Notice, that the output signal remains at 0 volts for the entire time (T0 - T8). Since the two input
signals are equal in amplitude and in phase, the difference between them (the base-to-emitter bias) is
always 0 volts. This causes a 0-volt output signal.
If you compute the bias at any time period (T0 - T8), you will see that the output of the circuit
remains at a constant zero.
From the above example, you can see that when the input signals are equal in amplitude and in
phase, there is no output from the difference amplifier because there is no difference between the two
inputs. You also know that when the input signals are equal in amplitude but 180 degrees out of phase,
the output looks just like the input except for amplitude and a 180-degree phase reversal with respect to
input signal number one. What happens if the input signals are equal in amplitude but different in phase
by something other than 180 degrees? This would mean that sometimes one signal would be going
negative while the other would be going positive; sometimes both signals would be going positive; and
sometimes both signals would be going negative. Would the output signal still look like the input signals?
The answer is "no," because figure 3-5 shows a difference amplifier with two input signals that are equal
in amplitude but 90 degrees out of phase. From the figure you can see that at time zero (T0) input number
one is at 0 volts and input number two is at -1 volt. The base-to-emitter bias is found to be +1 volt.