Lecture - 3: Difference Amplifiers

Dual Input, Unbalanced Output Differential Amplifier:

In this case, two input signals are given however the output is measured at only one of the two-collector w.r.t. ground as shown in fig. 2. The output is referred to as an unbalanced output because the collector at which the output voltage is measured is at some finite dc potential with respect to ground..

Fig. 2

In other words, there is some dc voltage at the output terminal without any input signal applied. DC analysis is exactly same as that of first case.

AC Analysis:

The output voltage gain in this case is given by

The voltage gain is half the gain of the dual input, balanced output differential amplifier. Since at the output there is a dc error voltage, therefore, to reduce the voltage to zero, this configuration is normally followed by a level translator circuit.

Differential amplifier with swamping resistors:

By using external resistors R'E in series with each emitter, the dependence of voltage gain on variations of r'e can be reduced. It also increases the linearity range of the differential amplifier.

Fig. 3, shows the differential amplifier with swamping resistor R'E. The value of R'E is usually large enough to swamp the effect of r'e.

Fig. 3


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