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MOS differential amplifier with cascode active load
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The differential-mode voltage gain is proportional to the output resistance looking into the active load transistor. The voltage gain can be increased, therefore, if the output resistance can be increased. An increase in output resistance can be achieved by using, for example, a cascode active load. This configuration is shown in figure.

enter image description here

The output resistance Ro is given by

R0=r04+r06(1+gmr04)gmr04r06

The small-signal differential-mode voltage gain is then

Ad=v0vd=gm(r02||R0)

The differential-mode voltage gain can be further increased by incorporating a cascode configuration in the differential pair as well as in the active load.

One such example is shown in figure. Transistors M3 and M4 are the cascode transistors for the differential pair M1 and M2. The differential-mode voltage gain is now

Ad=vovd=gm(Ro4||Ro6)

where Ro4gmro2ro4 and Ro6gmro6ro8

The small-signal differential-mode voltage gain of this type of amplifier can be on the order of 10,000.

enter image description here

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