LM675 -VEE fault?

Thread Starter


Joined Jul 28, 2010
I just bought two LM675s and want to use them in a bridge connection to get a high voltage amplifier. I tried building the circuit I was interested in and seen as it did not work I decided to test the op amp in a simple inverting or non-inverting amp configuration which did not work either.

I must be doing something wrong but I don't know what?

The problem:
When powering the LM675 from a dual power supply (+ and - together to form a floating ground and then + to Vcc and - to Vee) the moment the -Vee connection is switched ON the signal I'm measuring at the output drops to a negative DC value instead of 0 and no input signal (be it sin or rectangle) gets amplified?
I'm quite sure it's not a problem with the way I'm measuring the signal, as I've tried a regular o-scope (making sure the ground is connected to the common floating ground) as well as a PC oscilloscope PicoScope 3425 which is designed to measure floating signals and had the same problem.

Has anyone come across this before and might have a solution?

Thanks and Regards,


Joined Apr 20, 2004
One question is why you would create a floating ground with dual supplies? There is already a common ground present. The floating ground is only needed for a single supply setup.

Can you post the schematic?
Last edited:


Joined Apr 24, 2007
You have not provided nearly enough information.

But could it be that there is also DC on the input and you have so much gain (and enough DC on the input) that it pegs the output to its limit? Have you tried it with a capacitor in series with the input?

Or maybe the two power supplies were designed in such a way that you cannot combine them to make a dual-polarity supply.

Yes, you really need to post the schematic, and probably a photo. How are you getting a signal on the output before you have connected the -V supply? Why would you apply a signal to the input before connecting the -V supply? Have you tested each LM675 separately, with a standard "single amplifier" circuit?

Maybe try the LM1875 or LM3875?