Conduction angle identification: In a class A power amplifier, for how much of the input signal cycle does the collector (or drain) current flow?
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ALess than 180° of the cycle
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BExactly 180° of the cycle
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CLess than 360° of the cycle but more than 180°
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DFull 360° of the cycle
Answer
Correct Answer: Full 360° of the cycle
Explanation
Introduction / Context:Amplifier “class” describes the bias point and conduction angle, which in turn affects linearity and efficiency. Class A operation is the benchmark for linearity in many small-signal applications, though it sacrifices efficiency.
Given Data / Assumptions:
- Sine-wave input signal.
- Proper bias so the active device never cuts off during the cycle.
- Focus is on classic class A (not special topologies like push-pull class A/B hybrids).
Concept / Approach:In class A, the transistor is biased in the middle of its active region so that current flows continuously. The conduction angle is therefore the entire 360° of the input cycle. This maximizes linearity and minimizes crossover distortion but results in low efficiency because current still flows at zero signal.
Step-by-Step Solution:
Choose a quiescent point (Q-point) in the active region so the device never reaches cutoff or saturation over the signal swing.As input swings positive and negative, the collector/drain current varies around a nonzero quiescent value.Since current never drops to zero within the cycle, conduction angle = 360°.Verification / Alternative check:Compare with class B (conduction 180°) and class AB (between 180° and 360°). These reference points confirm the definition for class A.
Why Other Options Are Wrong:
- Less than or equal to 180° describes class B or class C, not class A.
- “Less than 360° but more than 180°” is class AB, not class A.
Common Pitfalls:Confusing classes due to push-pull implementations; even with two devices in push-pull, each class A device conducts for the full cycle.
Final Answer:Full 360° of the cycle.