Differential pair output

[STOIKOV]

in a differential-pair what's the difference between obtaining the output currents (I1,I2) as the picture shows, and doing a small signal analysis with superposition I mean with VDD in short-circuit to ground.

 

[Verba]

if you need the gain of the circuit, you need to calculate dI/dVin
small signal analysis will give you the same result.

 

[pthoppay]

The equation is correct, only thing is it gives the large signal analysis of the current flowing through each transistor.

i.e DC+AC components, as you can see when the "vin" is very small then the fourth order can be neglected w.r.t sciebtific approx and then to find the gain you do a derivation w.r.t "vin".

Iss/2 correposnds to the DC component or can be seen as the biasing current fixing each transistor in the saturation region.

Prakash.