Encyclopedia of radio electronics and electrical engineering / radio reception

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This name fully justifies the proposed design, since its sound volume is much higher compared to conventional detector receivers. With a good outdoor antenna and grounding, nearby powerful radio stations can be listened to even on a dynamic head.

The receiver uses the so-called key transistor detector - a full-wave detector made on transistors (Fig. 1). It is easy to see that the circuit of this detector resembles the circuit of a voltage converter used "on the contrary" - a high-frequency signal is fed to the output of the converter, and the detected voltage is taken from its input.

Fig. 1

The key transistor detector has a number of advantages over a conventional full-wave diode detector. First, its amplitude characteristic is more linear (curve 1 in Fig. 2) than that of a conventional detector (curve 3) and has a steep slope. When the transistors are turned on inversely (the emitter and collector terminals in Fig. 1 are reversed), a further increase in the steepness of the detector characteristic is observed, an increase in its linearity (curve 2 in Fig. 2). These advantages of a switching transistor detector are explained by the fact that the resistance of the open collector junction of the transistor is less than the forward resistance of the diode at the same values ​​of the input signal.

Fig. 2

Shown in fig. 2, the characteristics were taken at an input signal with a frequency of 200 kHz for GT108G transistors with a static current transfer coefficient of about 200. At lower values ​​of h21E (80 ... 150), the slope of the characteristic practically does not change, and at large values ​​it decreases somewhat. The characteristic of the diode detector was taken for a full-wave rectifier circuit with a midpoint on D9B diodes. For both detectors, the parameters of the transformer were the same: winding I - 75 turns of wire PEV-2 0,13, winding II - 90 turns with a tap from the middle of the wire PEV-2 0,11. windings III and IV (only for the key detector) - 15 turns of PEV-2 0,11 each. The windings are placed on two rings of size K7x4x2 folded together made of ferrite 600NN

On fig. 3 shows a practical circuit of the receiver using a key transistor detector. Such a receiver has been working for the authors (60 km from Moscow) for more than two years. With an outdoor antenna 20 m long and about 18 m high, the transmission of the first program (at a frequency of 173 kHz) is audible at a distance of several meters from the receiver.

Fig. 3

The oscillatory circuit of the receiver is made up of a winding Ia or Ib and a variable capacitor C1. When the switch S1 is in the position shown in the diagram, the receiver covers the frequency range 140...880 kHz (with various combinations of positions of the capacitor rotor and switch S2 engine). If switch S1 is set to position "II", the receiver operates in the range of 270...1600 kHz.

An output transformer T2 is connected to the output of the detector, the secondary winding of which is loaded on the dynamic head B1. When receiving low-power or remote radio stations, high-impedance headphones (TON-3, TON-1) are connected to the X2 connector. An external antenna is connected to socket X1, and a ground wire is connected to socket X2. The details of the receiver are placed in the housing of the broadcast loudspeaker "Orbita" (Fig. 4).

Fig. 4

Transformer T1 is made on a ferrite rod from the magnetic antenna of the Alpinist-405 radio receiver (Fig. 5). Winding Ia contains 125 turns of wire PEV-2 0,18 with taps from the 20th and 48th counting from pin 1; winding Ib - 36 turns of wire PEV-2 0,35 with taps from the 7th, 17th, 19th turns, counting from output 4. Winding II is placed evenly in two sections of the frame and contains 30 turns of wire PEV-2 0,31 ,10 with a tap from the middle. The base windings (III and IV) are wound on the corresponding halves of the winding II - each of them contains 2 turns of wire PEV-0,44 XNUMX.

Fig. 5

The trick of the transformer is made on the magnetic circuit Ш8х10 from permalloy. Winding I contains 1650 turns of PEV-2 0,1 wire, winding II - 165 turns of PEV-2 0,59. You can also use another small-sized output transformer with the appropriate transformation ratio. The dynamic head, in addition to that indicated in the diagram, can be 4GD-8E, 4GD-35. Transistors GT108G, GT109V work in the receiver. GT109G, GT115V - GT115D and other germanium transistors of any structure with a current transfer coefficient of 100 ... 200. Capacitor C1 - one section of the block of capacitors from the VEF-202 receiver. Switch S1 - sliding, from the Sokol receiver, 52 - biscuit with 11 positions (for example, 11P1N).

When setting up the receiver, it may be necessary to change the number of turns of the windings Ia and Ib of the transformer T1, depending on the antenna used, and also more accurately select their position on the ferrite rod to obtain the highest sensitivity with sufficiently good selectivity. If sound distortion is observed when receiving powerful radio stations, the capacitance of capacitor C2 should be reduced to 3000 ... 2000 pF.

Such a transistor detector can, of course, be used both in the simplest direct-amplification transistor receivers and in superheterodynes.

Authors: M. Balashov, V. Belyakov, Moscow

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