# OpAmp Questions

Discussion in 'General Electronics Discussion' started by Yoa01, Jan 5, 2013.

1. ### Yoa01

214
0
Jun 18, 2012
Hi!

I know that OpAmps are a bit complex, but I was wondering if you could help me out a bit. First, let me show you this image:

So I was reading both Wikipedia and a beginner series on OpAmps and decided to try these in a virtual space so as to not blow stuff up, hence the image of LTSpice.

A few notes: The schematics are what I am using for tests, but if it's too messy for you to read it uses a +-9V bipolar supply, and I use 1K resistors. I am using input voltages of 1V and 9V, 1V for easy math, 9V for practical uses (almost everything I build uses +-9V).

With the Non-Inverting amp, it works as you'd expect with lower voltages, but if Vin is 9V, Vout is only 8V (given that R1+2 are 1K, as shown). Shouldn't it be 18V?

I have a different issue with the Inverting amp: given Vin as 1V, Vout is actually -7.8V. Same with a 9V Vin. I think I simply messed this one up, but I'm not sure.

Thirdly, with the Comparator circuit (which really makes me sad that I got a bunch of comparators now), It seems to work, but is it normal for the output voltage to be slightly less than the supply voltages? Like, say, with the image you have 2V in the +input and 1V on the -input. LTSpice shows a voltage of 8.08V. It that normal?

Lastly (thanks for sticking with me), with the buffer, with only 1V Vin there is only a slight voltage drop, but with 9V it drops more than a volt to 8.07V.

I know there are a TON of different possibilities, even including the OpAmp that I used (not that LTSpice gives you much info on them), so any help is appreciated. Thank you!

2. ### duke37

5,364
771
Jan 9, 2011
1. You can not get more volys out than the power supply and it seems that with that particular op-amp, you will only get to within a volt of the power supply.

2.Inverting amp
Yes, you messed it up. The feedback must go from the output to the - input.

3. Comparator
Once again you are getting about the maximum voltage that you could expect. Secondly, compararors often need a pull up resistor on the output but here the op-amp will do its own pull up.

4. Buffer.
Seems to be working OK. There should be no measurable voltage drop.

3. ### Yoa01

214
0
Jun 18, 2012
Ok, so some of it is OpAmp dependant. Should have figured as much.

So the input and feedback go in the same input? What of the + input?
EDIT: nevermind, I see what's up. Thanks.

So, what's that, a resistor going between the V+ and the output? It acts as a stabiliser, I presume.

So it seems that this OpAmp isn't the best to be learning with. haha

Thanks.