JNTU B.Tech II ANALOG ELECTRONICS Previous Question Paper (September/October - 2023 - R16)

A bypass capacitor is used in a transistor amplifier to provide a low-impedance path for the AC signal to ground, effectively bypassing the emitter resistor (in common emitter configuration). This prevents negative feedback from occurring at signal frequencies, thereby maintaining high voltage gain for the amplifier while still allowing the emitter resistor to provide DC stability for the operating point.

A Cascode Amplifier is a two-stage amplifier configuration consisting of a common-emitter (or common-source) stage feeding into a common-base (or common-gate) stage. In this arrangement, the first stage acts as a transconductance amplifier, and the second stage acts as a current buffer, providing high voltage gain while minimizing the Miller effect.

The primary advantages of the cascode configuration include:

• Reduced Miller Effect: By isolating the input from the output, the input capacitance is significantly reduced, which greatly improves the high-frequency bandwidth of the amplifier.
• High Output Impedance: The common-base stage provides a very high output resistance, which allows for higher voltage gain and makes the circuit behave more like an ideal current source.
• Improved Reverse Isolation: The configuration minimizes the feedback from the output to the input, leading to superior stability and reduced interaction between stages.