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Assertion–Reason (BJT Amplifiers): The common-emitter (CE) amplifier is the most widely used BJT amplifier configuration. Reason: A CE amplifier introduces zero phase shift between input and output.

Difficulty: Easy

Correct Answer: A is correct R is wrong

Explanation:


Introduction / Context:
Common-emitter (CE) stages are the workhorses of BJT amplifier design. They provide substantial voltage gain, moderate input impedance, and reasonably low output impedance, making them very versatile. This assertion–reason item checks your understanding of both the popularity of CE and its phase characteristics.


Given Data / Assumptions:

  • Linear small-signal CE amplifier with resistive biasing and load.
  • Standard midband analysis (coupling/bypass capacitors treated as AC shorts).
  • No special phase compensation network added.


Concept / Approach:
CE amplifiers offer high voltage gain due to collector load and transistor transconductance, alongside convenient biasing. However, a hallmark of CE is signal inversion: the small-signal output at the collector is 180° out of phase with the input at the base. Therefore, any claim of zero phase shift is false for a typical CE stage.


Step-by-Step Solution:

Assess Assertion (A): CE is most widely used — correct due to favorable gain and impedance properties.Assess Reason (R): CE has zero phase difference — false; CE introduces approximately 180° phase inversion in the passband.Match: A true, R false ⇒ the correct choice is 'A is correct R is wrong'.


Verification / Alternative check:

Small-signal hybrid-π model shows v_o ≈ −g_m * v_π * R_C/parallel load, where the negative sign indicates inversion (≈180°).


Why Other Options Are Wrong:

If R were true, CE would exhibit no inversion, contradicting standard textbook results. Other combinations misstate either the popularity or the phase property.


Common Pitfalls:

Confusing CE (inverting) with emitter follower (common-collector), which has ~0° phase shift but unity gain.


Final Answer:

A is correct R is wrong
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