If a FET is used as the first AF amplifier in a transistor receiver, this will have the effect of

Solution (By Examveda Team)

A Field-Effect Transistor (FET) is a type of transistor where current flow is controlled by an electric field. It is commonly used in various amplifier stages of receivers because of its high input impedance and low noise characteristics, which make it ideal for the first stages of a receiver.

An AF amplifier (Audio Frequency amplifier) amplifies the audio signal after it has been demodulated from the radio frequency signal. It is a key part of the receiver that ensures the audio output is at a usable level for listening.

Now, let's go through each option:

Option A: Improving the effectiveness of the AGC
This option is incorrect. The AGC (Automatic Gain Control) is designed to maintain a consistent output level by adjusting the receiver's gain based on signal strength. Using a FET as the first AF amplifier does not have a significant impact on the AGC's effectiveness. While FETs do offer high input impedance and low noise, they do not directly enhance the operation of the AGC circuit, which primarily controls the RF and IF amplifiers.

Option B: Reducing the effect of negative-peak clipping
This option is correct. A FET in the first AF amplifier stage helps to reduce distortion, particularly negative-peak clipping, which can occur when the signal is too strong and the amplifier fails to handle it properly. The FET's characteristics—such as its smooth and non-linear transfer function—help to limit or reduce the clipping effect that can distort the audio signal. Therefore, using a FET in the first AF amplifier can improve the handling of high signal levels and prevent clipping.

Option C: Reducing the effect of noise at low modulation depths
This option is incorrect. The FET does provide low noise operation, but it does not specifically target noise at low modulation depths. Noise at low modulation depths is typically a result of poor signal-to-noise ratio (SNR) at lower signal levels, and while a FET could contribute to overall noise reduction in the receiver, it is not particularly effective in reducing noise specifically at low modulation depths.

Option D: Improving the selectivity of the receiver
This option is incorrect. Selectivity refers to the ability of a receiver to focus on the desired frequency and reject unwanted frequencies (adjacent channels). The selectivity of a receiver is primarily determined by the RF and IF stages, as well as the filtering employed in the receiver. The FET in the AF amplifier stage does not directly impact the selectivity of the receiver. Selectivity is more influenced by the intermediate frequency (IF) section, not the audio frequency (AF) amplifier.

Conclusion:
The correct answer is Option B: Reducing the effect of negative-peak clipping. The use of a FET as the first AF amplifier in a transistor receiver helps to reduce the distortion caused by clipping, particularly at higher signal levels.