AM Transmitters and Receivers Questions and Answers

Answer: A

The master oscillator sets the carrier frequency, and its stability determines whether the transmitter stays inside its channel, which is why it is crystal controlled or synthesised and buffered from everything downstream. Audio amplification is the modulator chain's job. Antenna matching is the output network's. And supply rectification is a power-supply function. Naming each block by the one quantity it controls makes a transmitter diagram readable at a glance.

Answer: B

A buffer presents a constant light load to the oscillator so that changes further down the chain cannot pull its frequency, which is the practical difference between a transmitter that holds its channel and one that drifts when the antenna is touched. It does not multiply frequency, which is a separate stage where one is needed. Modulation is applied at low or high level elsewhere. And audio filtering happens in the modulator chain. Isolation is a recurring requirement wherever a frequency-determining element meets a varying load.

Answer: C

High-level modulation varies the supply to the final stage, so every stage before it handles an unmodulated carrier and can run in efficient class C. That is the classic broadcast arrangement. Modulating the oscillator would produce frequency rather than amplitude modulation and would destabilise it. Modulating the first stage is low-level modulation, which then forces linear amplification downstream. And the coupling network is passive. Where the modulation is injected decides the efficiency of the whole chain.

Answer: D

Once the signal carries a varying envelope, all subsequent amplification must be linear, which rules out the efficient class C operation an unmodulated carrier permits. That is the whole trade against high-level modulation. The oscillator is never pulled in an amplitude scheme. Antenna tuning is unaffected by modulation. And it is high-level modulation, not low-level, that demands a powerful audio stage. Tracing the consequence of a design choice through the whole chain is what these questions test.

Answer: A

Class C is standard, because a constant-amplitude carrier tolerates the heavy distortion of a stage conducting for less than half a cycle, and the tuned output circuit restores a clean sinusoid while the efficiency approaches seventy per cent or more. Class A is linear but wasteful. Class AB and class B are the compromises needed once an envelope must be preserved. Matching the amplifier class to whether the envelope carries information is the single most useful rule in transmitter design.

Answer: B

It varies the supply, so the output amplitude follows the message while the stage stays in class C. The audio modulator must therefore deliver power comparable with the carrier, which on a large transmitter is a substantial amplifier or a switching supply. Base bias variation would distort rather than modulate cleanly. Retuning the output network at audio rates is impossible. And varying the drive gives grid or base modulation, a lower-power but less linear alternative.

Answer: C

Switching the supply and filtering the result produces the modulated voltage the output stage needs with switching-mode efficiency rather than a linear audio amplifier of comparable power. It is why modern high-power AM broadcast transmitters are far smaller than their predecessors. The oscillator and the output tuned circuit are both still required. And the carrier is retained, this being full-carrier AM. Replacing a linear stage with a switching one is the same move that transformed power supplies generally.

Answer: D

Feedback through the device's internal capacitance can make a tuned amplifier oscillate at its own tuned frequency, and neutralisation cancels that path with an equal and opposite one. It is essential in valve stages and in some transistor designs. Harmonic radiation is dealt with by output filtering. Oscillator pulling is prevented by buffering. And overheating is a thermal design matter. Each unwanted behaviour has its own specific countermeasure, and confusing them is a common examination error.

Answer: A

It transforms the antenna's impedance to the value the output stage needs for maximum power transfer, and usually filters harmonics on the way. Without it the stage would be mismatched, delivering less power and risking damage. Modulation happens upstream. Harmonic generation is deliberate only in a multiplier chain, which an AM transmitter does not use. And metering rectifiers are small separate circuits. Matching and filtering in one network is a standard economy of transmitter design.

Answer: B

A dummy load presents the correct resistive impedance and turns the output into heat instead of radiation, so a transmitter can be tuned and measured without interfering with other users or breaching its licence. It does not increase the power measured; if anything a good load makes the measurement honest. It replaces the antenna rather than the modulator. And it is deliberately resistive, with no reactive part. Testing into a known load is what makes transmitter measurements repeatable.