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Difference amplifier

pulasulaki singhaniya
Prodigy 10 points


If I want to subtract two voltage signals either I can use a difference amplifier built using OP-AMP or just connect the positive of 1 voltage source with the positive of the other voltage source(shown in image below)  and then ampify it which is very easy and simple and cheap than the former method. Then what is so special with difference amplifier?

  • 0
    TI__Guru* 93105 points

    Pulasulaki,

    Your schematic uses ideal voltage sources. One may be able to force current to flow in the opposite direction of the normal current flow with these ideal sources. For example lets leave VS1 at +5 V, make VS2 +3 V, and add a load from VF1 to ground. VS1 will have no problem forcing current to flow through VS2 in the opposite direction and 2 V will be developed across the added load resistor.

    Real voltage sources may not be content with such arrangements. Often they can only source current and cannot sink current. The difference amplifier can accommodate input voltages of the same, or opposite, polarity providing they are within the input ranges of the amplifier.

    There is another operational amplifier circuit called a summing circuit. Take a look at the National application note:

    http://www.ti.com/ww/en/bobpease/assets/AN-31.pdf

    http://en.wikipedia.org/wiki/Operational_amplifier_applications#Summing_amplifier

    These can be built using a low cost operational amplifier and resistors.

    Regards, Thomas

    PA -Linear Applications Engineering