Voltage to Current Converter explained with PMOS and NMOS transistors

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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.

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1 окт 2024

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Комментарии : 23

@ivolol 4 месяца назад

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

@STEMprof 4 месяца назад

@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 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Lu760gdQee0.html which is a Modified Wilson Current Mirror with PNP BJT Transistors.

@jg99997 4 месяца назад

Whats the point of the first stage? Why not just use e second stage alone?

@STEMprof 4 месяца назад

Good question. Without the first stage, we get a Negative Transconductance for V-I conversion.

@ricolauersdorf687 4 месяца назад

@@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.

@christopherventer6391 4 месяца назад

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.

@STEMprof 4 месяца назад

@@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 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-mNYq5QId1pE.html is a Hybrid Current Mirror with JFET BJT transistors -------- and ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-QY48IQXJIRI.html discusses a Temperature-Compensated Programmable Current Source Circuit Design with Zener Diode, BJT Transistors.

@STEMprof 4 месяца назад

@@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 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Rs7gEMk03dw.html Programmable Current Source Design with Op Amp, Zener Diode and Digital Potentiometer ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-R1x6B0TczWk.html

@STEMprof 4 месяца назад

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 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-LfbfJYrovN0.html Temperature-Independent Current sink with Op Amp, BJT, Zener, Schottky Diodes ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-hFbnjbddUvs.html Thermometer Circuit Design with Op Amp & BJT transistor ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-55YsraFE0rg.html Analog Computer Vector Sum Calculator with Op Amp and BJT Transistors ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-PIAsa0QNVns.html Instrumentation Amplifier with Electronic Gain Control ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-C4tghZ-q6Zs.html Programmable Current Source Design with Op Amp, Zener Diode and Digital Potentiometer ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-R1x6B0TczWk.html On-Chip BJT Current Source Design ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Rs7gEMk03dw.html Push-Pull Power Amplifier with Darlington Transistors ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-866MYibo8yE.html Analog Logarithm Computer ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-RpKEq5WyoLg.html Op Amp Analog Computer Differential Equation Solver ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-ENq39EesfPw.html Sawtooth Waveform Generator design with OpAmp, JFET, BJT ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-5zHXTx-Vl20.html op amps Circuit with feedback loops to design an analog computer that solves a second order differential equation ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HeZRtnRXpEI.html For more analog circuits and signal processing examples see: ru-vid.com/group/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt I hope these Circuit design and analysis videos are helpful.

@chaochang1305 4 месяца назад

So it seems that we don't need to care about the output of the op amp?

@christopherventer6391 4 месяца назад

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.

@STEMprof 4 месяца назад

@@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.

@STEMprof 4 месяца назад

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 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-NoNgQpbj77Y.html of a gain control Op Amp output converging in real time to proper voltage value to electronically control the resistance of JFET transistor.

@Chupacabras222 3 месяца назад

Very interesting circuit. Well explained.

@STEMprof 3 месяца назад

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 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-R1x6B0TczWk.html . I hope it is interesting as well.

@ats89117 4 месяца назад

Very useful voltage to current source.

@STEMprof 4 месяца назад

Glad that you liked this voltage controlled current source. Here are a few related circuit videos: On-Chip BJT Current Source Design ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Rs7gEMk03dw.html Programmable Current Source Design with Op Amp, Zener Diode and Digital Potentiometer ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-R1x6B0TczWk.html hope there are useful as well.

@jeanaimarre8605 4 месяца назад

Exceptional training course. Thank you !

@STEMprof 4 месяца назад

You are welcome! Additional circuit videos about Current Sources, Impedance Converters and Gyrators are posted in ru-vid.com/group/PLrwXF7N522y48AAPxFaQlowim4-8gYoWz playlist. Hope these videos are helpful as well.