Differential amplifier

A differential amplifier is a type of electronic amplifier which multiplies the difference between 2 inputs by some constant factor

Differential amplifier symbol

The inverting and non-inverting inputs can be distinguished by "-" and "+" symbols (respectively) placed in amplifier triangle. V_s+  and V_s-  are the power supply voltages; they are omitted from the diagram for simplicity, but of course should be present in actual circuit.

The Differential amplifier is a basic building block of the op-amp. The function of the differential amplifier is to amplify difference in between two input signals.

How can a differential amplifier be developed? Let us consider 2 emitter-biased circuits as shown in the figure 1.

Figure 1

The 2 transistors Q₁  and Q₂  have the same characteristics. The resistances of circuits are equal, that is R_E1 = R _E2, R_C1 = R_C2 and magnitude of +V_CC is equal to magnitude of -V_EE. These voltages are measured with respect to the ground.

To make a differential amplifier, 2 circuits are connected as shown in the figure 1. The 2 +V_CC and -V_EE supply terminals are made common as they are same. The 2 emitters are also connected and parallel combination of R_E1  and R_E2  is replaced by the resistance R_E. The 2 input signals v₁  & v₂ are applied at base of Q₁ and at base of Q₂. The output voltage is taken between the two collectors. The collector resistances are equal and hence denoted by R_C = R_C1 = R_C2.

Ideally, output voltage is zero when the 2 inputs are equal. When v₁ is greater than v₂ output voltage with the polarity shown appears. When v₁  is less than v₂, output voltage has the opposite polarity.

The differential amplifiers are of the various configurations. The 4 differential amplifier configurations are given as follows:

1.   The Dual input, balanced output differential amplifier.

2.   The Dual input, unbalanced output differential amplifier.

3.   The Single input balanced output differential amplifier.

4.   The Single input unbalanced output differential amplifier.

Fig. 2

These configurations are shown in the figure 2, and can be described as the number of input signals used and the way an output voltage can be measured. If use 2 input signals, configuration is said as dual input, otherwise it is a single input configuration. Conversely if output voltage is measured in between the two collectors, it is referred to as a balanced output as both the collectors are at same DC potential with respect to ground. If the output is measured at one of collectors with respect to ground, the configuration is called as unbalanced output.

A multistage amplifier with the desired gain can be obtained by using direct connection in between the successive stages of differential amplifiers. Advantage of direct coupling is that it removes lower cut off frequency imposed by coupling capacitors, and they are hence, capable of amplifying DC as well as AC input signals.

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