Maker Pro
Maker Pro

differential amplifier

rasmus

Nov 6, 2014
30
Joined
Nov 6, 2014
Messages
30
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 :(
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
It's almost impossible for them not to.

Perhaps you can explain your problem more clearly.
 

rasmus

Nov 6, 2014
30
Joined
Nov 6, 2014
Messages
30
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
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
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.
 

rasmus

Nov 6, 2014
30
Joined
Nov 6, 2014
Messages
30
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
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
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.
 

LvW

Apr 12, 2014
604
Joined
Apr 12, 2014
Messages
604
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.
.
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.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
4,878
Joined
Jun 21, 2012
Messages
4,878
Whoa! Are we talking about op-amps (assumed to have almost infinite differential gain) or unity-gain differential amplifiers?
 

rinchan6

Aug 27, 2013
42
Joined
Aug 27, 2013
Messages
42
Why not comparator? Ask google for full detailed overwhelming details :)
 
Top