Hi Guys, simple difference amplifier. Voltages in are around 3-4 volts. Gain is supposed to be 4.02x (I know it won't be that precise).

I'm using an LM358 as a test amp, it just doesn't work?? gain is wrong or no gain, just doesn't match what its supposed to do.

Something missing here?

 

What are the values of R6 and R7?

hgmjr

 

What are the values of R6 and R7?

hgmjr

I can't quite remember, I have a 10k pot in there and I adjust it until I get a certain voltage at the output of the opamp which is about 3-4 volts.

If the gain was exactly 4.02x then I would need about 1 volt delta on the inputs to get this voltage, however it just doesn't seem to work this way. I get something like an 80mV delta at the input which works out a gain of about 110x, which doesn't make sense.

 

If the gain was exactly 4.02x then I would need about 1 volt delta on the inputs to get this voltage, however it just doesn't seem to work this way. I get something like an 80mV delta at the input which works out a gain of about 110x, which doesn't make sense.

Can you explain how you got a gain of 110?

hgmjr

 

Have you double checked your resistor values?

You're sure you're measuring the input in the right location?