# Explanation of filter for op amp controlled MOSFET current sink [closed]

I have designed this circuit and someone suggested R and C to enhance the functionality of the circuit.

The circuit is basically a current controlled device. Depending on the voltage set by the potentiometer, the current varied through the circuit.

However, what is the aim of the R and C?

How could they be calculated?

Could you please explain to me in certain detail its functionality and advantages?

• You said that you designed the circuit. Why did you put those components in there to begin with? Jan 12, 2021 at 13:02
• @RyanPace: You might have drawn the circuit, but you didn't design it. Design is the process of defining requirements and calculating the values of the circuit elements that make sure the circuit meets its goals. If you have to ask what a part of it is for, you didn't design it. If you'd designed you'd know what each part is for and how to determine proper values. This is a language thing. Words have meaning, and "design" doesn't mean "drew a bunch of parts copied from somewhere else without understanding what it does."
– JRE
Jan 12, 2021 at 13:23
• I ... cannot explain the role of the R. It makes no sense whatever. Jan 12, 2021 at 16:41
• @BrianDrummond An R connected to the inverting input of an OPA is sometimes used to suppress instabilities. Particularly when driving complex loads as in this case. But this depends strongly on the characteristics of the OPA and must be selected thoroughly. Jan 19, 2021 at 12:46

I have designed this circuit.

You've designed it incorrectly so...

what is the aim of the R and C.

The circuit is likely to be unstable without them. That's the short story.

Slightly longer story: The MOSFET gate-source capacitance adds an extra phase shift to the output of the op-amp and drives it closer to becoming unstable. The added R and C are there to combat this by ensuring that at the point of instability, the overall open-loop gain is less than unity thus, it prevents the circuit turning into an oscillator.

The really long story involves detailed analysis of the data sheet for the op-amp. Because you haven't supplied an op-amp name or data sheet, I cannot provide the really long answer. Even if you did supply the data sheet, a really long analysis may be impossible if the data sheet is poor/old/both.

Could you please explain to me in certain detail its functionality and advantages?