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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Two two-transistor radio receivers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The hobby for simple transistor radios is not as developed now as it was in the 70s and 80s, but still, I think it can be interesting... For the benefit of the readers, I offer two diagrams of the simplest radio receivers operating in the medium wave (MW) range for headphones. The receiver circuit shown in Figure 1 is quite unusual. In fact, both transistors are connected as two amplifier stages with direct coupling between the stages. The bias voltage at the base of transistor VT1 is set by resistor R1, but the purpose of this bias is the loop coil L1, which, for direct current, compared to R1, has no resistance at all. But this coil is part of the magnetic antenna and the RF signal voltage of the radio station is induced in it. This voltage is supplied to the base of VT1 (since the coil is included entirely in its base circuit). What does this give? Firstly, there is no transition capacitor, and secondly, it’s more original.
The amplified RF voltage from the collector VT1 is supplied to the base of the cascade on transistor VT2. This cascade is interesting because it not only amplifys the low-frequency signal, but also detects it. This receiver works with headphones with a total resistance of at least 150 Ohms (electromagnetic headphones “Ton” or “TG”, “TK”). For a ferrite antenna, a ferrite rod with a diameter of 8 mm and a length of 40-50 mm is taken. Coil L1 contains 90 turns with a tap from the tenth. It is wound with PEV wire 0,2-0,35 turn to turn. Capacitor C1 - from the Yunost radio receiver, or any other from a small-sized receiver with AM bands. You can replace it with a large tuning capacitor like KPK-2 with a capacitance range of at least 25-150 pF. But the PDA is tuning, and after a dozen other settings it is covered wears out and becomes unusable, so with the KPK-2 we can only talk about a fixed tuning to one station. However, it is unlikely that this receiver will be able to “catch” more. Setting it up comes down to selecting the resistance of resistor R1 so that the loudest sound is obtained. The first setup to the station should be done by placing the receiver closer to the window of the room (placing it on the windowsill), especially if you are in a panel or block (reinforced concrete) house. The circuit of the second receiver is shown in Fig. 2. This receiver operates from a lower voltage power supply (1.5V). At its output, dynamic headphones from an audio player are connected (total impedance not lower than 32 Ohms), so the sound quality is better.
The oscillatory circuit here is connected at the input of the RF cascade (VT1) in the same way as in the circuit in Figure 1 - in the bias circuit of the transistor. The collector circuit VT1 includes high-frequency inductor L2. The amplified voltage from the output of the RF cascade is supplied through the isolation capacitor C3 to the cascade on diode VD1 and transistor VT2. Please note that diode VD1 is connected to the bias supply circuit to the base of transistor VT2. This offers several benefits. Firstly, there is a direct connection between the detector and the ultrasonic stage, which makes it possible to eliminate the need for a transition capacitor, and secondly, a constant voltage flows through the diode, which shifts the detection point to a region with a steeper I-V characteristic of the diode. Therefore, the detector is more sensitive, and a more affordable silicon diode can be used. This circuit uses exactly the same ferrite antenna as in the circuit in Fig. 1. Inductor L2 is wound on a ferrite ring with a diameter of 7 mm. It contains 50 turns of PEV 0,1-0,2 wire. Variable capacitor - the same as in the circuit in Figure 1. Silicon diode VD1 is used - KD521, KD522, KD503. But, if you take a germanium diode such as D9, D18, GD507, then the sensitivity of the receiver will be slightly better. In both circuits, you can use transistors KT315, KT342, KT312, KT316, KT3102 or foreign analogues. Both receivers were assembled for experimental purposes (“on cardboard”), so the boards for them were not wired. Author: R. Lyzhin
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