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telescopic opamp input common mode level .

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manissri

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hi all
why telescopin opamp has the input common mode level and out put common mode level is not same.
suppose the supply range is 0- 5 volts then the input common mode can not be 2.5v. and the output common mode level is at 2.5v.
where as in folded cascode the input common mode level and output common mode level can be at 2.5 volt for both input and output.
plz explain.
regrds
manish
 

In telescopic opamps, the output and input common mode levels are dependant on each other. The input common mode voltage is a Vt drop (lower or higher dependent on NMOS or PMOS input) different from the output common mode voltage. This is to ensure that the input diff pair is in saturation.

In Folded cascode opamp, the input and output common mode levels are independent of each other. The output common mode level depends on the NMOS and PMOS load transistors. The input common mode level depends on the input diff pair, the tail current transistor and the load transistor.

Hence in this case both the levels can be set to the same value.
 

"In telescopic opamps, the output and input common mode levels are dependant on each other. The input common mode voltage is a Vt drop (lower or higher dependent on NMOS or PMOS input) different from the output common mode voltage. This is to ensure that the input diff pair is in saturation. "

I don't agree with that.
The difference between these two range is not a Vt drop, it's a Vt drop minus a Vd,sat
 
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