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What is the operational amplifier compensation capacitor?
2018-11-21
 
1, about compensation capacitance.


There are some theoretical calculations, but at the mature stage of design, it seems that most people rely on previous debugging experience. Generally, the value of capacitance should take into account the frequency response of the system (simply, the larger the capacitance added, the narrower the bandwidth), and then the oscillation problem. If you have to calculate, you can see the transport of the op-amp. For example, the negative feedback amplifier circuit is to ensure that the product of the input resistance and the distributed capacitance equals the product of the feedback resistance and the capacitance you want to add...


2, two roles


1. change the phase shift of feedback network and compensate the phase lag of op amp.


2. compensating the influence of the capacitance of the input of the operational amplifier. )


Because the operational amplifiers we use are not ideal.


Generally, the operational amplifier used in practice has corresponding phase-shifting effect on a certain frequency signal. Feedback of such signal to the input terminal will make the amplifier circuit unstable or even oscillate. Therefore, the corresponding capacitance must be added to compensate the phase. Compensation capacitors are usually built in the operation, but they can also be added to the circuit if necessary. The value depends on the signal frequency and circuit characteristics.


The input parasitic capacitance Cs of linear amplifiers (i.e. amplifiers with negative feedback) will affect the stability of the circuit. The compensation measures are shown in the figure. The parasitic capacitance Cs of about a few PI method generally exists at the input end of the amplifier. This capacitance includes the input capacitance of the operational amplifier and the distributed capacitance of the wiring. It forms a lag network with the feedback resistance Rf, which causes the phase lag of the output voltage. When the frequency of the input signal is very high, the bypass effect of Cs makes the high frequency response of the amplifier worse. The upper limit of the frequency band is about:


Omega h=1/ (2 pi RfCs)


If the resistance value of Rf is large, the upper limit frequency of amplifier will decrease seriously. 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 and make_h higher than the practical frequency range, but this method will reduce the voltage amplification factor of operational amplifier (because of Av=-Rf/Rin). In order to keep the voltage amplification ratio of the amplifier circuit high, a more general method is to connect a compensation capacitor Cf to the Rf to make the phase compensation between the RinCf network and the RfCs network. RinCf will lead to phase advance of output voltage. Because the value of Cs can not be accurately known, it is impossible to completely compensate the phase advance and delay. Generally, variable capacitance Cf is used to minimize the additional phase shift by experiment and adjustment of Cf. If Rf=10k ohm, the typical value of Cf is 3 to 10pF. For voltage follower, the Cf value can be slightly larger.
 
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