MOS operation equivalent circuit and its characteristic analysis

In a bipolar op amp circuit, the transconductance gm is usually large, and the frequency of the zero point Z is much greater than the unity gain bandwidth GB, which will not affect the stability of the circuit, so the frequency compensation capacitorThe situation shown in Figure 3.1-2 will not appear. But the MOS op amp situation is different, because the transconductance of the MOS tube is low, and the frequency compensation capacitorthe resulting zero point Z is low, causing the instability of the op amp, so the frequency compensation of the MOS op amp needs to be further discussed.

The zero point of the op amp is due to the compensation capacitorthe feedforward effect. The so-called feedforward is to compensate the capacitance at high frequencyThe output signal of the first stage is directly fed to the output terminal of the second stage. Obviously, the signal passes through the compensation capacitorthe phase difference between the phase fed to the output terminal and the output phase after the signal is amplified by the second stage is 180°. In the bipolar op amp, in order to obtain high input impedance, the operating current of the first stage amplifier is very small, the transconductance (gm1) is very low, and the voltage gain is not high. In order to provide the open loop gain of the op amp and increase the driving capability . The second stage has a large operating current and a high voltage gain. Its transconductance gm is much larger than the first stage transconductance gm1, so the feedforward signal at the output of the second stage is much smaller than the signal amplified by the second stage , When it is comparable to the amplified signal, the total open loop gain of the op amp is much smaller than 1, so the compensation capacitorthe feedforward effect of has no effect on the stability of the op amp. In MOS operational amplifiers, in order to reduce noise and increase voltage gain, the transconductance gm1 of the first-stage amplifier is usually increased, and from the perspective of increasing the total gain of the operational amplifier, the working current of the first-stage amplifier and the work of the second-stage amplifier The currents are similar, that is, their transconductance and gain are very close. Since the transconductance of the MOS tube is not high, at this time due tothe signal generated by the feedforward effect on the output terminal cannot be ignored. When the frequency increases, its impact becomes more serious, and the additional phase shift of the op amp reaches -180° when it does not reach the unity gain, making the negative feedback change Into positive feedback. Regarding this situation, we can use the equivalent circuit diagram 3.1-3a of the op amp to illustrate, in order to illustratethe effect of C1 and C2 is ignored in the figure. At low frequencies, the capacitance  it forms an integral circuit with the amplifier, but at high frequencies, the capacitanceIt can be regarded as a short circuit. At this time, the second-stage amplifier tube can be regarded as a diode with the drain gate short-circuited. Therefore, the equivalent input resistance of the second stage becomes 1/gm1, which becomes the load of the first-stage amplifier. At this time, the total gain of the circuit is equal to the gain of the first stage, which is a gm1/gm2 and its phase is opposite to that at low frequencies. Figure 3.1-3b shows its amplitude-frequency characteristics. As mentioned above, in a bipolar op amp circuit, gm1》gm2, the gain is much less than 1 at high frequencies; while for MOS op amp circuits, gm1≈gm2, the gain is similar at high frequencies. Equal to 1. Therefore, the MOS operational amplifier circuit only uses simple frequency compensation capacitorsat times, its closed-loop working state is sometimes unstable.

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