# high voltage opamp distortion problem

Discussion in 'General Electronics Chat' started by nimaajbphs, Jul 13, 2011.

1. ### nimaajbphs Thread Starter New Member

Jun 1, 2011
14
0
Hi,
I am using a pa240cc (http://media.digikey.com/pdf/Data Sheets/Cirrus Logic PDFs/PA240.pdf) to amplify an input signal that is between 0 to 10 volts and map it to 0 to 100 V.
The amplification seems to work fine. however when a triangle wave is fed to the input the output looks like a sine wave. when i sine wave is fed to the input the out put is still sinusoidal however it is a bit distorted and it is phase shifted. Also the input frequency at 1kHz for both of these observations. I have attached the schematic for the circuit I'm using:

I suspect it might be some sort of compensation that i need to do however I am not an electrical engineer so I'm not sure how to go about resolving the problem. I've tried googling the issue but I'm not sure what keyword to use for the search. Any suggestions or tips are much appreciated.
Thanks.

Edit: I forgot to mention the supply voltage is 150 V measured from +VS to ground

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2. ### Jaguarjoe Active Member

Apr 7, 2010
770
90
It's almost 2am and I'm tired but, the slew rate on this thing is only 35v/μs. Maybe that's the problem.

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You are using a single supply with an opamp that needs a dual supply.
The common mode range is +Vs-14v to -Vs+12v. You are exceeding the lower common mode limit of the opamp.

You can either add another supply to get -Vs down to -12v or more, or add an offset of 12v or more to your input signal, and connect the ground side of the 10k resistor to that same 12v or more offset.

You need bypass capacitors connected from your +Vs lead to ground, and the -Vs lead to ground; 0.1uF and 1uF or greater. They must be rated for >= the supply voltage.

You don't show Cc connected. With a gain of 10 at 25°C, the "GAIN AND COMPENSATION" chart at the top right of page 3 indicates that you will need a value of 3pF. This cap will also need to be rated for your total supply voltage.

4. ### nimaajbphs Thread Starter New Member

Jun 1, 2011
14
0
hey thank you very much SgtWookie, The problem was the capacitor I was using was too large. It works fine now, except for my limit on the lower rail. =)

5. ### Jaguarjoe Active Member

Apr 7, 2010
770
90
5 graphs later, you'll see how the slew rate goes straight down hill as CC increases.