Here is a low cost AM receiver circuit that can be tuned from around 550 to 1100 KHz. Three transistors are used in this project. The transistors Q1 and Q2 are wired as a compound transistor pair in which Q1 is an emitter follower and Q2 is a common emitter amplifier. The emitter follower prevents the loading of tank circuit, while the common emitter amplifier necessary voltage gain. The L1 and C2 forms the tank circuit.The L1 also does the job of antenna.The series combination of R6 and R7 gives a regenerative feedback between output of the Q2 and tank circuit. The transistor Q3 performs the demodulation of the carrier signal. It also provides amplification the demodulated signal. The audio output is coupled out from the collector of Q3 via the capacitor C3.Capacitor C6 provides some noise filtering.
- The circuit can be assembled on a general purpose PCB.
- Use a 9V PP3 battery for powering the circuit.
- R7 can be used to adjust the sensitivity and selectivity of the circuit.
- A high impedance head phone can be used for listening the radio.
- For L1 make 60 turns of 26 SWG enameled copper wire on a 3/8 inch diameter, 3inch long ferrite rod.
- The moving plates of C2 must be connected to the base of Q1 and fixed plates must be connected to the junction of C3 and R6.Reversing the connection may decrease the stability.
Some voltage values at different points on the working circuit.
Q1 (b) 1.31V
Q2 (b) 0.71V
Q2 (c) 1.34V
Q3 (b) 0.62V