Here is the circuit diagram of direct coupled radio. The central and core part of this circuit consists of three transistors Q1 (HEP 641), Q2 (HEP 253) and Q3 (HEP 641) as shown in the circuit diagram. In this circuit, all the three transistors are made to work as audio amplifiers. Among these transistors Q1 has got an additional function. It also acts as a diode detector. The diode detection operation is taking place at the emitter-base junction of Q1 which operates as a PN junction diode. The radio filtering stage in detection process is provided by the base-emitter capacitance of Q1. The consistency in volume and least distortion are obtained by adjusting the potentiometer R1 (47 K).
- Use a good quality PCB to assemble this particular circuit.
- This an excellent direct coupled radio circuit ideal for listening to near by stations.
- Use a center tapped ferrite antenna coil for L1 that is used AM receivers.
- Use an appropriate battery for powering this circuit since it can reduce noise in the circuit (A 3V DC battery is used here)