In Reply to: Follow Up on differential amplifier posted by Jason on 08/28/03 at 5:44 AM:
In circuits A and B, Q1 and Q2 are common source amplifiers. Q3 and Q4 are not amplifiers, so the designation does not apply. AC ground occurs when there is no signal on the port. This can be accomplished with a bypass capacitor or a voltage source.
Your circuit B is not a good amplifier because both Q3 and Q4 have gate and drain connected to VCC. The amount of signal that can be developed at Q4 is just Vgs4 so the gain is 1.
The configuration I proposed is shown here: http://www.geocities.com/russlk/6tamp.gif
Note that M3 and M4 gates are connected to a voltage source, so they are common gate types. The reason for M3 and M4 is to provide isolation between the input and output. Consider this to be the solid state equivalent of a screen grid in a vacuum tube.
Your question 2: The small signal model is a linear version of the macromodel. The grounded port does not change, so I don't understand your confusion.
Question 3: Your equation is correct for circuit A, but the equation for circuit B is: -gm2*rsg4. Since rsg4 is very low, the gain is low.
The Pmos current mirror is used with a Nmos amp because the Nmos current mirror (as in circuit B) does not work.