In a superheterodyne transistor receiver, what is the main advantage of using a Field-Effect Transistor (FET) as the very first RF amplifier stage? Explain the practical benefit at the front end of the receiver rather than later stages.

Introduction / Context:
At the front end of a radio receiver, the first active device largely determines the overall noise figure. A Field-Effect Transistor (FET) is often chosen as the first RF amplifier because of its high input impedance and inherently low noise when properly biased. This question probes why a FET is preferred at the very first stage rather than deeper in the chain.

Given Data / Assumptions:

• Superheterodyne topology with a dedicated RF amplifier ahead of the mixer.
• The first stage dominates the cascaded noise figure.
• FET exhibits high input impedance and low gate current.

Concept / Approach:

Friis’ formula shows the overall noise figure is most sensitive to the first stage’s noise and gain. Putting a low-noise, moderate-gain FET first reduces the system noise temperature and improves sensitivity, particularly for weak signals near the noise floor. While selectivity and AGC matter, they are not the primary benefits of choosing a FET as the first device.

Step-by-Step Solution:

Verification / Alternative check:

Practical receivers use low-noise JFETs or MOSFETs in the RF front end; measured minimum discernible signal (MDS) improves compared to bipolar-only solutions under similar matching and bias conditions.

Why Other Options Are Wrong:

• Improve selectivity: primarily a function of RF/IF filtering and Q, not the device type.
• Improve AGC effectiveness: AGC is a control strategy; device choice does not inherently improve it.
• Reduce detector clipping: a detector issue, not a first-stage device choice.
• Increase LO deviation: unrelated to the RF amplifier device.

Common Pitfalls:

Assuming the front-end device affects selectivity directly; in reality, filtering topology dominates selectivity, while the first device dictates noise performance.

Final Answer:

Reduce the effect of noise (better noise figure) at the receiver front end