Input transconductor dimensions

[mirror_pole]

guys i have a noob question. If i design on OTA does it make sence to chose a small gm? I always thought that the gm of the input transconductor is huge to get more gain, but then i saw a design where the transistor dimensions on the input stage were pretty small. Are there other benefits to chose small/big gm then for gain considorations?

 

[Dominik Przyborowski]

Noise/linearity tradeoff.

Notice that number of low gm OTAs are working in open loop configurations, so they have to has gm in well defined range.

 

[deep_sea]

A good starting point is to see how much GBW is required. If your dominant pole is at the output (5T OTA, folded and telescopic cascode), then you have
\[ GBW= \frac {gm1} {2* \pi * CL} \]