# Single supply op amp question

Discussion in 'General Electronics Chat' started by Setanta, Apr 8, 2013.

1. ### Setanta Thread Starter New Member

Mar 12, 2012
17
4
Can anyone tell me what's wrong with the single supply inverting op-amp circuit attached?

I'm trying to learn a bit about op amps. A split supply works as expected so i presume I'm doing something wrong. Anyone enlighten me?

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2. ### patricktoday Member

Feb 12, 2013
157
42
Hello, you'll need a capacitor between your input signal and the 10k input resistor. That way, if there is no input signal, the output will sit at 10V; and the circuit will only amplify the fluctuations of your signal's ups and downs without being concerned with the DC center point of your input signal.

3. ### #12 Expert

Nov 30, 2010
16,704
7,354
As far as I can see, the graph is wrong, unless you are inputting a DC current into the input resistor.

Oh wait, you are. The input is assumed to be ground referenced and the input pin of the op amp is +10 volts. That's where the DC offset is coming from.

4. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
Use Google and find the datasheet for LM324. Then look at the circuit examples. That can give you some ideas It is bed time for me soon

5. ### Setanta Thread Starter New Member

Mar 12, 2012
17
4
Yup the cap did it, thanks guys. I'll need to do some more reading to understand it a bit better i think. One question though. As patricktoday mentioned though the output now sits at 10v and the fluctuations are amplified around that.

What if that voltage is to high though (or low). Can we control the output point without modifying the power supply? Perhaps a better way of putting it is can we replicate the output voltage of a split supply?

Last edited: Apr 8, 2013
6. ### patricktoday Member

Feb 12, 2013
157
42
You can alter the voltage divider at your (+) input to set the bias point to any value; you could wire it up to a pot if you'd like. But you can't exactly replicate a split supply because your signal cannot go below zero and is limited by the op amp's maximum voltage swing to the negative and positive.

7. ### Setanta Thread Starter New Member

Mar 12, 2012
17
4
Ah ok, I get it. Thanks for that.

8. ### crutschow Expert

Mar 14, 2008
13,505
3,376
If you want the output to be centered at 0V then you can also use a capacitor to couple the output voltage. You also need a resistor on the capacitor output to ground to establish the zero voltage value. But the maximum peak-to-peak output is still limited to be somewhat below the supply voltage.