Analyze CMOS operational amplifiers with low output impedance and strong drive capability

Figure 3.4-5 is a low output impedance op amp. The output stage adopts Class A and Class B push-pull amplifiers, which can drive a low resistance load of 200 ohms. In order to reduce the distortion, the output stage is in a micro-conduction state in static state, and its current bias is determined by M6~M9. M12 is used as an equivalent resistance (1/gm1) in series with the compensation capacitor Cc to eliminate the zero point of the circuit and improve stability. As explained in Chapter 2, the disadvantage of this circuit is that the output amplitude is relatively small.

Figure 3.4-6 is also a low output impedance op amp. The output stage uses a transistor (Q3) follower, which can drive a large load capacitance (100~150 picofarad) and a small load resistance (3 kohm), and the unity gain bandwidth is 8 MHz, and has a phase shift margin of 60 degrees.

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