In this Common Source CMOS amplifier, the current is set by PMOS (3), it's 2* Ibias due to 1:2 mirror. However NMOS (1) is also a current source with gate at Vi, why the current in the branch is not set by this?

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1 Answer 1


I've used this topology ---- constant current source load for bipolar amplifiers ---- to achieve gains of 10,000x (80 dB) in one stage. The output was buffered using a JFET to avoid bias currents.

The "vi" was part of a negative feedback loop, thus "vi" was servo-loop adjusted to hold the gain node near the mid-swing for adequate linearity.

Summary: Fet#1 cannot be operated as constant current.


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