Hi,

I wanted to know what happens to the speed of a digital circuit when the gain is increased?
do we have to sacrifice noise margin to get better speed?

 

Don't think gain is a concept that really applies to digital circuits. Obviously the transistors used have gain but this only applies during the transition between states. Where it spends as little time as possible. When in the 0 or 1 state, the transistors are off or saturated and have very little gain.

Speed is down to how quick a gate can change from 0 to 1. Depends on stray capacitance and the resistance of the transistors mainly.

Don't think noise margin is related to speed either. Perhaps someone else would like to comment?

 

The only logic family I can think of that exhibits any change is 4000 CMOS, and the logic switches faster as the Vcc is increased.

Gain in logic is fixed - assuming we are discussing logic IC's and not some discrete DTL or RTL circuit - and the switching point is clearly defined. That switching point might be considered as "noise margin", but that is more an analog concept.