Differential amplifiers

 Give differential amplifier using BJT, FET?

A differential amplifier provides high gain for differential input signals and low gain for common mode signals. Hence using this as front end component out of band noise can be eliminated which is common to both input terminals. The differential amplifier implemented using BJT’s are shown below

Differential amplifier BJT

Differential amplifier BJT

The resistance in the emitter lead plays a crucial role in the functioning of differential amplifier. This resistance has to be high in order that the CMMR of differential amplifier to be high. Alternatively in the improvised versions of differential amplifiers a constant current source is kept in emitter lead to provide very high resistance (Theoretically infinite). The same differential amplifiers implemented using MOSFET’s is shown below

Differential amplifier MOSFET

Differential amplifier MOSFET

The MOSFET differential amplifier provides high input resistance but less gain compared to differential amplifiers implemented using BJT’s.

What is CMMR of differential amplifier using BJT? What is the effect of degeneration resistance on CMMR?

The CMMR of differential amplifier using BJT with emitter resistance at the common terminal where the emitters of both the BJT’s meet and output taken across the collector terminals is given as

CMMR = |Ad/Ac| = gm*Re

where Ad (differential gain) = -gm*Rc, Acm (common mode gain of each stage) = -Rc/(2*Re) and the overall common mode gain of differential amplifier will be double of Acm.

It is obvious from the expression of CMMR that increase in emitter degeneration resistance increases CMMR.

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