[Moved]: Push pull Class AB source follower

[predator89]

I have tried to design a Class AB push-pull source follower. Attached the schematic and its transient analysis.
Input is 0-3V AC. The output waveform expected is 0-40V AC. But I am not getting the proper output waveform. and it is clipped at the lower end.
Please suggest me how can I improve the response of my circuit.

 

[Audioguru]

Re: Push pull Class AB source follower

Your negative "Cadence" schematic shows "45" which I assume is a 45V supply voltage. Why doesn't it say 45V? The circuit is symmetrical so then Vout should be half the supply voltage which is 22.5V but yours is only 9V so the bottom of waveforms are clipped.

The circuit will have high crossover and harmonic distortion because negative feedback is missing.

 

[predator89]

Re: Push pull Class AB source follower

Why doesn't it say 45V?

Yes i have given 45V as VDD. I have turned on DC voltage notations on cadance schematic, so it just shows 45 instead of 45V.

The circuit is symmetrical so then Vout should be half the supply voltage which is 22.5V but yours is only 9V so the bottom of waveforms are clipped

Then can you suggest me how i can increase the voltage to 22.5 V.

 

[dick_freebird]

Re: Push pull Class AB source follower

I think you'll always have the VT(phv) pedestal on the low end,
but right off the bat I wonder why I'm seeing 5-7V Vgs operating
points. That's not a good place to be for analog, so far above VT
you have to be falling off gm a fair bit, and you will not be very
load-current-insensitive (i.e. not a good follower) because your
output can swing either side of the pilot device Vgs, pretty wide.
You don't have a "pivot" at VT, you're trying to match something
that's soft and squishy. I'd size Ws up maybe 100X and see how
things look in comparison, that's an easy stab. The only thing
you're likely to lose is some bandwidth.

You also want a look at lambda / rout, how flat are these HV
devices? You're hugely mismatching Vds, pilotutput, and
this can push the Vgs matching in this diode:saturation pair.

 

[AMS012]

Re: Push pull Class AB source follower

Why dont you increase the size of the PMOS and try?

 

[Audioguru]

Re: Push pull Class AB source follower

The gate-source threshold voltage rating of a Mosfet is when it is barely conducting, not when it is being used in a circuit.

 

[erikl]

Re: Push pull Class AB source follower

The gate-source threshold voltage rating of a Mosfet is when it is barely conducting, not when it is being used in a circuit.

BTW: This is called operation in moderate inversion mode and - inter alia - provides the highest possible gm/Id values. Very often used in LP, LN circuits, but admittedly not necessary nor advised for the present circuit.