MOSFET Differential amplifier
(a) Zero both input. Divide the tail supply into equal parallel current I’0/2 in parallel with a resistor 2 RQ. The circuit obtained for M1 is shown on the left in Fig. 2. The circuit for M2 is identical. Now make a Thevenin equivalent as shown in on the right in Fig. 2. This is the basic basic bias circuit.
Small Signal or AC Solutions
The solution assume that the two FET’s are matched
Common-Mode Rejection Ratio
The CMRR for the BJT differential amplifier was defined with the output taken from only one side of the diff amp. To illustrate another way of defining the CMRR, it will be assumed that the output is taken differentially between the two outputs. In this case the CMRR is doubled.