grosser wrote on Jun 28th, 2006, 12:54am:hello
can you suggest me how to increase transconductance gmx of Mx transistor without increasing bias current?
what circuit solution i should use?
regards
Hi,
You can certainly increase the Gm of Mx by using larger W/L ratio. Your bias current is set by the lower transistors and so this can be kept constant. In a differential pair, one can easily apply some small amount of +ve feedback to boost the Gm. You can look up the paper by Castello et al. (JSSC, June '90) and see if you can adapt his ideas somehow. However, be aware that circuits with +ve feedback need to be designed very carefully or else there will be instability. If nothing else works and you have access to good NPN BJTs, use them. MOS devices can never beat BJTs in terms of Gm/I. You will obviously need some way to compensate for the base current there.
The change in Gm vs. L is not apparent from the square-law model, and you cannot derive it thus. Short-channel devices have reduced Gm owing to velocity saturation which tends to shift the I-V curve from square-law towards a linear one. Look up any good device physics book (Tsividis for example).
Regards
Vivek