Electronic – op amp+mosfet = current source.Why do we need a feedback resistor


The feedback resistor is needed to compensate for the error of the input currents? How to choose the resistance R2.

Circuit source

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Resistor R2.

Can I use this circuit, op-amp with differential input voltage range = +/- 0.6V? I'm not sure. I think not

Best Answer

R2 (10k R4 in my diagram) is there to form together with C1 (1nF capacitor) a Miller Integrator to prevent unwanted oscillation. And yes, this circuit will sometimes oscillate, mainly due to poor PCB/breadboard design. And here you have a real world example (the breadboard one).

Without the Miller capacitance: circuit diagram and trace showing oscillation

And after I add the Miller capacitance into the circuit: Circuit diagram and trace, this time showing flat output



Today I test this circuit again. And the result are: For RG = 0 Ohms; RF = 10k Ohms without Miller capacitance circuit oscillate (I_load from 1mA to 1A).

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But surprise surprise If I short RF (10K) resistor the oscillations magically disappear (even if RG = 1K ohms).

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So, it seems that the main cause of a oscillation in my circuit was a feedback resistor. I suspect that RF together with opamp input capacitance and some parasitic capacitance add a pole (lag) to the circuit and the circuit start to oscillate.
I even change the opamp to "much faster one" (TL071).And results was almost the same except the fact that he frequency of oscillations was much higher (713kHz).

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