3-13Figure 3-8.—Single-input, differential-output differential amplifier.This circuit’s operation is the same as for the single-input, single-output differential amplifier justdescribed. However, another output is obtained from the bottom of R2. As the input signal goes positive,thus causing increased current through Q1, R2 has a greater voltage drop. The output signal at the bottomof R2 therefore is negative going. A negative-going input signal will decrease current and reverse thepolarities of both output signals.Now you see how a differential amplifier can produce two amplified, differential output signals froma single-input signal. One further point of interest about this configuration is that if a combined outputsignal is taken between outputs number one and two, this single output will be twice the amplitude of theindividual outputs. In other words, you can double the gain of the differential amplifier (single output) bytaking the output signal between the two output terminals. This single-output signal will be in phase withthe input signal. This is shown by the phantom signal above R5 (the phantom resistor connected betweenoutputs number one and two would be used to develop this signal).DIFFERENTIAL-INPUT, DIFFERENTIAL-OUTPUT, DIFFERENTIAL AMPLIFIERWhen a differential amplifier is connected with a differential input and a differential output, the fullpotential of the circuit is used. Figure 3-9 shows a differential amplifier with this type of configuration(differential-input, differential-output).

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