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High voltage output stage design

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predator89

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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.
 

Audioguru

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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.
 

predator89

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

SunnySkyguy

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We have confusion what inputs and outputs you have.
Single Supply or Dual?
AC out or DC coupled?
rise time?
Load Capacitance?
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.
 

dick_freebird

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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
load, something like 1E-6s/50E-12F=>Rout=20Kohms
(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?
 

predator89

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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
Load: 1M
Freq: 100Khz

I am still confused with which design to go for high voltage output stage.
Please help me with the schematic if possible.
 

dick_freebird

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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.
 
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