JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRONIC CIRCUIT ANALYSIS Previous Question Paper (March - 2022 - R15)

The Emitter follower, also known as a Common Collector (CC) amplifier, consists of a transistor where the input signal is applied to the base and the output is taken from the emitter. The collector is connected to the supply voltage, making it common to both input and output circuits.

Circuit Operation:

1. The base-emitter junction is forward-biased by the input signal.
2. The emitter voltage follows the base voltage closely, minus the base-emitter voltage drop (V_BE ≈ 0.7V).
3. Because the output (emitter) tracks the input (base) with a gain of approximately unity, it is called an emitter follower.
4. It provides high input impedance and low output impedance, making it ideal for impedance matching applications.
5. There is no phase shift between input and output signals; they are in phase.

An emitter-coupled differential amplifier consists of two identical transistors with their emitters tied together and connected to a common constant current source. The inputs are applied to the two bases, and the output is typically taken differentially between the two collectors.

Operation:

1. When identical input voltages are applied (common-mode signal), the current from the constant current source splits equally between the two transistors, resulting in zero differential output voltage.
2. When different voltages are applied (differential-mode signal), the transistor with the higher base voltage conducts more current, causing its collector voltage to drop, while the other transistor conducts less, causing its collector voltage to rise.
3. The circuit effectively amplifies the difference between the two input signals while rejecting signals common to both inputs, a property known as high Common-Mode Rejection Ratio (CMRR).
4. The constant current source ensures that the sum of the emitter currents remains constant, providing stability against variations in supply voltage and temperature.