CRAC Class C (2025) — Questions 1041–1050

A triode amplifier provides gain in any of its three basic configurations. In the common-cathode (共阴极) stage the small-signal grid voltage controls both plate current and plate voltage (A). In the common-grid (共栅极) stage the grounded grid blocks grid–plate capacitance while the cathode input current change is reflected as a plate-voltage change (B). In the common-plate or cathode-follower (共屏极) stage a grid- voltage change drives a corresponding cathode-current change (C). All three describe valid amplification mechanisms. D is false: a voltage amplifier does not "automatically" become a power amplifier merely because the input grows — power amplification depends on the stage's design and load, not input level.

A 小信号放大器 (small-signal amplifier) is defined by its operating condition: the operating point (Q-point) remains in the linear region at all times, so the active device behaves linearly. B, C, and D try to define it by arbitrary voltage or gain thresholds, which is incorrect — the defining feature is the linear operating point, not a fixed voltage or gain number.

A 大信号放大器 (large-signal amplifier, i.e. a power amplifier) is one whose operating point can swing outside the linear region, deliberately driving the device into cutoff/saturation or nonlinear operation to deliver power. B/C/D are arbitrary voltage or gain thresholds and do not define large-signal operation.

电源效率 (plate/power-supply efficiency) is lowest for the 甲类 放大器 (Class A amplifier), which conducts for the full 360° cycle and idles at a high quiescent current; its maximum theoretical efficiency is only about 25% (with a transformer) or 50% (inductor load). Class B/AB/C are progressively more efficient.

Among conventional Classes A/B/AB/C, the 丙类放大器 (Class C amplifier) has the highest efficiency because its conduction angle is well under 180° and it idles at essentially zero current. (Class D switching amplifiers are even higher, but among the listed options Class C is the most efficient.)

The 甲类放大器 (Class A amplifier) conducts for the full 360° and the device never cuts off, so the output waveform tracks the input with the least distortion. Class B has crossover distortion, Class AB slightly less, and Class C is highly distorted without a tuned/linearizing network.

The 丙类放大器 (Class C amplifier) has the largest waveform distortion because it conducts for less than half a cycle and clips the signal heavily; it only reproduces a clean waveform after a tuned (frequency-selective) network restores the fundamental. Class A is the least distorted, so A is the correct choice.

An audio power amplifier must reproduce the full waveform with low distortion yet deliver useful power. The 甲乙类放大器 (Class AB amplifier) is the standard choice: it has far less crossover distortion than pure Class B and much better efficiency than Class A, making it the practical audio PA. Class C cannot linearly amplify audio; pure Class B suffers crossover distortion; Class A is inefficient for power.

An audio small-signal amplifier needs linearity and low distortion, with little concern for efficiency, so the 甲类放大器 (Class A amplifier) is the natural fit — it stays in the linear region and reproduces audio cleanly. Class C is unusable for audio; Class B/AB are power-stage choices, not small-signal pre-amp choices here.

CW is a constant-amplitude (on/off) emission, so both a near- Class-B-biased 甲乙类 (Class AB) stage (A) and a near-Class-A- biased Class AB stage (B) are suitable and commonly used. According to the official key, C (Class C) and D (Class D) are not selected here; note that in practical CW design Class C and switching Class D are also widely used for constant-amplitude keyed carriers, but the examination marks only A and B for this item.