Differnetial Amplifier - An introduction

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:

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

Fig. 1.1 
  • The characteristics of two transistors Q2 and Q1 are identical.
  • The circuits have equal resistances, that is RE1 = RE2, RC1 = RC2 and the magnitude of –VEE and +VCC 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 +VEE and +VCC provides terminal are formed common because they are similar.
  • We exchange the parallel combination of RE1 and RE2 with resistance RE and the two emitters are connected also.
  • The two input signals v1 & v2 are applied at the base of Q1 and at the base of Q2. The output voltage is taken between two collectors.
  • The resistances of the collectors are equal and thus denoted with the help of RC = RC1 = RC2.
  • In ideal situation, the output voltage is zero when the two input values are equal. In case v1 is more than v2 then the output voltage with polarity shown appears. Also, if v1 is more than v2 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.
  1. Dual input, balanced output differential amplifier.
  2. Single input unbalanced output differential amplifier.
  3. Single input balanced output differential amplifier.

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