Here is the circuit diagram of simple operational amplifier radio that can be employed to hear nearby stations. This circuit is based on quad operational amplifier LM 324. This operational amplifier radio consists of crystal radio and sensitive audio amplifier. The audio amplifier receives stations using with a minimal 15 foot antenna. For little weaker signals a germanium diode such as 1N34A is more appropriate. A rectified output signal is produced at the cathode of the diode. But the carrier frequency of the rectified signal is removed with the help of a 300pF capacitor.
The audio frequency obtained through antenna is given to the non-inverting input of the op-amp through capacitor C2. The op-amp acts as a high impedance stage here. The output from the first op-amp is given to another op-amp which increases the voltage level about 50 times and is DC coupled to the first through the 10K resistor. High impedance headphones can be used as Z1.
Resistors: R1 (100K), R2 (1M), R3 (1M), R4 (10K), R5 (100K), R6 (100K), R7 (470K), R8 (470 ohm)
Capacitors: C1 (30-360 pF variable), C2 (0.1 uF), C3 (300 pF), C4 (100uF)
Active Components: IC1 (LM 324), T1 (SL 100), Z1 (Head phone), D1 (1N 34)
- For antenna use a 50 cm long copper wire.
- For inductor make 210 turns of no: 28 enameled copper wire on a 1 cm diameter and 10 cm long plastic pipe.
- Use a 9V battery as power source for reducing noise.
- In order to keep the DC voltage at the transistor emitter between 3 and 6 volts, adjust the values of 100K and 1 mega ohm resistance, as they may not be close in value.