Differential amplifier MOS with active current load

Hi everyone !

I would like to know why in the differential mode of the diff amplifier MOS with active current load, the output impedance is looked from the drain and in the common mode of this diff amplifier, the impedance is looked from the source ?

I would be very grateful if you can help me !

Regards

This are the equations I forgot to write.

Differential gain:

Av = -gm * (ro4 // ro2); where ro4 is the impedante looking into the drain of the PMOS transistor (active load) and ro2 is the impedance looking into the drain of the NMOS (one of the input terminal of the differential amplifier MOS)

Common gain:

Acm = - (1 / gm4) / (2 * ro6); where ro6 is the impedance looking into the drain of the NMOS transistor (bias) and 1 / gm4 is the impedance looking into the source of the PMOS transistor (active load).

Why in the common gain the equation has 1 / gm4 instead of ro4 ? This make feel confuse.

Why in the differential gain, used the impedance looking into the drain of the PMOS and NMOS and in the common gain used the impedance looking into the source of the PMOS ?

Regards,