A simple regenerative shortwave receiver
This short wave radio receiver is intended to receive signals in range of 3.5...14 MHz with AM/CW/SSB modulation.
The circuit diagram (see the figure 1) includes a radiofrequency part (VT1, VT2, VD1, VD2) and an audio part (VT3, VT4).
Figure 1
Figure 2
The radiofrequency part consist of two RF inverted amplifiers and the detector. The first amplifier with the FET provides high input impedance that doesn't load the resonant tank L1C1. The second stage is based on a bipolar transistor VT2, the stage also inverts the signal. The potentiometer R4 and the trimming capacitor C3 form an adjustable feedback circuit.
A detector circuit is based on diodes VD1, VD2, it's a voltage doubler circuit. Two stage audio amplifier circuit based on transistors VT3 and VT4. Headphones are the load of the audio amplifier, they are connected to X1 socket.
The coil L1 is frameless, it has 20 turns of the enameled copper wire with the diameter of 1.5 mm (AWG 15). The coil diameter is 30 mm, it is tapped at the fifth turn from the cold end.
The transistor VT1 can be replaced with almost any N-Channel FET that has zero gate voltage drain current of several mA, just like 2N3819, MPF102, J310, etc. transistors. Any PNP RF BTJ transistor can be used as VT2, the gain of this stage doesn't matter, this stage should provide a current of several mA through the potentiometer R4, the current can be adjusted by matching the value of R3. Moreover, the second stage can be based on a NPN transistor (see the figure 2).
As it was mentioned earlier, the potentiometer R4 is used for the feedback control. If the wiper of R4 is in the lower position, the feedback is absent, the receiver works as a tuned radio frequency receiver, it has a very low sensitivity. Moving the wiper up, the quality factor of the resonant tank L1C1 increases, the receiver can receive weak signals, its sensitivity is increased. Moving the wiper further up, the receiver falls into oscillation, it allows receive CW and SSB (USB or LSB) signals.