Figure 3-7.Single-input, single-output differential amplifier.
When the input signal developed by R1 goes positive, the current through Q1 increases. This
increased current causes a positive-going signal at the top of R3. This signal is felt on the emitter of Q2.
Since the base of Q2 is grounded, the current through Q2 decreases with a positive-going signal on the
emitter. This decreased current causes less voltage drop across R4. Therefore, the voltage at the bottom of
R4 increases and a positive-going signal is felt at the output.
When the input signal developed by R1 goes negative, the current through Q1 decreases. This
decreased current causes a negative-going signal at the top of R3. This signal is felt on the emitter of Q2.
When the emitter of Q2 goes negative, the current through Q2 increases. This increased current causes
more of a voltage drop across R4. Therefore, the voltage at the bottom of R4 decreases and a negative-
going signal is felt at the output.
This single-input, single-output, differential amplifier is very similar to a single-transistor amplifier
as far as input and output signals are concerned. This use of a differential amplifier does provide
amplification of a.c. or d.c. signals but does not take full advantage of the characteristics of a differential
SINGLE-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
In chapter one of this module you were shown several phase splitters. You should remember that a
phase splitter provides two outputs from a single input. These two outputs are 180 degrees out of phase
with each other. The single-input, differential-output, differential amplifier will do the same thing.
Figure 3-8 shows a differential amplifier with one input (the base of Q1) and two outputs (the
collectors of Q1 and Q2). One output is in phase with the input signal, and the other output is 180 degrees
out of phase with the input signal. The outputs are differential outputs.