohms. My R1 = 330 and R2= 814.
V1=3.22 and V2=3.36. I'l double check my measurements and calculation.
With this method should one of the resistors be close to 50 ohms?
73 Ken VA3ABN
Great video. Subscribed. Just wondering what you used for your funct generator.
This was a home-made function generator that i designed and built 25 years ago. Unfortunately, I have lost the documentation of the design, so I'd have to do some reverse engineering to recover the design.
***** Don't sweat it. Just thought I noticed the same kind of distortion on the peaks of the sine wave as I have on my function generator I built with an XR2206. Might be just seeing things though. In any case, great work on your videos. And if you get a minute, I'd love to hear your input (read: see a video) about good grounding techniques (esp. in regards to audio electronics.) Thanks!
Niiiiiiiice lesson... thanks man
Hello Alan, my other question is when you have multiple test equipment such as an oscilloscope, frequency counter, signal generator, and they all have different impedance output such as 50 ohms, 75 ohms, 600 ohms or 1 meg ohms. What coax cable is appropriate when connecting the test equipment ???
There should only be one termination on the line. Other devices should be in high impedance mode. The cable should match the source and terminating resistance. So signal generator is 50 ohm source, freq counter is high impedance connected to a tee, o scope at other end terminated in 50 ohms, using 50 ohm coax between each.
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.
Hi Alan, quick question...if I'm not using a t-connector with the bananna jack adaptation, where exactly in the circuit should I connect the probe? The ground clip? Do these locations change when I add the load?
The probe should connect across the "load", whether the load is an open circuit (first part of the test) or a resistor (second part of the test). Ground on the probe should be connected to the ground/shield/common from the signal source.
thanks alan! That's what I thought. One note of interest, this approach works with a multimeter as well. I tried the math out with RMS voltages and it works out correctly so long as you consistently apply RMS (or even just peak) to every voltage variable.
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?
Than you for thus video. So, in order to measure the output impedance of my sound card must I use the 1 Kohm resistor and the formula you sent as? The measure woudn t be correct if
I only use my multi-meter Omhs parameter? Thank yuo so much.
You can't use an ohmmeter to measure output impedance because it depends on the active properties of the circuit. The ohmmeter *only* works on unpowered devices.
Nice video...
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.
Awesome!
Please tell us how to measure inductance at high freq. When LCR meter only measures up to 1khz?
Did you see my video on how to measure inductance with a scope and some basic parts?
Measure Capacitors and Inductors with an Oscilloscope and some basic parts
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!
nice video :) will this method work with square wave generator say from 555 timer or we can only measure the impedance of sine wave generator using this method
It should work well with a square wave output too. Just keep in mind that many logic circuits have a different output impedance for the high state and low state.
Many 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?
Please understand that I really enjoyed your video and you are a good teacher. On top of that did your thumbnail clearly show that your video was about low frequency. But I am looking for high-frequency impedance measurements, do you know someone with your teaching skill level that is covering that subject? :-)
High frequency impedance measurements are typically done with a VNA (vector network analyzer)
Hi, Alan
I guess my question is about the differences (if any) between… 'Output Impedance’ vs. 'Internal Resistance’, ... or ( 'Source Impedance' *).
Are each of these terms completely synonymous? or is there enough of a difference between them where they are all needed in order to properly describe slightly different things at work.
---
In the class I’m taking we did an experiment to determine the INTERNAL RESISTANCE (R_int) of a voltage supply.
The 'SPECIAL CASE’ section of your video here basically covers the same process and formula we used… with the minor difference that… V-open / 2 = R_int (in our experiment).
In the experiment we did I used a potentiometer as the R_load,… and adjusted it until V_load = V_open/2.
After which, the POT was measured to determine it’s resistor value… and we approximated that to being (R_int).
[ ... we ultimately did this for a variety of R_load values… and calculated the Power Differences as R_load values shifted away from matching what R_int is; … and more calculation were done to determine dB gains and losses-relative to the values of the matched pair ( R_int = R_load ). ]
I guess In hindsight… we were doing a little more than just finding out what R_int was… it was also about maximizing power transfer, …and maybe starting to flirt with impedance matching too? though, I’m still not 100% solid on understanding everything that impedance is. I have been reading about it (a lot)… but still need more real experience with it, and intuitive understanding… in terms of circuit building, and more hands on testing.
Thanks for the Great Channel, Alan… I really LOVE IT. and it’s helping me a lot in my attempts to learn about electronics.
Take Care,
Jim
* My mention of the 'Source Impedance' comes from watching your impedance matching videos, which I'm just now starting to look at.
.
In general, the "internal resistance" and "output impedance" in this context are basically the same thing.
Thanks you.