CRAC Class B (2025) — Questions 1031–1040

An ideal op-amp (理想运算放大器) has infinite open-loop gain (A) and infinite input impedance with both inverting (−) and non-inverting (+) inputs (B). C and D wrongly equate "ideal" with a specific transistor type or unlimited output swing; the ideal model is about gain/impedance, not device technology.

An integrated operational amplifier (集成运算放大器) processes continuous analog signals, so it is an analog integrated circuit (模拟集成电路). MMIC (B) is for RF/microwave; digital ICs (C) and PLDs (D) handle logic, not linear amplification.

Suffixes such as DIP (dual in-line package), SOP/SSOP (small- outline package), QFP (quad flat package), SIP (single in-line), SOT (small-outline transistor) denote the physical package form (封装形式) of the IC, not temperature, logic levels, or frequency. 测量与仪表 Measurement and Instruments

A DC voltmeter reads the average (mean) value. A bipolar symmetric square wave that spends equal time at +V and −V has an average of zero, so the reading is zero (A).

A moving-coil AC meter rectifies the waveform and is calibrated to read the RMS value of a sine wave (using the sine form factor ≈1.11). A square wave has a rectified average equal to its peak (Vpk); multiplying by 1.11 yields about 1.11·Vpk, which exceeds the peak (A). It is neither the peak (B), peak-to- peak (C), nor the average (D).

A 0-to-Vpk square wave at 50% duty has an average = Vpk/2. A DC meter reads this average. For this waveform the RMS value also equals Vpk/2 (since RMS² = (Vpk²·0.5 + 0²·0.5) = Vpk²/2). So the reading is half the peak and equals the RMS value (A).

An ohmmeter supplies a small current to measure resistance. If a large capacitor is across the circuit, it initially acts like a short (low reading) while charging, then its impedance rises as it fills, so the reading climbs toward infinity (A). A large inductor would instead show a rising-then-settling or initially high reading, not this pattern; an AC source or fault (C/D/B) does not explain the gradual rise.

A sound electrolytic capacitor can be checked with an analog ohmmeter (it should show a charging swing to high resistance, A) and, more directly, with a digital multimeter's capacitance function if available (B). A current range (C) is unsuitable, and a charger plus short-circuit test (D) is dangerous and not a proper test.

The behavior — an initial charging deflection on first connection, then settling, and little or no movement on a repeat connection (the component having retained charge) — is consistent with an electrolytic capacitor measured on an ohmmeter (A). The other options make absolute ("肯 定/definitely") claims that are not supported and are incorrect as the intended answer.

Ohms-per-volt = the meter's input resistance per volt of range; a higher figure means higher input resistance, so the meter draws less current and disturbs the circuit less while measuring voltage (A). Lower ohms/volt means more loading (B wrong); it does not describe overload strength (C/D wrong).