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In a common-emitter (CE) amplifier, the quiescent collector current I_C and collector-to-emitter voltage V_CE are defined as the values measured under which signal condition?

Difficulty: Easy

Correct Answer: When the AC input signal is zero (no signal condition)

Explanation:


Introduction / Context:
The quiescent (Q-point) defines the DC operating state of a transistor amplifier in the absence of any applied time-varying signal. It is the reference around which the signal swings occur, ensuring linear amplification with minimal distortion if properly placed on the load line.


Given Data / Assumptions:

  • CE amplifier with some biasing network setting base current.
  • No input AC applied when defining quiescent values.
  • Supply voltage and resistors fix the nominal operating point.


Concept / Approach:
By definition, the quiescent state is the steady DC condition: I_C = I_CQ and V_CE = V_CEQ with the input signal set to zero. Small signals then cause symmetric excursions about this point. Using 'very low amplitude' is not the formal definition; it may approximate the quiescent behavior but is not exact. Negative half-cycle only is irrelevant to defining a DC state.


Step-by-Step Solution:

Set AC source = 0 (short for AC analysis; no time-varying input).Solve DC bias network for base current, then collector current I_CQ.Compute V_CEQ from supply and voltage drops across resistors and transistor.This pair (I_CQ, V_CEQ) is the Q-point.


Verification / Alternative check:

Load-line method: intersection of transistor characteristic (at chosen base bias) and the collector resistor line gives the quiescent values.


Why Other Options Are Wrong:

'Very low signal' is not a definition; it is an approximation. 'Negative half-cycle' has no role in DC definition. 'Either (a) or (b)' dilutes the precise definition.


Common Pitfalls:

Trying to define the Q-point during signal swing; mixing small-signal incremental analysis with the DC operating point.


Final Answer:

When the AC input signal is zero (no signal condition)
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