You can see that when the input signals are equal in amplitude and 180 degrees out of phase, the
output signal is twice as large (40 volts peak to peak) as it would be from either input signal alone (if the
other input signal were held at 0 volts).
Figure 3-4 shows the two-input, single-output, difference amplifier with two input signals that are
equal in amplitude and in phase.
Figure 3-4.Input signals in phase.