How can selectivity be achieved in the front-end circuitry of a communications receiver?

By using a preselector.

Initial filter for the incoming radio signal from the antenna.

See Wikipedia's article on Preselector

Also, see the AWG RF site for the article RF PRE-SELECTOR: AN OVERVIEW OF RF PRE-SELECTOR

And, see the Ham Radio Secrets site for the article on A Simple Low-cost HF Preselector

For more information, please see the Industrial Electronics site for the article Guide to RF Circuit Electronic Design--RF amplifier and preselector circuits

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What is the primary purpose of an RF amplifier in a receiver?

To improve the receiver’s noise figure.

When the antenna receives the signal, it is amplified but only for the specified range of frequencies.

See Wikipedia's article RF power amplifier

Also, see the Electronics Notes site for the article on Superhet Radio RF Amplifier & Tuning

And, the Easy Electronics site for the article RF amplifier – Working, Circuit Diagram, and Advantages

For more information, please see the EEEGuide site for the article Transistor RF Amplifier Circuit:

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How much gain should be used in the RF amplifier stage of a receiver?

Sufficient gain to allow weak signals to overcome noise generated in the first mixer stage.

For more information, please see the Digi-Key Electronics site for the article on Understanding the Basics of Low-Noise and Power Amplifiers in Wireless Designs

See Wikipedia's article RF power amplifier

Also, see the Electronics Notes site for the article on Superhet Radio RF Amplifier & Tuning

And, the Easy Electronics site for the article RF amplifier – Working, Circuit Diagram, and Advantages

For more information, please see the EEEGuide site for the article Transistor RF Amplifier Circuit:

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Too much gain in a VHF receiver front end could result in this:

Susceptibility of intermodulation interference from nearby transmitters.

Please see the Info Tech Open site for the article on Design Concepts of Low-Noise Amplifier for Radio Frequency Receivers

See Wikipedia's article RF power amplifier

Also, see the Electronics Notes site for the article on Superhet Radio RF Amplifier & Tuning, and also the article on Radio Signal to Noise S/N Ratio, SNR

And, the Easy Electronics site for the article RF amplifier – Working, Circuit Diagram, and Advantages

For more information, please see the EEEGuide site for the article Transistor RF Amplifier Circuit:

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What is the advantage of a GaAsFET preamplifier in a modern VHF radio receiver?

High gain and low noise floor.

View YouTube video from the Maersky channel, titled JIM M-75 Low Noise Wide Band GaAs FET Pre-Amplifier

See the Repeater-Builder site for the article Build a good GaAs FET preamp for 2 meters, 222 or 440 MHz.

Examine the wa5vjb site the well-illustrated pdf document, called GaAs FET Pre Amp Cookbook #3, Kent Britain WA5VJB.

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In what stage of a VHF receiver would a low noise amplifier be most advantageous?

Front end RF stage.

When the antenna receives the signal, it is amplified but only for the specified range of frequencies.

Please see the Info Tech Open site for the article on Design Concepts of Low-Noise Amplifier for Radio Frequency Receivers

See Wikipedia's article RF power amplifier

Also, see the Electronics Notes site for the article on Superhet Radio RF Amplifier & Tuning, and also the article on Radio Signal to Noise S/N Ratio, SNR

And, the Easy Electronics site for the article RF amplifier – Working, Circuit Diagram, and Advantages

For more information, please see the EEEGuide site for the article Transistor RF Amplifier Circuit:

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