The role of positive feedback capacitors in op amps

bigbat

The role of positive feedback capacitors in op amps [Copy link]

This is a circuit that converts the voltage of the PWM circuit through a piezo filter. After the conversion, the output is output using an op amp voltage follower circuit.

1. What is the function of capacitor C3 in this circuit?

2. What is the function of resistor R3 in the circuit?

The positive feedback circuit is equivalent to a comparator, but because the negative feedback is relatively strong, the entire circuit is always a voltage follower. So what is the use of this capacitor?

呜呼哀哉

Without R3, it is a second-order Sally-Kay low-pass filter. You can check the Sally-Kay circuit for its specific function. There is a lot of information.

maychang

The second floor said it well, this circuit is a second-order Sallen and Key low-pass filter.

What is the function of capacitor C3 in this circuit?

Without C3 (open circuit), this is a first order low pass filter (formed by R1R2 and C2).

maychang

What is the function of resistor R3 in the circuit?

Provide a DC path to ground for the op amp's non-inverting input to prevent the op amp's input bias current from saturating the op amp. This resistor is only useful when there is no DC path at the PWM end.

乱世煮酒论天下

I have seen this circuit structure before and always thought it was a low-pass filter structure, but it is different from the common low-pass filter structure. I also looked up information saying that it is a low-pass filter structure for current signals. Today I saw that it is to reduce the input bias current? But is the selection of this resistor so large? Assuming that a bias current of 1nA has a voltage drop of 2mV on a 2M resistor?

gmchen

Typical low-pass filter circuit.

The specific order depends on the internal resistance of the signal source. If the internal resistance of the PWM signal source is very low (voltage source), it is a 2nd order low pass filter; if the internal resistance of the PWM signal source is high (the same order of magnitude as R1 and R2 or higher), it is a 3rd order low pass filter.