Problem with decreasing gain in folded cascode opamp

[lhlbluesky]

ro=1/(lambda*id), lambda~1/L,so the larger the size of L, the larger the gain;
but when i simulate a opamp (foded cascode),i find that when i make the size of the transistors(the lower most two for nmos input) twice the original value,the gain decreases rather than increase a little;confused,what's the reason?
pls explain to me.

 

[dineshbabumm]

a strange problem

Gain decreases! Fine.. It means your gm decreases.. Check the gm value before u increase teh length and after u increase teh length! Also verify if your resistance has increased or decreased(it wil increase only if u had used same current.. i mean the same current u used before increasing the length.. In that case surely you ro must increase and gm must be the same(if u havent changed your overdrive and used same current..)) In that situation surely your gain must increase!

Hope it helps!

 

[malizevzek]

Re: a strange problem

you have to be careful - make sure which transistors need to have large ro and which ones large gm. If you increase L, you do increase the output resistance, but your gm falls down. In order to avoid that, you have to increase W proportionally. Where gm is not critical, you can leave "the old" W, but have to increase Vgs in order to have the same Ibias.

 

[lhlbluesky]

a strange problem

thanks all