JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II ELECTRONIC CIRCUIT ANALYSIS Previous Question Paper (November/December - 2020 - R15)

Amplifier coupling schemes are used to connect stages of a multi-stage amplifier to transfer signals while maintaining appropriate DC bias conditions. The primary methods are:

• RC Coupling: Uses a resistor and capacitor network. The capacitor blocks DC while allowing AC signals to pass. It is cost-effective and offers good frequency response in the audio range but is unsuitable for DC amplification.
• Transformer Coupling: Uses an impedance-matching transformer. It provides excellent impedance matching and high efficiency, often used in power amplifiers, but is bulky and expensive for low-frequency applications.
• Direct Coupling: Connects the output of one stage directly to the input of the next without reactive components. It is ideal for amplifying very low frequencies and DC signals, though it suffers from thermal instability and requires complex biasing.
• Impedance Coupling: Uses an inductor and capacitor (LC) combination. It is similar to RC coupling but uses an inductor to provide high impedance at the signal frequency, offering better gain at specific frequencies.

A Common Emitter (CE) amplifier with an unbypassed emitter resistor (R_E) introduces negative feedback, which improves stability and linearity at the cost of reduced voltage gain. In the circuit, the emitter resistor is placed between the emitter terminal and ground.

The voltage gain (A_v) is given by:

Where r'_e is the internal emitter resistance. The presence of R_E provides series-series negative feedback, which:

• Increases Input Impedance: The effective input resistance becomes Z_in(base) = β(r'_e + R_E).
• Improves Stability: The gain becomes less dependent on transistor parameters (β) and temperature variations.
• Reduces Distortion: Negative feedback linearizes the transfer characteristic, reducing non-linear distortion.
• Increases Bandwidth: The gain-bandwidth product remains constant, but the gain reduction results in a wider frequency response.

The circuit diagram consists of a BJT in CE configuration with a collector resistor R_C, base bias resistors R₁ and R₂, and the emitter resistor R_E. An input coupling capacitor connects the source to the base, and an output coupling capacitor connects the collector to the load.