TYPICAL DIFFERENTIAL AMPLIFIER CIRCUIT
7-30. Figure 7-6 shows the schematic of a typical differential amplifier. This circuit
requires two transistors to provide the two inputs and two outputs. If you look at one input
and the transistor with which it is associated, you will see that each transistor is a CE
amplifier for that input (input 1 and Q1; input 2 and Q2). R1 develops the signal at input 1
for Q1 and R5 develops the signal at input 2 for Q2. R3 is the emitter resistor for both Q1
and Q2. Notice that R3 is NOT bypassed. This means that when a signal at input 1 affects
the current through Q1, that signal is developed by R3. The current through Q1 must flow
through R3. As this current changes, the voltage developed across R3 changes. When a
signal is developed by R3, it is applied to the emitter of Q2. In the same way signals at
input 2 affect the current of Q2, are developed by R3, and are felt on the emitter of Q1. R2
develops the signal for output 1 and R4 develops the signal for output 2.
Figure 7-6. Differential Amplifier
7-31. Even though this circuit is designed to have two inputs and two outputs, it is not
necessary to use both inputs and both outputs. Remember, a differential amplifier was
defined as having two possible inputs and two possible outputs. A differential amplifier
can be connected as a single-input; single-output device; a single-input, differential-output
device; or a differential-input, differential-output device.
SINGLE-INPUT, SINGLE-OUTPUT, DIFFERENTIAL AMPLIFIER
7-32. Figure 7-7 shows a differential amplifier with one input (the base of Q1) and the
output (the collector of Q2). The second input (the base of Q2) is grounded and the second
output (the collector of Q1) is not used.
7-33. 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.
23 June 2005