Three transistor radio with high sensitivity
The receiver, described in this article, is designed for the Medium Wave band. It has very good sensitivity and smooth regeneration control. The circuit diagram of the receiver is shown in Figure 1. This is the regenerative circuit, the capacitor C5 and the potentiometer R1 are used to adjust the regeneration (the coarse and fine adjustment).
This receiver uses a loop antenna (L1). The sensitivity of the receiver is totally depends on the quality of the loop antenna. To create the loop antenna, take a piece of cardboard and make a frame measuring 5.5 inches by 7.5 inches (140x190 mm). The copper insulated wire (AWG 23 or 0.55 mm in diameter) is wound around the frame in one layer. The coil L1 has 23 turns tapped at 8 turn.
The dimensions of the receiver depends on the loop antenna size. The receiver can be built more compact, but the size of the loop antenna has to be not less than 4x5 inches (100x125 mm).
Figure. 1.
T1 - 2N135; T2 - CK722; T3 - 2N107;
R1, R6 - a potentiometer of 15K; R2 - 1.5K; R3 - 100K; R4 - 10K; R5 - 220K;
C1 - 5..365 pF; C2 - 5..50pF; C3 - 0.1 μF; C4, C5 - 1 μF; C6 - add if it needs and match the value;
L1 - a coil with 23 turns tapped at 8 turn, wound on a frame of insulated material 5.5x7.5 inches with a wire of 0.55 mm in diameter (AWG 23);
L2 - 2.5..5 mH;
Supply voltage: a battery - 3 V or 1.5 V;
High impedance headphones
The PCB used in this design has the same dimensions as the loop antenna. Therefore, the density of components placed on the PCB is very low. The inductance of the coil L2 can be 2.5...5 mH. The higher inductance, the higher sensitivity, but in the same time it increases a tendency to self-oscillation of the circuit. Because of this, it is better to place both coils L1 and L2 on opposite sides of the PCB, and arranged them to keep their axis perpendicular.
The circuit can be powered from 1.5 V battery, but it is better to use 3 V supply (two 1.5 V battery). If used 6 V power supply, the circuit will provide better sensitivity and higher output level, but in the same time the circuit will have a tendency to self-oscillation.
The transistor T1 is a germanium PNP 2N135, it can be replaced with a modern silicon device, just like 2N2907 or similar. Probably it takes to match the resistor R2.
The receiver is intended to work with high impedance headphones (≥ 2000 Ω). In case of use a modern headphones the audio output will be not so loud. So it may be better to replace the audio amplifier (T2, T3) with an amplifier based on integrated circuit LM386, but it requires higher supply voltage (5 V and more).
This circuit was described in Radio-Electronics magazine for December of 1960 year.