Direct gain HF receiver. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Direct gain HF receiver. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Direct amplification receivers are not very sensitive and selective, therefore, direct amplification schemes are very rarely used for reception in HF broadcasting bands. Usually a direct amplification receiver is a simple means for receiving local radio stations on medium or long waves.

And yet, there are ways to increase the sensitivity and selectivity of the direct amplification receiver, one of which is the introduction of adjustable positive feedback in the RF amplifier. The method consists in that the level of positive feedback (POF) in the RF amplification stage is set so that it is on the verge of excitation. By operating the input circuit settings and the POS depth adjustment, it is possible to achieve sensitivity and selectivity sufficient for confident reception of remote radio stations in the KB broadcasting bands.

Receivers made according to this scheme were widespread before the Great Patriotic War. Their widespread use was the result of a significantly lower price than a superheterodyne receiver of similar sensitivity and selectivity. The main drawback is the need to carefully set the POS depth for receiving each specific radio station.

A schematic diagram of a receiving path of this type, made on a more modern element base, is shown in the figure.

Direct Gain HF Receiver

The first amplifying stage is made on a field-effect transistor VT1. The input signal is selected by the L1-C2-C3-C4 circuit and fed to its gate. The transistor is connected according to the scheme with a common drain. Positive feedback is carried out from the gate to the source through the capacitor C5. The depth of the POS is set by a variable resistor RP1, which regulates the degree of connection through C5 between the gate and source VT1. Capacitor C7 reduces the likelihood of uncontrolled self-excitation.

The second amplification stage is made on the VT2 transistor, according to the scheme with a common emitter. C9 is necessary to reduce the tendency of the cascade to self-excite.

The third stage on the transistor VT3 performs the functions of the third stage of the URF and the detector. The detected signal is filtered by capacitor C11. Resistor RP2 serves as a volume control. From it, a signal can be applied to the input of any ULF.

The receiver covers two broadcasting KB bands - 41 and 49 meters.

As a loop coil L1, a ready-made high-frequency choke with an inductance of 5 μH is used. The coil does not adjust. Variable capacitor C4 - one section of a variable capacitor 2x10 ... 495 pF with an air dielectric. Such capacitors were used in tube radios and receivers. If there is no such thing, any other from the receiver will do. But it is desirable, nevertheless, with an air dielectric. If it is a popular capacitor with a capacity of 2x5 ... 260pf, you need to connect both of its sections in parallel. Or change C3 accordingly.

Antenna - a piece of mounting wire about two meters long.

KT315 transistors can be replaced with KT3102, SST310 transistor with J310, T310 or KP302, KP303.

Experiments on the reception of KB were carried out in rural areas (the region of Lake Ladoga). At night, it was possible to receive many long-distance HF broadcasting stations.

Author: A.Ivanov

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According to this scheme, you can also make an all-wave receiver, you just need to connect the switch and coils of medium and long waves, as well as the VHF unit from an unusable transistor receiver. In VHF mode, it will be superheterodyne.

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