In Reply to: Differential Amplifier posted by Jason on 08/24/03 at 12:02 PM:
Q3 and Q4 comprise a current mirror. The drain currents of Q3 and Q4 are equal because the gate voltages are equal. You could not do this with n-type because the drains have to go to VCC and driving the source would not be constant current. There could be some bandwidth advantage in using n-type for Q3 and Q4 because driving the source is low Z which minimizes drain-gate feedback. Putting the load in the drain circuit of Q3 or Q4 also has minimum drain-gate feedback because the gates are grounded to AC.
Using N-type and P-type would be unbalanced and makes no sense.
This is not a differential amplifier, by the way, because Q2 has drain and gate connected. The signal path is Q1 gate to Q1 source to Q2 drain. There is no gain because Q2 gate cannot rise above the source more than required to maintain Ib.