The Trf Receiver is the simplest type of AM radio receiver. The block diagram of a Tuned Radio Frequency (TRF) receiver is shown in the figure. Infinite number of transmitters installed throughout the world radiates radio waves in space. In general, these transmitters radiate different frequencies. Electromagnetic waves surrounding an antenna will induce currents of their frequency in the antenna. A provision should be there in the receiver to select only the desired RF Signal out of a number of frequencies to the receiver. This function of selecting the desired RF signal and rejecting the rest is achieved by the tuned voltage amplifiers in the RF amplifier stage. Tuned RF amplifiers contain a parallel LC tuned circuit. The desired RF signal is selected by the tuned circuit.

When the RF signal reaches the receiving antenna, a very weak voltage is induced in it. It is not possible to extract the audio signal from this voltage. It is necessary first to amplify the RF signal to a required level. This is achieved in a radio receiver with the help of tuned RF amplifier. Thus RF amplifier serves two purposes.

1. Selection of desired RF signal

2. Amplification of the selected RF signal to a suitable value. Usually two or three tuned RF amplifier stages are used.

The amplified RF signal is applied to the detector or demodulator stage where the audio signal is extracted from the audio signal. Diode detectors are the most common detector used for AM detection.

The demodulated signal amplitude will be very small in amplitude. In order to drive a loudspeaker, it must be first amplified. The audio amplifier stage includes the audio voltage and power amplifier. The audio voltage amplifier will be a class A amplifier and the power amplifier will be a class B push-pull amplifier. The voltage and power level of the audio signal from the output of the detector is raised in this stage. The signal gets sufficient energy to drive the loudspeaker. The output of the audio amplifier stage is applied to the loudspeaker. It will reproduce the original sound by converting the electrical audio frequency waves into sound waves.

Advantages of a TRF receiver

1. It is simple to design.

2. Its alignment is very easy.

Disadvantages of a TRF receiver

1. Poor selectivity and hence insufficient adjacent channel rejection.

2. Poor sensitivity

3. Instability of gain and bandwidth variation over the tuning range.

TRF receiver is suffered from the variation of the bandwidth over the tuning range. So the receiver will pickup adjacent stations as well as the one to which it is tuned. i.e., the selectivity of the TRF receiver varies with frequency. In TRF receiver the amplification of the signal also varies with the frequency. So the TRF receiver suffers from the instability of gain. These entire problems can be solved using superheterodyne receivers.

Significance of tuned amplifiers in radio receivers

Infinite number of transmitters installed throughout the world radiates radio waves in space. In general, these transmitters radiate different frequencies. Electromagnetic waves surrounding an antenna will induce currents of their frequency in the antenna. A provision should be there in the receiver to select only the desired RF signal out of a number of frequencies to the receiver. This function of selecting the desired RF signal and rejecting the rest is achieved by the tuned voltage amplifiers in the RF amplifier stage. Tuned RF amplifiers contain a parallel LC tuned circuit. The tuned RF amplifier contains a parallel tuned LC circuit. The RF amplifier also amplifies the selected RF signal to a suitable level. Also four frequencies are present at the output of the mixer.

1. Local oscillator frequency

2. The selected RF frequency

3. Local oscillator frequency + RF signal frequency

4. Local oscillator frequency – RF signal frequency

From these four signals, we have to select the difference frequency which is equal to 455 KHz. The fixed tuned amplifiers in the IF section are tuned to 455 KHz to select only this frequency and provides a high gain. Thus tuned amplifiers play a very significant role in radio reception.