# Feedback mechanism intuition

There is a circuit taken from the presentation below.

Suppose at some point the VFM voltage rises from V0 to V0 + 0.01 V so V_minus of the op-amp rises too. So what causes the voltage on VFM to return to V0?

https://cgit.osmocom.org/osmo-small-hardware/plain/yang/doc/yig_and_yang.pdf • The change in the supply currents for the opamp does the sensing work, altering the bjt vbes. Oct 1 at 17:56
• You can find uses not entirely unlike this here and here for example. Oct 1 at 18:15

The internal circuitry of the op amp.

If V_minus rises above V_plus then there would be a difference voltage between the op amp's inputs which drives the input and causes the op amp's internal circuitry to act in a way which would drive the op amp's output (Vop) downwards.

This is negative feedback in action.

• Hello user350400,to whatBIAS state i need to put the NPN and PNP ,there is forward active or saturation.What state do you think the NPN and PNP are operating in? Thanks. Oct 1 at 18:50
• @lub2354 That's a different question. Please ask a separate question to try and obtain answers to that. Oct 1 at 19:05

There is no "V0" on your drawing, so the question is not clear.

Consider this -

If we call Vfm the "output" voltage of the circuit, then the overall circuit is a non-inverting voltage amplifier. The unknown stuff to the left is the input voltage, Vfm is the output voltage, R4 + L1 form the series impedance of a negative feedback loop, and R2 is the shunt impedance.

At DC, L1 is simply a resistance in series with R4. With these values, you can calculate the circuit gain at DC. If Vin is steady at a non-zero value, the output will be greater than Vin and steady. After being attenuated by the feedback loop components, the voltage at the inverting input (what you call V_minus) will equal Vin, and neither will be 0 V.

• Hello AnalogKid,to what BIAS state i need to put the NPN and PNP ,there is forward active or saturation.What state do you think the NPN and PNP are operating in? Thanks. Oct 1 at 18:51