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Design >> Analog Design >> folded cascode...
https://designers-guide.org/forum/YaBB.pl?num=1245912685

Message started by somisetty on Jun 24^th, 2009, 11:51pm

Title: folded cascode...
Post by somisetty on Jun 24^th, 2009, 11:51pm

hi all,

can u tell me why the name folded cascode?

why miller compensation not required in folded cascode OTA?

Thanks

Title: Re: folded cascode...
Post by raja.cedt on Jun 25^th, 2009, 1:14am

hi,
because it is single stage..so how you use miller compensation...generally in two stage miller compensation is required but the same two stage gain you will get in single stage if you use folded.

Thanks,
Rajasekhar.

Title: Re: folded cascode...
Post by nobody on Jun 25^th, 2009, 2:32am

The input pair is folded so that the input range is improved. Sometimes, the miller effect will have effects on PM.

Title: Re: folded cascode...
Post by raja.cedt on Jun 25^th, 2009, 5:40am

you mean miller effect at the input tx gate..any how your drain is low impedance because of cascoding..no problem

Thanks,
rajasekhar.

Title: Re: folded cascode...
Post by nobody on Jun 25^th, 2009, 7:42pm

Basically, you assume the impedance frequency is 1/gm, which is right @ high frequency and the gain is gm,inp/gm,cascoded.
However, the impedance is not low @ low frequency and you can try to derive the gain @ low frequency and see what you have.

Title: Re: folded cascode...
Post by raja.cedt on Jun 25^th, 2009, 9:30pm

yaa..its correct now you catch the correct point..pole at the input device is non dominant ..so it will be come into action at high frequency..so whats the problem..

Thanks,
Rajasekhar.

Title: Re: folded cascode...
Post by nobody on Jun 25^th, 2009, 11:46pm

It is my fault saying about the miller effect on PM. The miller effect will show up in SC ckt and has the influence on the loop gain. Sorry about that. The non-dominant pole is basically determined by PMOS or NMOS input pair.