**Introduction:**Differential amplifier is a basic building block of an op-amp. The main task of a differential amplifier is to amplify the difference between two input signals.

Construction of Differential Amplifier:

Construction of Differential Amplifier:

- Let us consider two emitter-biased circuits as represented in fig. 1.1

**Fig. 1.1**

- The characteristics of two transistors Q
_{2}and Q_{1}are identical.

- The circuits have equal resistances, that is R
_{E1}= R_{E2}, R_{C1}= R_{C2}and the magnitude of –V_{EE}and +V_{CC}are equal. We can measure the voltages with respect to ground. - To make a differential amplifier, the two circuits are attached together as represented in fig. 1.1. The two +V
_{EE}and +V_{CC}provides terminal are formed common because they are similar. - We exchange the parallel combination of R
_{E1}and R_{E2}with resistance R_{E}and the two emitters are connected also. - The two input signals v
_{1}& v_{2}are applied at the base of Q_{1}and at the base of Q_{2}. The output voltage is taken between two collectors. - The resistances of the collectors are equal and thus denoted with the help of R
_{C}= R_{C1}= R_{C2}. - In ideal situation, the output voltage is zero when the two input values are equal. In case v
_{1}is more than v_{2}then the output voltage with polarity shown appears. Also, if v_{1}is more than v_{2}then the polarity of output voltage is opposite. - The configurations of differential amplifiers are different.
- The configurations of four differential amplifiers are provided as:

- Dual input, unbalanced output differential amplifier.

- Dual input, balanced output differential amplifier.
- Single input unbalanced output differential amplifier.
- Single input balanced output differential amplifier.

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