Radio 1991, 10
This regenerative receiver (see figure) is based on the well known two-transistor emitter-coupled oscillator. With an antenna WA1, connected to the tunable resonant tank L1C3 this receiver can receive, detect and amplify signals of radio stations. The resistor R4 controls the oscillation mode of the oscillator. The audio signal is developed across the load resistor R2. A stage based on low-noise FET transistor VT3 is an audio amplifier.
VT1, VT2 - GT309B (germanium transistors, forward current transfer ratio hFE = 60...180, transition frequency ft = 120MHz);
VT3 - KP303B (Ucutoff = 0.5...3.0 V at Uds=10 V and Id=0.01 mA);
When the resistor R4 is adjusted to the point just before oscillating starts, the device can be used as a regenerative detector for reception of AM radio stations working in LW, MW and SW bands. If the regeneration is adjusted just past the oscillation point, the device becomes a direct conversion receiver that receives CW and SSB signals. If setup the resonant tank L1C3 to the VHF band, it is possible to receive an FM radio station because the oscillator frequency will be locked by the frequency of the FM signal. In this case it's better to connect the antenna to the emitters of the transistors VT1, VT2.
This circuit can be used in the IF section or as an FM demodulator in a radio receiver. For this purpose wind around an IF coil of the mixer 3..4 turns of wire, one end of the wire connected to ground, and the second end connect through a resistor of 75 ohms to the input of the universal detector (but remove components WA1, C1, L1C3). In this configuration the IF resonant tank of the receiver will works as resonant tank L1C3 of the oscillator.
The proposed method of detection allows to design various detecting devices by using of a variety of audio amplifiers, changing the way of connection the resonant circuit, reducing the supply voltage down to 1.5 V and so on.