telescopic and folded Opamps

folded opamps

Hello everybody:

I am writting here to discuss the difference between telescopic and folded cascode opamps. Please post your understandings. Also, can anybody tell me why folded cascode opamp is composed of different type of transistors while in telescopic cascode opamp, transistors are same?

Regars,

Telescopic opamp:
1 - only one type of transistor presented in signal path (usually NMOS)
2 - Lesser current legs thus smaller power conumptions
3 - Faster since it can have only NMOS in signal path (actually it is the fastest opamp structure among those well known ones)
4 - Higher supply voltage needed since 5 MOS stack to each other
5 - Input and output common mode level is not easy to be control to
equal
6 - Lower noise since lesser transistor is used

Folded-Cascode opamp:
1 - Two type of transistor presented in signal path
2 - More current legs thus higher power conumptions
3 - Slower since it must have PMOS in signal path
4 - Lower supply voltage requirement since 4 MOS stack to each other
5 - Input and output common mode level can easily controlled to be the same
6 - Higher noise

you can read the text book for any further information. Grey Meyer is one of the books.

razavi speaks very well about telescopic and folded cascode opamps...try that....

razavi or allen has detail explaination in their book about amplifier.

for practice, I prefer to D. Johns and K. Martin's.

there is problem in using the telescopic OTA as buffer which is not the case for folded cascode.