Transistorised Differential Amplifier

Transistorised Differential Amplifier

The transistorised differential amplifier basically uses the emitter biased circuits which are identical in characteristics. Such two identical emitter biased circuits are

The two transistors Q₁ and Q₂ have exactly matched characteristics. The two collector Resistors R _C1 and R _C2 are equal while the two emitter resistances R _E1 and R _E2 are equal.

R_C1 = R_C2

And

R_E1 = R_E2

The magnitudes of + Vcc and – V _EE are also same. The differential amplifier can be obtained by using such two emitter biased circuits. This is achieved by connectinemitter E₁ of Q₁ to the emitter E₂ of Q₂. Due to this, R _E1 appears in parallel with R _E2and the combination can be replaced by a single resistance denoted as R _E. The base B₁of Q₁ is connected to the input 1 which is V _S1 while the base B ₂ of Q₂ is connected tothe input 2 which is Vs2. The supply voltages are measured with respect to ground. Thebalanced output is taken between the collector C1 of Q1 and the collector C₂ of Q ₂. Suchan amplifier is called emitter coupled differential amplifier. The two collectorresistances are same hence can be denoted as R _C..

The output can be taken between two collectors or in between one of the twocollectors and the ground. When the output is taken between the two collectors, none of them is grounded then it is called balanced output, double ended output or floatingoutput. When the output is taken between any of the collectors and the ground, it is calledunbalanced output or single ended output. The complete circuit diagram of such a basicdual input, balanced output differential amplifier is shown in the Fig.

As the output is taken between two output terminals, none of them is grounded, it is called balanced output differential amplifier.

Let us study the circuit operation in the two modes namely

i) Differential mode operation

ii)Common mode operation