Inverting Op-amp Always outputting .6V

zwigby

Ok So I am pretty baffled at the moment. I am trying to get my inverting op-amp to work LM324. I have 15V supply going in. The + is going to ground with 5.7k resistor. The - has Rin and Rf both 10k. So this should output -1 * Vin. Vin is supplied by +2.8V from 2 AA batteries. But all I can get out is .6V. Thanks for any input.

berkeman

You are powering the opamp between 15V and ground, and want to get a negative output voltage out of it?

berkeman

If you change your supplies to +/-9V (like use two 9V batteries), it should work a bit better.

Welcome to the PF, BTW!

vk6kro

If you connect the LM324 like this:


with the + input connected to a reference point that is half the supply voltage, the output should move between 0 and the + supply voltage whe the input moves from near the + supply voltage to zero.

The 324 has to have a single supply and it cannot accept input very close to the + supply voltage.

berkeman

Oopsies. Thanks for catching that vk6kro!

vk6kro

Actually you were right, Mike. Checked the data sheet. It doesn't have to have a single supply, but it is usually used like that.

Incidentally in the sticky above,
http://www.physicsforums.com/showthread.php?t=315139

the link
More simple schematics :http://www.uoguelph.ca/~antoon/gadgets/gadgets.htm

seems to have gone faulty or been shifted. Been like that for a while.

berkeman

Got it, thanks. I was able to find where the "gadgets" poster moved to after university, and updated the link. Man, what a lot of schematics!

uart

FYI guys. When an OpAmp is said to feature "single power supply operation" it usually just means that the input common mode range extends all the way down to (and perhaps even very slightly below) the negative rail.

As long as you respect both the maximum supply range (Vcc-Vee) and the input common mode range then it matters not whether you use a +/- supply or just a single supply. For example +/- 6V is no different to single supply 12V as far as the opamp is concerned (for any opamp, and not just for a single supply type), provided that the input common mode range restrictions are obeyed. The only difference is that with a single 12V supply you are more likely to want to reference your input signal to the -ive rail (0 volts) instead of the center. Hence the input common mode range is normally the only real consideration as to whether and opamp is described as "single power supply" or otherwise.

Averagesupernova

I was wondering about all this. I've never heard of an op-amp that is 'single supply only'. The inputs and output are designed to work within the supply rails. I don't think the op-amp typically cares where what we call 'ground' is at relative to these supply rails.

dlgoff

I attended a seminar that National put on when they first introduced the LM124/LM224/LM324. I still have their little application booklet, Quadzilla. Here's what the have to say, which has already been said here: Boy does this bring back memories.