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<p>
This circuit outputs the difference in voltage between two input
signals. In this case, the first input is a 60 Hz signal, and the
second input is a 60 Hz signal with 120 Hz <a href="http://en.wikipedia.org/wiki/Square_wave">square wave</a> added in. (In
this simulation, the two signals are added simply by connecting the two
sources in series, which is convenient but not realistic.)
<p>
The second input signal is
driving two resistors, which act as a voltage divider, and the + input
of the op-amp is connected between them, where the voltage is equal to
half the second input signal. The <a
href="http://en.wikipedia.org/wiki/Operational_amplifier">op-amp</a>
attempts to keep its – input at the same voltage as the + input. The two
resistors on top act as a voltage divider, making the – input
halfway between the first input signal and the op-amp output.
<p>
If V<sub>+</sub> = 1/2 In<sub>2</sub> = V<sub>–</sub> = 1/2
(In<sub>1</sub> + output), then the output = In<sub>2</sub> -
In<sub>1</sub>, or the difference between the two inputs.
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