Voltage to Current Converter explained with PMOS and NMOS transistors
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- Опубликовано: 14 окт 2024
- A high-side Voltage to Current V-I converter circuit also known as Voltage-controlled current source is discussed in this video. How the voltage to current conversion works and how to choose circuit components values are explained to realize the target maximum output current and VTC which is DC transconductance coefficient for the conversion in this circuit. We also desire to maximize the conversion efficiency to more than 99% while maximizing output voltage compliance. This circuit is implemented with two operational amplifiers, one low-power N-channel MOSFET transistor and one high-power P-channel MOSFET transistor. The conversion of input voltage to output current is controlled by three resistors in this circuit. The output resistor is a low-impedance high power resistor with a value of of on the order of one ohm. The resistor tolerances are also discussed due to practical considerations. The stability or compensation feedback capacitor and resistors are also discussed for the op amps, NMOS and PMOS transistors.
Very interesting circuit. Well explained.
Thank you! Glad that you like Voltage-controlled current source video. A related example is the Programmable Current Source Design with Op Amp, Zener Diode and Digital Potentiometer ruclips.net/video/R1x6B0TczWk/видео.html . I hope it is interesting as well.
Exceptional training course. Thank you !
You are welcome! Additional circuit videos about Current Sources, Impedance Converters and Gyrators are posted in ruclips.net/p/PLrwXF7N522y48AAPxFaQlowim4-8gYoWz playlist. Hope these videos are helpful as well.
Very useful voltage to current source.
Glad that you liked this voltage controlled current source. Here are a few related circuit videos: On-Chip BJT Current Source Design ruclips.net/video/Rs7gEMk03dw/видео.html
Programmable Current Source Design with Op Amp, Zener Diode and Digital Potentiometer ruclips.net/video/R1x6B0TczWk/видео.html hope there are useful as well.
Your channel is solid gold ❤
Glad that you like this engineering channel, Thank you 🙏
Second opamp also need to be able to accept input to the top rail, right? Otherwise I'd think an easier choice would be to give them some higher supply like 12V. And first opamp input needs to be ground-sensing.
I thought that maybe you started discussing what the figure of voltage compliance should be, but didn't really summarise a final conclusion for that. I'd guess it'd be (10V - 1V - ~1V) = ~8V.
The design also seems to rely on a good quality 10V PSU, that can remain solid from 0-1A; that doesn't seem trivial either.
Appreciate the time spent on practicalities of compensation
@ivolol Good questions and thanks for the reminder regarding voltage compliance that I also briefly started discussing it at 4:46 in the video (mentioning voltage drops across resistor R3 and saturation voltage across source-drain of power PMOS). You summarized it well 10V - 1V - ~1V = ~8V is a practical assumption. Output voltage across the Load can vary from zero to about 8 volts in this circuit with proper design. For the supply, of course extra care is needed including considering say 100 uF (or higher) Bypass caps on the supply to minimizes supply noise and variations. If supply voltage variation is considerable then it should at least have a minimum value around 10 volt in order to insert a voltage clamper component like properly sized Zener Diodes. An example is discussed in the video ruclips.net/video/Lu760gdQee0/видео.html which is a Modified Wilson Current Mirror with PNP BJT Transistors.
Voltage to Current V-I converter circuit also known as Voltage-controlled current source is discussed in this video. For more analog circuit examples see: Wilson Current Mirror and Current Source Design ruclips.net/video/LfbfJYrovN0/видео.html
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So it seems that we don't need to care about the output of the op amp?
Mosfets are voltage-controlled devices and would not draw any significant current from the op amp output due to their very high gate input impedance. So the mosfets and resistors are the ones whose current handling are of concern.
@@christopherventer6391 Well-said, as long as the proper negative feedback and stable circuit is in place, then Op Amps outputs will converge to proper voltage values via negative feedback that will enforce virtual short for the positive and negative input terminals. thank you.
www.youtube.com/@chaochang1305 Good question, as long as the proper negative feedback and stable circuit is in place, then Op Amps outputs will converge to proper voltage values via negative feedback that will enforce virtual short for the positive and negative input terminals. Here is another example ruclips.net/video/NoNgQpbj77Y/видео.html of a gain control Op Amp output converging in real time to proper voltage value to electronically control the resistance of JFET transistor.
Whats the point of the first stage? Why not just use e second stage alone?
Good question. Without the first stage, we get a Negative Transconductance for V-I conversion.
@@STEMprof My simulation says that we can more or less give signal into inverting input of opamp which then drives p-mos and measures the drain with noninverting input. I think ive also done similar things on breadboard.
Perhaps for temperature stability, but I would not think the mosfets would be close enough in temperature if the currents they provide are very different.
@@ricolauersdorf687 Thanks for sharing your insights and simulations results of your alternate wiring with single Op Amp and power PMOS. While, feedback wise, what you suggested is functional, the potential issue is it grabs voltage from the drain of PMOS which means the feedback will depend on the load value rather than direct dependence on the output current of the circuit. Here are a few more circuit videos ruclips.net/video/mNYq5QId1pE/видео.html is a Hybrid Current Mirror with JFET BJT transistors -------- and ruclips.net/video/QY48IQXJIRI/видео.html discusses a Temperature-Compensated Programmable Current Source Circuit Design with Zener Diode, BJT Transistors.
@@christopherventer6391 Thanks Christopher for sharing your thoughts regarding thermal stability. While as @ricolauersdorf687 and @jg99997 noted, there are alternative ways to realize this circuit, the preferred design is the dual-stage design given that the BOM cost is super minimal since the NMOS transistor is a cheap low power transistors and given that the cheap CMOS Op Amp packages usually come in dual or quad op amp. The single stage design can work but it comes with its own drawbacks including more sensitivity to op amp noise and dependence on output load value in case of grabbing feedback voltage from the drain of PMOS. There are of course many other ways that a programmable current source/sink can be designed for instance: On-Chip BJT Current Source Design ruclips.net/video/Rs7gEMk03dw/видео.html
Programmable Current Source Design with Op Amp, Zener Diode and Digital Potentiometer ruclips.net/video/R1x6B0TczWk/видео.html
thank you
My pleasure! More circuit examples are posted in the circuit videos playlist ruclips.net/p/PLrwXF7N522y48AAPxFaQlowim4-8gYoWz .