MOS output stage circuit and source follower features

MOS output stage circuit

When designing the output stage of an operational amplifier, it is necessary to consider its ability to drive a load (in MOS circuits, the load is mainly a capacitor), output dynamic amplitude, output impedance, and frequency characteristics. A good output stage circuit should have strong drive capability, low output impedance, large output amplitude and better frequency characteristics.

1. MOS source follower

Usually the output circuit always adopts the form of a follower. The simplest source follower is shown in Figure 2.6-1a, where M1 is the source follower and M2 is its active load. Due to the low transconductance of the MOS tube, the output resistance of the circuit is much larger than that of the bipolar circuit. MOS output stage circuit. In addition, due to the substrate bias effect, the voltage gain of the source follower is only 0.8-0.9, and the maximum dynamic fortune of the output is reduced.

From the small-signal AC equivalent circuit, the source follower voltage gain can be expressed as

whereis the transconductance of M1,are the output impedances of M1 and M2 respectively,

It can be seen that if there is no substrate bias effect, Av=1.

In order to reduce the given resistance and eliminate the substrate bias effect, the nPn tube can also be used in the MOS circuit to replace the M1 tube in the figure, as shown in Figure 2.6-1b. MOS output stage circuit. The Pn tube here is a parasitic nPn tube in the MOS circuit. Its collector region is an n-type substrate, the base region is a P well, and the emitter region is an n'diffusion region, which is formed at the same time as the drain-source region of the n-channel MOS tube.

Relevant information

CMOS complementary output stage circuit

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