When the op amp is used as a voltage follower, what is the role of the resistor and capacitor connected to the negative feedback terminal?

普拉卡图

When the op amp is used as a voltage follower, what is the role of the resistor and capacitor connected to the negative feedback terminal? [Copy link]

As shown in the figure: C9, R54 are connected to the negative feedback loop, what is their function?

I looked up some information this morning:

Resistor part:

Statement 1: When biasing, the resistance of the inverting and non-inverting terminals should be consistent (I personally think this statement is unreliable)

Statement 2: Some op amps, such as CFA, require resistors to be added for negative feedback

Statement 3: Since some op amps have diodes inside (some special structures), they are also required to connect this resistor

I tend to believe in statement 2 and statement 3.

Capacitor part:

Statement 1: When biasing, the capacitance of the inverting and non-inverting terminals should be consistent (I personally think this statement is unreliable)

Statement 2: Filtering

Statement three: Adjust the negative feedback phase to compensate for the response delay caused by the input capacitor and suppress the self-oscillation of the op amp.

Above, I tend to believe in statement 3

However, this circuit diagram is for voltage sampling. When the input frequency is not high

I don't know if the capacitor is just reserved. I personally think it won't have much impact if the capacitor is not added.

Back to the original question: What is the function of C9 and R54 when they are connected to the negative feedback loop? Please leave a message to discuss. Thank you!

chunyang

This is related to the model of the op amp. You need to look at the relevant content in the device manual. The CFA statement does exist. Generally, when a VFA general-purpose op amp is used as a follower, there is no need to add a feedback resistor. If it is added and there is a vibration elimination capacitor, if the design is correct, it means that the op amp is special and phase compensation is required. In addition, your statement 3 also exists.

When applying, be sure to read the device manual of the op amp carefully to determine whether such a design is necessary.

chunyang

However, if statement 3 is true, R54 should not be too large. Too large a value may cause instability.

jimtien

In both cases, resistors should be added. As for capacitors, they will affect the bandwidth of current feedback op amps.

普拉卡图

chunyang posted on 2022-8-2 15:52 This is related to the op amp model. You have to look at the relevant content in the device manual. CFA does exist. Generally, VFA general-purpose op amps do not need feedback when used as followers...

Okay, thank you teacher!

普拉卡图

jimtien posted on 2022-8-2 21:44 2,3 In both cases, resistors should be added. As for capacitors, they will affect the bandwidth of current feedback op amps

Ok, thanks for your answers!