# OpAmp problem

Discussion in 'General Electronics Chat' started by Wtech, May 1, 2009.

1. ### Wtech Thread Starter New Member

Apr 27, 2009
8
0
I am using opAmp LM358 as difference amplifier. I use single DC supply 5V and GND. I give AC signal at inputs of OPAmp. The gain of opamp is approximately 18. According to this configuration the output of opamp should have +ve side of AC signal with multiply by gain and ve side is 0V because of supply. But opamp worked like comparator, its output is 0v and 5v. Can any help me to identify the problem. Thanks
see the attach image,

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2. ### StayatHomeElectronics Well-Known Member

Sep 25, 2008
864
40
A difference amplifier usually has the configuration as on this web site:

http://www.play-hookey.com/analog/difference_amplifier.html

Your setup looks more like an inverting amplifier with a gain of 5.55, determined by R2/R1 or 100k/18k. However, you don't have the ground reference of the inverting amplifier.

For simulatioin purposes, try pulling node 4, the (+) terminal of the op amp, to ground through a large value of resistance. That is, put a large resistor, >1MEG, between node 4 and GND.

Apr 27, 2009
97
1
Wtech, you need some DC level on the noninverting input to lift the output off the ground and position it somewhere in the middle of 0-5V range. The DC input will be amplified (1+R1/R2) times, so make sure it is not too high. You can check my post about the differential amplifier here

http://masteringelectronicsdesign.com/solving-the-differential-amplifier-part-1/

The schematic and calculations should help you. There are two articles, part 1 and 2, each one solving the differential amplifier in a different way.

Last edited: Dec 6, 2009
4. ### Wtech Thread Starter New Member

Apr 27, 2009
8
0
I am using AC source at input of OPAMP, and as my requirement i haven't add DC level at input. only i need that when node 4 goes high with respect to node 3 the output should equal to gain*voltage at node 4. and when node 4 goes low with respect to node 3 the output should equal to GND because i supply GND at pin 4 of OPAMP.
As the basic property of AC source, when node 4 goes high with respect to node 3, conversely node 3 goes low with respect to node 4 at the same time.

5. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
You need to ground the + input.