confusion in circuit diagram of folded cascode and its biasing.......!

[ASHUTOSH RANE]

hi,

I am designing folded cascode op amp

i have doubt about the current source transistors( M12 M21 M22) and the current mirror transistors (M16-M18)

I am biasing the M12 M21 M22 with current biasing and current mirror transistors (M16-M18) with voltages biasing .......(using sources at present )

in different books i have seen different biasing schemes followed ....

i have basic doubt for folded cascode amplifier that for which transistors we should fix the voltage at gate and for which transistor we should use Current mirror to bias .....??????????????????????????:shock::roll:8-O

what if all transistors are biased with voltage sources ??

 

[ASHUTOSH RANE]

 

[ASHUTOSH RANE]

how do i generate Vbn Vbp Vcascn Vcascp Vbcm??? if opamp is fully differential....

 

[jimito13]

You can use current mirrors (simple or cascode if you have available headroom) for the various points you mention above.With current mirrors you fix the current in every branch and you also configure the voltage at the desired level.

Using ideal sources for biasing is not a realistic or practical technique since you can't evaluate your OTA's performance over corner (process,temperature,power supply variations) and mismatches (Monte Carlo simulations for this) and even for nominal state you don't have a fast way available to see how your design targets (gain,unity gain freq. etc) change with the variation of a current Ix at the x branch let's say.

As for Vbn Vbp Vcascn Vcascp Vbcm i already answered.

 

[dgnani]

how do i generate Vbn Vbp Vcascn Vcascp Vbcm??? if opamp is fully differential....

look up bias generators and high-swing cascode mirror bias
this **broken link removed** is a good start, then you need a second branch of mirrors to bias the other side (the mirror load side in the pdf) of your fully diff design