Encyclopedia of radio electronics and electrical engineering / radio reception

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Direct gain receivers were very popular with radio amateurs until the 90s. Then it's not like that anymore. And yet, maybe someone will be interested in this scheme.

The receiver is built according to the direct amplification scheme. It receives radio stations in the range of 25-52 meters, blocking the main part of the HF broadcasting band. The circuit is based on only three transistors, but thanks to the adjustable PIC in the radio path, very good sensitivity and selectivity can be achieved, despite the adjustment with just one circuit.

The receiver gives the best results in areas where there are no powerful radio stations on the MW band. This is due to the fact that a powerful medium-wave radio station can significantly "clog the air" and getting rid of its influence by receiving HF signals with such a simple scheme can be very difficult.

The schematic diagram is shown in the figure.

There is no input loop. The signal from the antenna W1, which can be used as any conductor, for example, a piece of a mounting wire, through a decoupling capacitor C1 is fed to the first stage of the URF on a transistor VT1, connected according to a common base circuit. The operating point of the transistor is set by the ratio of the resistances of the resistors R2 and R3, which determine the voltage at its base. The amplified signal from the collector through the coupling coil L1 is fed to the L2-C4 circuit, which is a means of tuning the receiver to the station. The circuit uses a variable capacitor from a superheterodyne receiver. This capacitor has two sections of 6-240pF. These sections are connected in parallel. The result is a variable capacitor with a capacitance overlap of 12-480 pF. This is enough to cover the above range, but you can use a capacitor with a lower maximum capacitance, in which case the overlap will be limited to the low-frequency part of the KB range. From the RF circuit, the signal is fed to the VT2 base.

Through the coil L2, the DC bias voltage received from the divider R2-R4 also enters the base VT5. Diode VD1 included in the emitter circuit VT2 is a detector. Moreover, due to the fact that a constant emitter current VT2 flows through this diode, the detection point is shifted to a steeper section of the diode CVC.

The detected low-frequency signal is taken from the VT2 collector and fed through the volume control R7 to one stage-by-stage ULF to VT3. B1 is one earphone (headphone).

Now about PIC (positive feedback). It happens from the VT2 emitter to its base through the circuit. The signal from the emitter VT2 through R6 and C4 goes to the collector VT1, that is, to the communication coil L1. The depth of the PIC is regulated by a variable resistor R7. With this resistor, you can adjust the state of the receiver from the minimum sensitivity to the occurrence of generation. The optimal mode in terms of maximum sensitivity and selectivity is obtained at the boundary at the self-excitation threshold of the receiver.

Coils L1 and L2 are wound on a frame glued from whatman paper. This is an empty sleeve with a diameter of 20 mm and a length of 40 mm. First, the coil L2 is wound. It contains 12 turns of winding wire with a diameter of about 0,5 mm (for example, PEV 0,47). Then, on the surface of L2, you need to wind L1, with the same wire, 5 turns. Both coils are wound in the same direction. The beginning of the windings are marked in the diagram with dots.

L3 - choke wound on a ferrite ring with a diameter of 7 mm from the material 400NM, 400NN, 600HN, 600NM. It has 200 turns of thin winding wire (for example, PEV 0,12).

The receiver is powered by a 9V battery.

The receiver was made for purely experimental purposes, because it is assembled on a breadboard, and a printed circuit board was not developed for it.

The adjustment consists in setting the collector current of the transistor VT2 within 0,6-0,7 mA by selecting the resistance of the resistor R5.

In the extremely lower position of R6 according to the circuit, the circuit should switch to self-excitation, that is, to the generation mode. If this does not happen, then the L2 coil is incorrectly soldered (swap the connection points of its outputs).

On the KB band, radio stations occupy small, in percentage terms, sections of the scale, so the tuning is very sharp. On the axis of the variable capacitor, you need to put on a plastic pulley, preferably of a larger diameter, and rotate it very, very slowly. Otherwise, you will simply skip radio stations without noticing them, and you will get the impression that there is no reception. In the process of tuning, two organs work - C4 and R6, you tune the range with a capacitor, and choose the optimal mode with a resistor. The process of tuning into a radio station is complicated, but very interesting.

I was able to use this unit, using an antenna in the form of a mounting wire stretched diagonally across the room, to receive stations from North America and Western Europe, and even Australia. Of course, the reception quality is, to put it mildly, strange. Especially at the threshold of generation, but intelligibility is quite normal.

Author: A.Ivanov

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