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Chapter: Electronic Devices and Circuits - Multistage Amplifiers and Differential Amplifier

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Transistorised Differential Amplifier

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

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.

RC1 = RC2

And

RE1 = RE2


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

 

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