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]

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.

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]

The 324 has to have a single supply

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,
https://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:

The LM324 op amp operates in a conventional manner-the output voltage responds to a differential input voltage, and offers many application advantages in single power supply systems. The new operating feature of most significance is the fact that the input common-mode voltage range includes ground and the output voltage can swing to ground, even when powered by a single power supply. This, in addition to the wide operating range of power supply voltage (3 to 30Vdc), will obsolete the requirement for the standard ±15Vdc power supplies for many linear systems-the +5Vdc supply which is used with most logic circuits is adequate for many applications. Finally, the low power supply current drain makes the LM324 a useful product for battery powered equipment or CMOS systems.

Boy does this bring back memories.