CRAC Class C (2025) — Questions 1031–1040

The figure (LK0594.jpg) shows a shunt capacitor to ground at the output of a series resistor (or L-section R-C), which passes low frequencies and attenuates high frequencies — a 低通滤波 (low-pass filter). A is correct; it is not a band-pass, high-pass, or protection element.

The figure (LK0605.jpg) depicts an oscillator whose feedback is taken via a coupling/transformer winding (inductive coupling) between the tuned collector circuit and the base — an 电感耦合 式振荡器 (inductively-coupled oscillator). A is correct; Colpitts/Hartley use a tapped capacitor/inductor divider and RC-feedback uses resistors/caps.

For a MOSFET switch, 截止 (cutoff) = bias too low to conduct (off); 饱和 (saturation, as a switch) = fully enhanced so the channel is as low-resistance as it gets and extra gate voltage barely lowers Rds(on) (on). A correctly defines both; B wrongly says resistance keeps dropping (that is the linear/triode region, not saturation); C/D are imprecise.

Each FET configuration provides voltage/current gain: common-source (栅极→漏极) gives large drain voltage/current change from small gate change (A); common-gate (源极→漏极) gives drain-voltage change from source-current change (B); common-drain/source-follower (栅极→源极) gives source- current change from gate change (C). D is false — a small- signal amplifier does not "become" a PA at large drive; it clips.

Bipolar transistor gain by configuration: common-emitter (基极 →集电极) gives large collector voltage/current swing from small base current (A); common-base (发射极→集电极) gives collector-voltage change from emitter current (B); common- collector/emitter-follower (基极→发射极) gives large emitter- current change from base current (C). D is false for the same reason as the FET case — large drive causes clipping, not automatic PA operation.

Input impedance: common-base is lowest (≈re), common- emitter is medium (≈β·re), common-collector (emitter follower) is highest (≈β·(re+RE')). So lowest→highest is CB, CE, CC. A is correct.

Current gain: common-base ≈ α (<1, no current gain); common-emitter ≈ β (tens to hundreds); common-collector ≈ β+1 (≈β, the largest). So lowest→highest is CB, CE, CC. A is correct.

Voltage gain: common-collector (emitter follower) ≈ 1 (lowest); common-base is moderate (≈gm·Rc); common-emitter is highest (≈gm·Rc with full gain, no local degeneration). So lowest→highest is CC, CB, CE. A is correct.

Output impedance: common-collector (emitter follower) is lowest (≈re+source/β); common-emitter is medium; common- base is highest (≈ro·... large). So lowest→highest is CC, CE, CB. A is correct.

The 射极跟随器 (emitter follower) is the 共集电极放大电路 (common-collector amplifier): the output is taken from the emitter, which "follows" the base voltage. A is correct; it has unity voltage gain and is used for buffering/impedance matching.