Op amp input compensation capacitor

The input parasitic capacitance Cs of a general linear amplifier (i.e., an amplifier circuit with negative feedback) will affect the stability of the circuit. The compensation measures are shown in the figure. The input end of the amplifier generally has a parasitic capacitance Cs of about several picofarads. This capacitance includes the input capacitance of the op amp and the wiring distributed capacitance. It forms a lag network with the feedback resistor Rf, causing the output voltage phase lag. When the frequency of the input signal is very high, the bypass effect of Cs makes the high-frequency response of the amplifier worse, and the upper limit frequency of its frequency band is approximately:

ωh=1／(2πRfCs)

If the resistance of Rf is large, the upper frequency limit of the amplifier will be seriously reduced. At the same time, the additional lag phase introduced by Cs and Rf may cause parasitic oscillation, which will cause serious stability problems. A simple solution to this problem is to reduce the resistance of Rf so that ωh is higher than the frequency range of actual application, but this method will reduce the voltage gain of the operational amplifier (because Av=-Rf/Rin). In order to keep the voltage gain of the amplifier circuit high, a more common method is to connect a compensation capacitor Cf in parallel to Rf so that the RinCf network and the RfCs network form a phase compensation. RinCf will cause the output voltage phase to advance. Since the value of Cs cannot be accurately known, the phase advance and lag cannot be fully compensated. Generally, a variable capacitor Cf is used to minimize the additional phase shift by experiment and adjustment of Cf. If Rf=10kΩ, the typical value of Cf is 3~10pF. For voltage followers, the Cf value can be slightly larger.