Figure 7-8. Single-input, Differential-output Differential Amplifier
7-39. Now you know how a differential amplifier can produce two amplified differential
output signals from a single input signal. Another point about this configuration is that if a
combined output signal is taken between outputs number 1 and 2, this single output will be
twice the amplitude of the individual outputs. In other words, you can double the gain of
the differential amplifier (single-output) by taking the output signal between the two output
terminals. This single-output signal will be in phase with the input signal. This is shown by
the phantom signal above R5 (the phantom resistor connected between outputs number 1
and 2 would be used to develop this signal).
DIFFERENTIAL-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIER
7-40. When a differential amplifier is connected with a differential input and a
differential output, the full potential of the circuit is used. Figure 7-9 shows a differential
amplifier with this type of configuration (differential-input, differential-output). This
configuration normally uses two input signals that are 180 degrees out of phase. This
causes the difference (differential) signal to be twice as large as either input alone. This is
just like the two-input, single-output difference amplifier with input signals that are 180
degrees out of phase.
7-41. Output number 1 is a signal that is in phase with input number 2, and output
number 2 is a signal that is in phase with input number 1. The amplitude of each output
signal is the input signal multiplied by the gain of the amplifier. With 180-degree out-of-
phase input signals, each output signal is greater in amplitude than either input signal by a
factor of the gain of the amplifier.
23 June 2005