Introduction / Context:
Amplitude limiters are essential in FM receivers to suppress amplitude variations caused by noise, since FM conveys information in frequency variations rather than amplitude. However, not all assumptions about limiter design are correct.
Given Data / Assumptions:
- Limiter operates before FM demodulator.
- Leak-type biasing forces class C operation.
- Threshold limiting ensures nearly constant output amplitude.
Concept / Approach:
The limiter works by clipping the amplitude of incoming signals beyond a threshold. It does not require the output circuit to be tuned because its primary function is amplitude stabilization, not frequency selection.
Step-by-Step Solution:
Leak-type bias forces transistor into cutoff for part of the input cycle.Class C operation enables strong limiting.When input exceeds threshold, output amplitude is clipped, maintaining constant level.Output tuning is unnecessary as limiter function is amplitude-based.
Verification / Alternative check:
Practical FM limiter circuits do not rely on tuned outputs; they rely on nonlinearity.
Why Other Options Are Wrong:
Leak-type bias is indeed essential for limiter action.Class C biasing is correct description of limiter operation.Threshold action is the very purpose of amplitude limiting.Noise suppression is partial but not complete, so not the “not valid” answer.
Common Pitfalls:
Assuming amplitude limiter is also a tuned amplifier—it is not.
Final Answer:
Output must be tuned
Discussion & Comments