Block Diagram & Working Principle of Superhetrodyne Radio Receiver

The Superhetrodyne circuit is used in almost all the transistor radios. In this type of circuit the frequency of incoming signals is changed into a fixed frequency known as 'Intermediate Frequency IF'. The main work of amplifying the signal and selecting the desired station, is done in IF section. This permits higher amplification per stage and better selectivity.

The main component of circuit are:
1. Frequency Converter (Mixer and Oscillator)
2. I.F. Amplifier
3. Detector
4. Audio Pre-amplifier
5. Driver
6. Output
7. Power Supply


1. Frequency Converter (Mixer and Oscillator)

It has two section Mixer and Oscillator. Usually only one transistor is used for both section.
The Oscillator produces oscillations at a frequency, which is higher than the frequency of the station being received. The difference equals to the intermediate frequency.
Out of the signals intercepted by the aerial, a tuned circuit used at the input of the mixer stage selects the signals from the desired stations. The mixer mixes these signals and the oscillations from the oscillator. The mixing of these signals results in the production of their sum (f1 + f2) and difference (f1 - f2) frequencies. The difference is IF signal. It contains the same modulation as contained by the received signals. The IF signals are then given to the IF amplifier.

2. I.F. Amplifier

It amplifies the I.F. signals. It provides a large part of the gain and selectivity. Two stages are used in this section to obtain the required amplification. Tuned transformers known as IFT are used as a collector loaf and for interstate coupling in the IF amplifier. The amplified IF signal are given to the detector.

3. Detector

The detector separates the audio signal from the modulated carrier. A diode is generally used for detection.

4. Audio Pre-amplifier

The audio signals received from the detector are given to audio pre-amplifier stage. This stage amplifies these signals and given them to the driver stage.

5. Driver

The signals obtained from the pre-amplifer are not strong enough to drive the output stage. The stage amplifies these signals and gives them to the driver stage.

6. Output Stage

The output stage amplifies the audio signals obtained from the driver and develops sufficient power to drive the loudspeaker. To obtain maximum economy in battery consumption, push pull output stages using two transistors in class B circuit are normally used.

7. Power Supply

Most of the transistor radios operate on a few cells/battery. The supply voltages from 3 to 9 volts are common. Some transistors radio can be operated from batteries as well as mains. In these diode rectifier converts transistor the voltage into D.C.
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