# Coupling high voltage to low voltage stages

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#### predator89

##### Member level 1
I want to connect a high voltage output stage to low voltgae input stage of 2 stage op amp.
Can anyone suggest me which output stage topology would be better for this.

#### AMS012

##### Full Member level 3
Common source with a resistor load will give the maximum swing.

#### predator89

##### Member level 1
Common source with a resistor load will give the maximum swing.

But my main confusion is how would i connect the low voltage output to the high voltage mos transistors

#### crutschow

Need more info on what you are trying to do with the circuit.

#### predator89

##### Member level 1
Need more info on what you are trying to do with the circuit.

I am trying to build a high voltage amplifier. And i started with a low voltage input stage with supply of 3.3 V and want to connect it to high voltage output stage. So that i can get a swing of 0 to 40V AC for the input of 0 to 3 V AC.
So i have difficulty in deciding which topology to go for.

#### crutschow

So we're into the 20 question mode. :wink:

#### crutschow

The frequency response required?

#### predator89

##### Member level 1
The frequency response required?

I need a gain of 10X .. and phase margin of 60 and GBW of 1 MHz.
My idea was to implement opamp and then try using negative feedback to control gain.

#### crutschow

GBW doesn't tell me the frequency response required at the output.
Don't tell me what you think you need, tell me what you actually need.

#### Easy peasy

You need a diff amp circuit to convert the high o/p voltage into something your low voltage opamp can deal with, search the net, plenty of examples...

#### FlapJack

I think you at least must give the input frequency, is it fixed or variable.

#### Audioguru

I do not think you want to connect a high voltage output stage to a low voltage input stage. That is saying it backwards.
I think you have an input stage that operates from a 3.3V supply and you want it to drive an output stage that has a much higher output voltage swing.

You said you want the input to be 0V to 3VAC but you did not say 3V RMS or 3V p-p. What is it?
You said you want an output swing of 40V but did not say 40V RMS or 40V p-p. What is it?

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