Operational amplifier receiver. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Operational amplifier 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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In the amateur radio literature, there were many descriptions of various direct amplification receivers, which are distinguished by the simplicity of the circuit diagram, on the one hand, and quite satisfactory technical characteristics, on the other. The use of an operational amplifier in the high-frequency part of the receiver makes it possible to reduce its size and thereby somewhat complicate the power circuit. As a rule, in receivers of this type, the Krona battery is used as a power source. However, these batteries have a low capacity and are in short supply. At the same time, rechargeable batteries 7D-0, 1 are widely used. The use of a rechargeable battery removes the problem of frequent replacement of the power source, but forces the use of a charger, usually made in the form of a separate device.

In view of the foregoing, a small-sized direct-amplification radio receiver was designed, manufactured and tested, designed to receive powerful radio stations in the medium wave range, which showed good results. A feature of this receiver is the use of one microcircuit as a high-frequency amplifier and detector - the operational amplifier K140UD1A, as well as placement in the same housing with the receiver of the charger for the battery type 7D-0, 1. The charger is connected to the mains without removing the battery from housing using the built-in mains plug.

The circuit diagram of the receiver is shown in fig. one.

Radio signals are received using a magnetic antenna in the form of a ferrite rod with two windings placed on it - a contour coil L1 and a communication coil L2. The receiver operates at a fixed frequency of the received signal, which is determined by the inductance of the circuit and the total capacitance of the constant capacitance capacitors C1 and C2 and trimmer capacitor C3. Capacitors C1 and C2 are selected with opposite temperature coefficients of capacitance TKE, which increases the reception stability. The specific values of their capacities are determined by selection when tuning the receiver to the desired radio station. The tuned capacitor C3 serves to compensate for a slight detuning of the circuit under the influence of various factors, for example, due to the aging of the magnetic antenna core. The high-frequency signal, isolated by the L2C1C2C3 circuit through the coupling coil L1, is fed through the limiting resistor R1 to the input of the operational amplifier between its inverting and non-inverting inputs.

The detection of the radio signal occurs due to the nonlinearity of the current-voltage characteristics of the transistors that make up the microcircuit. The low-frequency component of the detected signal is isolated on the load resistor R2, which acts as a volume control, and the high-frequency component is closed to a common wire by capacitor C6. Capacitors C5 and C4 prevent self-excitation of the operational amplifier at high frequencies. With a unipolar supply, the optimal mode of the microcircuit is achieved by connecting pin 4 to the inverting input. In this case, the output potential of the operational amplifier is equal to half the voltage of the power supply. From the volume control, through the coupling capacitor C7, the signal is fed to the input of a transformerless audio frequency amplifier, which is assembled on three transistors. Transistor VT1 operates in the pre-amplifier stage and is connected according to a common emitter circuit. Its load in the collector circuit is the resistor R4, from which the amplified signal is fed to the input of the output stage. The output stage of the audio frequency amplifier is assembled according to a push-pull transformerless circuit using transistors of different conductivity VT2 and VT3.

To prevent the appearance of "step" type distortions, a slight bias is created between the bases of the output transistors by turning on the diodes VD1 and VD2 in the forward direction. The load of the output stage is a dynamic head VA1 of type 0,25GD-19, connected through a separating capacitor C9. To improve the operation of the amplifier and stabilize its mode, negative voltage feedback was introduced into its circuit through resistor R5 from the amplifier output to the base of the input transistor. Capacitor C8 prevents self-excitation of the amplifier at high audio frequencies. The charger is a full-wave rectifier, assembled in a bridge circuit on VD3-VD6 diodes, powered directly from a 220 V AC mains.

The voltage to the rectifier is supplied through a current limiter, consisting of capacitor C12 and resistor R6. Resistor R7 ensures the discharge of capacitor C12 when the charger is disconnected from the mains. The operating voltage of the capacitor C12 must be at least 400 V. The battery is constantly connected to the charger. Capacitors G10 and SP serve to reduce the output impedance of the power supply at audio and radio frequencies. The Q1 receiver's power switch is integrated with the volume control. The receiver is assembled from widely used radio components, which are mounted on a common board. For the magnetic antenna, a ferrite rod with a diameter of 8 mm and a length of 110 mm made of M400NN ferrite is used. Loop coil L2 contains 70 turns of LESHO brand LESHO 7X0,07, coupling coil L1 - 5 turns of PEV wire with a diameter of 0,12 mm. Both coils are wound turn to turn on cuffs glued from thin paper with BF glue. The coils should move with little effort along the ferrite rod. Fixed resistors are of the MLT type, capacitors C1 and C2 are of the KT type: one is gray and the other is blue, or one is blue and the other is blue with a red dot. Electrolytic capacitors - type K50-35, C12 type BMT-2, other capacitors type KM.

The body of the radio receiver is glued from separate parts made of colored plexiglass 4 mm thick. It has the form of a case with a removable back wall, which moves in the grooves milled in the upper and lower walls of the case before gluing. The dynamic head is mounted on short M3 studs, fused in a heated state into the front panel for half its thickness from the inside, with nuts. The mains plug consists of two brass bushings with an internal thread. The bushings are hot-fused into the holes of the bottom wall of the housing flush with it. Two threaded brass pins are stored in the receiver's battery compartment and are screwed into the threads of the bushings when the battery is being charged. The board with the circuit elements rests on the support ledges, made of pieces of Plexiglas and glued to the corners of the case.

The board is pressed against the stops by a removable rear wall. The receiver is easy to set up. If it is assembled from serviceable parts and without errors, it starts working immediately after power is applied. If, after switching on, a loud whistle is heard from the speaker, it is necessary to swap the conclusions of the communication coil and select the optimal distance between it and the contour coil. By moving both coils along the rod, they achieve maximum volume and fix the coils with a drop of melted wax or paraffin.

The coils are installed in the position of the volume control corresponding to the maximum. The most difficult operation is to tune the input circuit to the selected radio station. To do this, the tuning capacitor C3 is set to the middle position, the capacitors C1 and C2 are turned off, and a variable capacitor is connected instead. By rotating its rotor, the receiver is tuned to the radio station and the capacitance is approximately estimated from the angle of rotation of the rotor. Dividing it in half, determine the capacitances of capacitors C1 and C2. After their installation, the final adjustment is made with a tuned capacitor C3. To charge the battery, slightly open the back wall, remove the pins of the mains plug, screw them into the bushings and connect the receiver to the mains socket. Charging the battery according to his passport should last 15 hours. After that, the receiver is disconnected from the mains, the pins are unscrewed from the bushings and removed into the battery compartment.

Author: V.Bykov

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