Introduction / Context:
Automatic gain control (AGC) is a standard feature of superheterodyne radio receivers. Its purpose is to keep the audio output approximately constant despite large variations in received RF signal levels, improving listener comfort and preventing overload or distortion.
Given Data / Assumptions:
- Simple diode-detector based AGC typical of broadcast receivers.
- AGC voltage is derived from the detected (envelope) amplitude after the IF or detector stage.
- AGC voltage is fed back to control the gain of RF/IF amplifiers.
Concept / Approach:
As the received signal strength increases, the detector output envelope becomes larger. The AGC network filters and rectifies this to a DC control voltage whose magnitude rises with signal level. This increased AGC voltage then biases gain-controlled stages (RF/IF) to reduce their gain, thereby leveling the overall output. Thus, stronger signals produce more AGC action (a larger control voltage).
Step-by-Step Solution:
RF signal → RF/IF amplifiers → detector/envelope → AGC rectifier/filter.As input field strength ↑, detector output ↑.AGC network outputs higher DC control voltage.Higher AGC voltage reduces RF/IF gain, stabilizing audio output level.
Verification / Alternative check:
Tuning from a weak to a strong station produces a measurable increase in AGC voltage and simultaneous reduction in IF gain—standard lab observation.
Why Other Options Are Wrong:
Highest AGC between stations: between stations the detector mainly sees noise; AGC is typically low to permit sensitivity.Faster time constant ⇒ “more accurate”: too fast produces pump-ing and distortion; proper time constants are a trade-off.Audio stage gain control: in most designs AGC acts on RF/IF stages, not the audio power amplifier.AGC does not selectively reduce noise; it reduces overall gain including both signal and noise.
Common Pitfalls:
Assuming AGC boosts weak signals (it generally reduces gain for strong signals rather than amplifying weak ones).
Final Answer:
An increase in signal strength produces more AGC (stronger control voltage)
Discussion & Comments