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MLCC Murata Chip Capacitor What is an op amp compensation capacitor?
2018-11-02
 
Two roles

1. Change the phase shift of the feedback network to compensate for the phase lag of the op amp

2. Compensate for the effect of the capacitance at the input of the op amp (in fact, the final phase is compensated...)

Because the op amps we use are not ideal.

Generally, the operational amplifier used in practice has a corresponding phase shift effect on a signal of a certain frequency. Such feedback of the signal to the input terminal will make the amplifier circuit unstable or even oscillate. For this reason, the corresponding Murata capacitor must be added to compensate the phase. . The compensation capacitor is usually built in the op amp. Of course, it can be added to the circuit if necessary. The value depends on the signal frequency and circuit characteristics.

If the resistance of Rf is large, the upper limit frequency of the amplifier will be seriously degraded, and the additional lag phase introduced by Cs and Rf may cause parasitic oscillation, which may cause serious stability problems. A simple solution to this is to reduce the resistance of Rf so that ωh is higher than the frequency range of the actual application, but this method will reduce the voltage amplification of the op amp (due to Av = -Rf / Rin). In order to keep the voltage amplification factor of the amplifying circuit high, a more general method is to connect a compensation capacitor Cf on the Rf to make the RinCf network and the RfCs network form phase compensation. RinCf will cause the output voltage phase to lead. Since the value of Cs cannot be accurately known, the phase lead and hysteresis cannot be fully compensated. Generally, the variable capacitor Cf is used to minimize the additional phase shift by experiment and adjustment of Cf. If Rf = 10 kΩ, the typical value of Cf is 3 to 10 pF. For a voltage follower, the Cf value can be slightly larger.


In addition to the above-mentioned unstable factors, there are other unstable factors, some of which come from the integrated chip itself. Some are derived from system circuits (such as the coupling problem of the internal impedance of the power supply). Sometimes it is difficult to solve the instability problem by using many methods, but the problem can be solved by using the appropriate compensation method. E.g. When the amplifier does not require a wide frequency band and an optimal slew rate, the overcompensation method for the integrated op amp will achieve good results, such as increasing the compensation capacitor by a factor of 9 or the multiple required to achieve stability, for μA301 For an op amp, the effect is generally better.

 
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