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?
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AWhen the AC input signal is zero (no signal condition)
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BWhen the AC input signal is very low amplitude
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CWhen the AC input signal is negative half-cycle only
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DEither (a) or (b)
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EDuring saturation with a large input
Answer
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)