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.

Dual Input, Unbalanced Output Differential Amplifier

• These configurations are represented in fig 1.2 and are defined by the number of input signals used and the manner in which output voltage is calculated.
• In case of two input signals, the configuration is known as dual input, otherwise it is a single input configuration.
• On the other hand, in case the output voltage is determined between two voltages, it is known to as a balanced output because the dc potential of both the collectors are same with respect to ground, the configuration is known as unbalanced output.
• A multistage amplifier having a required gain can be determined with the help of direct connection between successive stages of differential amplifiers.

• The benefit of direct coupling is that the lower cut off frequency is removed which is imposed by the coupling capacitors, and thus they have the ability of amplifying both ac input signals and dc input signals.