Amplifier categories — match each stage type to its frequency range or operating class List I A. IF amplifier B. RF amplifier C. AF (audio-frequency) amplifier D. Class operation (e.g., Class A/B/C) List II 1. Amplifier frequencies in the range of about 100 kHz to 5 MHz 2. Current in the output flows only during a portion of the positive half of the input cycle 3. Amplifier frequencies in the range from near 0 Hz up to a few megahertz (low-to-mid frequencies) 4. Amplifier frequencies in the range of about 40 Hz to 15 kHz

Introduction / Context:
Receiver and transmitter chains use AF, IF, and RF amplifiers at different stages. Separately, “class of operation” (A/B/C, etc.) describes the conduction angle of the active device. Matching these concepts helps relate block diagrams to device physics.

Given Data / Assumptions:

• Typical broadcast/communication frequency ranges.
• AF roughly spans human hearing (≈40 Hz to 15 kHz in many systems).
• IF stages commonly lie in the hundreds of kilohertz to a few megahertz, but the list item 3 summarizes a broad low-to-mid-frequency region that includes common IFs.
• Class operation is characterized by conduction angle (e.g., Class C conducts less than 180 degrees).

Concept / Approach:
We map RF amplifiers to a representative 100 kHz–5 MHz band, AF amplifiers to 40 Hz–15 kHz, IF amplifiers to the broader low-to-mid band that includes common intermediate frequencies, and class operation to the definition involving partial-cycle conduction (typical of Class B/AB/C).

Step-by-Step Solution:

Verification / Alternative check:
Standard superheterodyne receivers employ IFs in the hundreds of kHz to tens of MHz. AF power amplifiers target audio bandwidth. RF pre-selectors and power stages operate across designated RF bands.

Why Other Options Are Wrong:

• Swapping AF with RF bands misplaces the physical meaning of the stages.
• Linking “class operation” to a frequency band instead of conduction angle confuses biasing with spectrum.

Common Pitfalls:
Treating list frequency bounds as universal—actual system IFs may be outside the example ranges; however, the relative ordering remains correct.

Final Answer:
A-3, B-1, C-4, D-2