Functions of the RF amplifier in a radio receiver front end Why is an RF amplifier stage commonly included ahead of the mixer in superheterodyne receivers?

Introduction / Context:
The RF amplifier in a receiver front end provides controlled gain and helps with noise figure and image rejection. It prepares the signal for mixing while reducing the impact of mixer noise and improving matching to the antenna system.

Given Data / Assumptions:

• Standard superheterodyne architecture.
• Reasonable quality RF amplifier with appropriate bandwidth and selectivity.
• Goal is robust reception across a range of signal strengths.

Concept / Approach:
Adding low-noise gain early improves the cascade noise figure (Friis formula). A tuned RF amplifier also provides preselection that suppresses images and out-of-band interferers. Proper input matching improves power transfer from the antenna into the receiver chain.

Step-by-Step Solution:
Sensitivity improves because the RF stage boosts weak signals before the mixer and IF.SNR improves since low-noise first-stage gain dominates system noise.Impedance matching and preliminary selectivity reduce unwanted signals and reflections.Thus, all listed benefits are valid.

Verification / Alternative check:
Receiver design texts emphasize a low-noise, high-linearity front end to set overall noise performance and dynamic range, confirming these roles of the RF amplifier.

Why Other Options Are Wrong:
The single-benefit choices ignore other equally important roles. Protection alone is not the main purpose; gain, noise, and selectivity are primary.

Common Pitfalls:
Assuming the RF stage is optional; while some minimal receivers omit it, most quality receivers include it for the reasons above.

Final Answer:
All of the above