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Design >> Analog Design >> Folded Cascode : Sizing of Devices
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Message started by raduga on May 16^th, 2007, 5:32am

Title: Folded Cascode : Sizing of Devices
Post by raduga on May 16^th, 2007, 5:32am

Hi All,

I am designing a floded cascode amplifier. It is a diff. input and single ended
output. I have one doubt there. If I would like to keep the output voltage
at the middle of the o/p swing range then how you should size the 4 transistors
quickly to get that point. Can any one give me his/her methdology.

Thanks in advance.

Regards

Raduga

Title: Re: Folded Cascode : Sizing of Devices
Post by mosman on May 18^th, 2007, 1:05am

the size of transisitors is based on the gain and BW. basically we want to get tahe gain as larger as possible. usually, gain equals gm*Rout. so you should increase the rout and gm. the cascode configure's rout is inverse to the biasing current, and propotianl the Longth. for gettting the larger Rout decreasing the biasing current of out colume is a good idea. meanwhile, you need increase the input biasing tail current to increas the gm. but too large ratio is very good. some experencided designer advizes the ratio should be 3~4.

best regards,