# differential amplifier

Discussion in 'General Electronics Discussion' started by rasmus, Nov 20, 2014.

1. ### rasmus

30
0
Nov 6, 2014
Hi

How does one make a differential amplifier show/output both negative and positive voltage differences. Example when the negative input is higher then the positive input and vice verse ?

Sorry if i haven't made myself clear

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,849
Jan 21, 2010
It's almost impossible for them not to.

Perhaps you can explain your problem more clearly.

3. ### rasmus

30
0
Nov 6, 2014
If an opamp is supplied with 20v, and has the positive input at 1 volt and the negative input at 5 volts, it will show 4v at the output (no problem). But if the input voltages at the negative and positive is replaced with each other the output will show full supply voltage or 0 volt instead of showing -4v

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,849
Jan 21, 2010
OK, you are assuming that the differential amplifier has a gain of 1.

The differential amplifier will operate correctly as long as the input voltages are within the range of the supply voltages (the output might get clipped to the supply voltage, but that's expected).

If you want negative input voltages then you must ensure that your negative supply voltage is more negative than the most negative input.

You might, for example have a supply that is +/- 18V. That will allow (for example) one input to be -6V and the other -1V and get an output of -7V.

5. ### rasmus

30
0
Nov 6, 2014
That is exactly what i am after, how do you supply +/- voltage to a opamp, is there any diagram ?

I understand that without a negative voltage you cant get a negative voltage output and vice versa

8,393
1,272
Nov 28, 2011

30
0
Nov 6, 2014
8. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
Connect the positive supply rail to the positive power pin of the op-amp. (Often this is pin 7.)
Connect the negative supply rail to the negative power pin of the op-amp. (Often this is pin 4.)
The ground or 0V rail of the power supply becomes the 0V reference of the whole circuit.

You should also have decoupling capacitors, typically 0.1 µF, one between the positive supply pin of the op-amp to 0V and one from the negative supply pin of the op-amp to 0V.

9. ### LvW

604
146
Apr 12, 2014
Yes - and more than that, the ground reference is provided as follows:
* For inverting applications the pos. input terminal is grounded (0V)
* For non-inv. applications the feedback network is grounded.

10. ### hevans1944Hop - AC8NS

4,683
2,191
Jun 21, 2012
Whoa! Are we talking about op-amps (assumed to have almost infinite differential gain) or unity-gain differential amplifiers?

42
1
Aug 27, 2013