CRAC Class C (2025) — Questions 1051–1060

FM (frequency modulation) is a constant-amplitude mode, so the amplifier need not be linear in amplitude — any class that preserves the envelope works. A near-Class-B Class AB (A) and a near-Class-A Class AB (B) both give clean constant-amplitude output, and the 丙类放大器 (Class C amplifier) (C) is classically used for FM/CW PAs because its envelope stays constant. Class A (D) is inefficient and not the intended answer here.

SSB (single sideband, 单边带 SSB) carries information in the envelope amplitude, so the PA must be linear to avoid distortion and splatter. A near-Class-B 甲乙类 (Class AB) stage (A) and a near-Class-A Class AB stage (B) are the standard linear SSB PAs. Class C (C) and Class D (D) are highly nonlinear and would destroy the SSB envelope, so they are incorrect.

Class A (甲类, A) conducts 360° and stays in the active region (no cutoff, no saturation). Class B (乙类, B) conducts 180° and does not saturate. Class C (丙类, C) conducts less than 180° and is driven into saturation at peaks. These three statements (A, B, C) are correct. D is wrong: 丁类 (Class D) is a switching amplifier — its devices are driven hard between cutoff and saturation (not linear), even though each may conduct ≤180°.

Only Classes whose devices can be arranged to track the full waveform linearly suit audio. Class A (甲类, A) with one transistor reproduces both half-cycles (A correct). Class B (乙类, B) needs two transistors in push-pull, each handling one half- cycle (B correct). Class C (丙类, C) clips most of the cycle and cannot recover the lost audio — wrong. Class D (丁类, D) is a switching (pulse-width) stage; although it can reproduce audio via filtering, the official key marks only A and B for this item (D's "totem-pole linear amplification" description is not the standard correct statement).

All four classes can amplify RF, but only the linear ones (甲类/乙 类, Classes A/B, and AB) preserve amplitude information, so they fit any RF mode (A correct). Classes C and D are nonlinear/switching and discard amplitude detail, so they only suit constant-amplitude RF such as FM/CW (B correct). C and D wrongly restrict A/B and C/D to only one kind of source signal; any class can amplify a signal fed to it.

Class AB (甲乙类) sits between A and B: a small quiescent bias current keeps the conduction angle just over 180° (A correct), eliminating Class B crossover distortion while keeping efficiency above Class A (B correct). C is wrong because the conduction angle is slightly more than 180°, not exactly 180°. D is wrong — Class AB is widely used for FM (constant- amplitude) and does not inherently produce disappearing carriers or spurious sidebands.

A single-ended Class B/AB final is usable because its harmonics are removed by the output 选频网络 (frequency-selective/tuned network) to satisfy 杂散发射 (spurious emission) limits (A correct). Its higher-order intermodulation creates spurs near the operating frequency, which are reduced by 负反馈 (negative feedback) (B correct). C is wrong — an antenna is not a harmonic filter and must not be relied on. D is wrong — spurious emissions are regulated and cannot be ignored.

Distortion increases as conduction angle shrinks and the device is driven harder: Class A (甲) is least distorted, then Class B (乙), then Class C (丙), and Class D (丁) switching has the largest raw waveform error before output filtering. So the ascending order is A, B, C, D.

Efficiency rises as the device spends more time near cutoff: 丁 类 (Class D, switching) is highest, then 丙类 (Class C), then 乙类 (Class B), and 甲类 (Class A) is lowest. So the descending order is D, C, B, A.

A single-ended Class B/AB RF stage clips half-cycles, but because RF is narrow-band the 选频网络 (tuned network) reconstructs the missing fundamental from the remaining energy (A correct). The same trick works for any narrow-band signal — even narrow-band audio at 1 kHz ±100 Hz could use a tuned single-ended stage (B correct). C is wrong (RF linearity still matters for SSB/AM); D is wrong (single-ended RF can be Class AB, not necessarily Class A).