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Folded cascode op amp with AB output stage

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Newbie level 4
Jul 1, 2005
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folded cascode with class ab output stage

I've designed a folded cascode op amp that used 3.3V supply and it operates properly (according to spec). However, when I included the class AB output stage, the op amp is not functioning...and some of the transistor at the input stage are operating in triode region. Does anyone had any idea about this?

Design spec:
Vsupply = 0~3.3V
GBW = 10MHz
Output swing = Rail to rail
Av > 70dB

(Referring to the op amp configuration) If I want to remove the cascode transistor for the current sourcing of the differential amplifier ( M8 - in order to acquire voltatge headroom), should I remove transistor M17? *(this is for the case of low voltage op amp)

Attached here is the simulation file of the op amp. I'm still learning; if I did any mistake please inform me. I really appreciate that.

cascoded op amps

I think you`d better upload the schematic instead of netlist...

class ab output stage opamp

pay attention to you bias circuit
check the dc point
bias circuit cascode using class ab output stage

I'm sorry, I'm used to it since I used HSpice simulation.

folded cascade class op amp

reduce I1,I2 and try.
check the dc point

M8 need not remove if CMMR is not the issue
low voltage op amp output stage biasing

From the diode connection of M22 and M21, the power supply for 3.3 is small margin.

If you can reduce the stack transistor number , it may be some changing.

Use the diode connection to bias the class ab controlling to get more margin.
cascode output stage

In my opinion, the diode connected transistors biasing the transistors controlling the AB loop, M29 and M30, are two big. This makes the the voltage at the source of M29 and M30 does not allow the current sources to operate in saturation.

Also transistors of the N-current mirrors are too small, making their Vdsat to be quite high, which also contributes to make them go out from saturation.

I would decrease the size of diode connected transistors and increase the size of transistors in the current mirrors.
folded cascode matching

Which are the really critical transistors in the folded cascode with class ab output stage in terms of matching.......



folded cascode op amp with class ab output stage

i would assume the following sizing considerations:

sizing i/p pair depending on how much current you want...and gm..coz gm depends on i/p pair..
cascode transistors...need to have matching vgs's so large w/ off is noise performance....
active current mirrors need small w/l for good off..o/p swing..also we need large ro for good gain...

what about the class ab structure and the rest of the circuit??

any suggestions,comments and advice are always welcome!!! rocks!


hi Chikuma,
Did you upload wrong nestlist? Why the nestlist not correspond to the schematic???
I tryed simulated your nestlist with Hspice (use Model Bsim,level 49) but the result was wrong,lots of transistors were cut-off!!
Can you upload yor Model ?Thanks!!

Your input stage need rail to rail input.

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