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# High voltage output stage design

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#### predator89

##### Member level 1
I have a signal coming from opamp (0-3 V) and want a high voltage output upto 40 V using HV mos.
I have confusion of whether to use class AB or B push pull output. Also please help me with schematic if possible.

A Mosfet can easily amplify 0-3V up to 0-40V. You did not say how much current and did not say the waveform or is it DC?
Class-AB has very low distortion and is used in audio amplifiers. Class-B has crossover distortion and is used for switching something on and off.

It is a AC signal. But how can i use both low voltage and HV mosfets for Class AB configuration?

What frequency AC? How much current does the 40V output have to deliver?
Frank

100Khz freq and have to drive load of 1M

With such a high load impedance a class A should do. You still have not said if its a sine wave of 40V P-P, 40 V RMS or a pulse of 0 -> +40 V.
Frank

We have confusion what inputs and outputs you have.
Single Supply or Dual?
AC out or DC coupled?
rise time?
Rail-Rail out from 3V? or 5V? @ 10k load or 100k?
40V? or +/-20V?
Voltage gain=? 10? 12? 13? 15?
Single supply is more complex with biasing, unless AC coupled.

High DC impedance is one thing, but 100kHz with good
waveshape fidelity would need a fair bit of output current
if there's any significant load capacitance. This needs to
be expressed before you can say that a Class A output
will be acceptable power-wise.

Figuring that, say, I wanted a 1MHz 3dB full power
bandwidth to ensure 100kHz is clean, and I had a 50pF
(leading to a 2% DC amplitude error if outside the
feedback loop) and a Class A idle current of 2mA
or so (higher than the 40uA that the DC load and
supply voltage alone, suggest). What is real Cload,
though?

I am designing a high voltage amplifier for a DAC coverter. For this i have designed a opamp and now designing high voltage output stage which i would be connecting in negative feedback. to the opamp.
Requirment for my high voltage amplifier is:
Single input
Input: 0-3V AC
Output: 0-40V AC
Freq: 100Khz

I am still confused with which design to go for high voltage output stage.

If you have the supply headroom (like 42V) you might
think about a voltage-to-current converter like an
emitter-degenerated NPN, throwing an output proportional
current to the high side, = a PNP mirror sourcing to the
load (which you may want to make lower impedance
by some parallel element). Very simple * stable in itself,
and the voltage-current transform may help stability /
settling over a voltage buffer type approach.

Now, you can find "40V" op amps (some higher, some
single-supply) and rigging one of these in A=15 ought
to not be too tough, stabilization issues aside. I think
the first op amp would want to be left as-is and just
make the buffer amp external to that loop, depending
on its local feedback only.

predator89

Points: 2