Simple Three Transistor Radio Receiver for Medium Wave Band.
The circuit diagram of this simple three transistor radio receiver has been published in the magazine "Practical Wireless", December of 1968 year, and it been reprinted in other magazines, just like USSR magazine for radio amateurs "Radio".
This is reflex circuit (see Figure) with regeneration adjustment. The ferrite loopstick antenna is made of standard 8mm ferrite rod with length of 50..70mm. The coil L1 is wound with 65 turns of litz wire 7 x 0.1 mm and tapped at 13 turn from bottom. With this loopstick antenna the radio receiver will receive Medium wave AM radio broadcasting band.
It the first stage the transistor T1 works as a high frequency amplifier and it works also as an audio amplifier (it is the reflex stage). From a detector (diodes V1, V2) the audio signal goes to the transistor T1. By using the potentiometer R1 we can adjust amplification of the receiver to the maximum, but slightly off from this point, to avoid the circuit breaks into self-oscillation, because in this case we couldn't listen AM radio stations (but we could receive CW and SSB radio stations). By adjusting the trimmer C4 we can adjust the positive feedback between the input and output of the first stage, so this feedback compensates for losses in the resonant tank circuit L1C2, it leads to an improvement of Q-factor. Adjust the trimmer C4 to get self-oscillation when potentiometer R1 is set in that position, where the control of the regeneration is very smooth.
The choke L2 is a high frequency load of the first stage, this choke is wound on the ferrite ring 10x6x4 with 100..200 turns of copper enameled wire of 0.1 mm in diameter (AWG 38).
The audio signal from the detector (V1, V2) goes to the base of the transistor T1 and this stage amplifies the signal. The resistor R3 is the audio load of this stage, because the choke L2 is the low impedance for audio frequency currents. The next stage, made of transistors T1 and T2, further amplifies the audio signal.
In this radio we can use modern silicon PNP transistors BC557 or similar, and if we change the polarity of the power supply, diodes and electrolytic capacitors then we can use NPN transistors BC547 or similar. Diodes V1, V2 - germanium GD9 or similar. High impedance headphones is the load of the audio stage.
In some cases we need to adjust the current of the last stage by matching value of the resistor R4 to get a voltage across resistor R7 equal to the half voltage (4.5V) of the power source (9V). Instead of the headphones we can use a resistor with value of 5.1k, and in this case it is possible to connect an audio amplifier to the receiver by using a 10μF capacitor.