Differential pair output

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STOIKOV

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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.
 

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

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    STOIKOV

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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.
 

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