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 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 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 R_C = R_C1 = R_C2.
• 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.

2. Dual input, balanced output differential amplifier.
3. Single input unbalanced output differential amplifier.
4. Single input balanced output differential amplifier.