Bias Circuit

Unit 8. MOSFET Differential Amplifier
Stage
The MOSFET differential amplifier stage is a relatively high-gain stage with grounded dc inputs,
which requires no bypass capacitor. The stage also has the important advantage of having two
inputs, one inverting and one noninverting. This is essential in operational amplifiers where
one or the other (or both) is used, depending on the application. The two inputs also provide
for operating the stage in the differential mode, where the input is not referenced to ground.
The differential amplifier stage discussed here is an all-resistor circuit with source and drain
resistors. In a modern integrated circuit, these are replaced with transistor current sources.
However, the principle of operation is the same, and the resistor version can be implemented
in our laboratory project.
Various segments of the differential amplifier comprise amplifier stages from all three terminal
configurations: the common source, the common gate, and the source follower (common
drain). In this unit, a detailed theoretical analysis of all three stages is undertaken (based on
Level 1 SPICE). The linear models used include, progressively, the output conductance, gds,
and the body-effect transconductance, gmb.