Radio receiver with field effect transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radio receiver with field effect transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Numerous publications in the journal "Radio" on simple detector receivers and direct amplification receivers indicate that the interest of radio amateurs in this technique does not fade away. Even though VHF FM is now victorious, simple DW-MW AM receivers can also find their place, especially for long-range reception at a later time. The author of the article decided not to stand aside and offers her own version of a direct amplification receiver.

The radio receiver is only on five transistors, but it has a very high sensitivity and high sound volume, developing a power of 0,5 W on an 8 ohm head. The receiver has a threshold AGC system.

This experimental loudspeaker receiver is intended mainly for the modernization of single-program loudspeakers or for experiments at home, at school or radio circles, since its mock-up can be made in a short time.

The signal of the radio stations of the LW or MW range is received on the magnetic antenna WA1 (Fig. 1), is selected by a selective circuit consisting of a coil L1 and a variable capacitor C1, and through the capacitor C2 enters the first stage of the URF, assembled on a field-effect transistor VT1. This stage has a very high input impedance and hardly shunts the oscillatory circuit, which makes it possible to do without a coupling coil. The field-effect transistor amplifies the signal well in terms of power, but not enough in terms of voltage, so the second stage is made on a bipolar transistor with a high current transfer coefficient. This makes it possible to obtain the range of the signal amplitude at the detector input in fractions of a volt.

(click to enlarge)

The detector is assembled on high-frequency germanium diodes VD1, VD2. Capacitors C7, C8 filter the low-frequency voltage from high-frequency signal ripples. A threshold AGC system is made on the VD3 diode and the R6C5 chain. At a high signal level, the VD3 diode opens and the negative voltage, supplied to the VT1 gate through the R2 resistor, closes this transistor, as a result of which the cascade gain decreases.

The low-frequency voltage from the volume control R8 is supplied to the UZCH, assembled on transistors VT3-VT5. The use of a field effect transistor in the input stage made it possible to obtain an amplifier with a high input impedance and a large gain in power and voltage with a very simple circuit. In order for the ultrasonic frequency converter to work on a low-resistance dynamic head, an emitter follower on transistors VT4, VT5 is needed. A small positive bias voltage is applied to the VT3 gate from a tuned resistor R12. Resistor R9 sets the quiescent current of transistors VT4, VT5. Germanium diodes VD4, VD5, fixed with springs on the cases VT4, VT5, are necessary for temperature stabilization of this current.

To improve the selectivity of the receiver and further increase the sensitivity, you can introduce a small positive feedback, as shown in Fig. 2.

FET radio receiver

The feedback coil L2 contains 2 turns of a thin mounting wire wound on the rod of the magnetic antenna WA1. The polarity of the L2 connection is determined when setting to increase the receive volume. Capacitor C * is installed if it is necessary to reduce the excessive increase in the quality factor of the antenna in the high-frequency part of the range.

The receiver uses MLT-0,125 resistors, tuning resistors - SPZ-386. Variable resistor R8 - any small-sized, with a resistance of 33 ... 100 kOhm, better with a power switch. Non-polar capacitors used KM-5, K73-9, K10-17. KPI C1 - any type with a maximum capacitance of 240 ... 500 pF, preferably with a backlash-free vernier and better with an air dielectric, since it does not create cod during restructuring. Oxide capacitors - K50-16, K50-35. Diodes VD1, VD2 can be replaced by 1D507A, D18, D20, VD3 - any of the silicon KD503, KD510, KD521. VD4, VD5 - any of the D2, D9, GD507 series. Transistor VT1 is desirable to choose a low cutoff voltage for good AGC performance. VT2 - any of the KT3102, KT342, KT315 series, preferably with a current transfer ratio of at least 150. VT3 - any of the KP501 series. VT4, VT5 - any of the indicated series. Dynamic head for a power of 1 ... 4 W with a resistance of 8 ... 10 Ohms, for example, 1GD-36 or similar. For the antenna, a 400NN ferrite rod with a diameter of 8 or 10 mm and a length of 18 ... 25 cm is suitable. The L1 coil is wound on a cardboard frame. For the MW range it contains 55 turns, for the LW range 150-200 turns. It is desirable to wind the coil with LESHO 7x0,07 or LESHO 21x0,07 litsentrat.

The receiver can be mounted on the printed circuit board shown in fig. 3. So that it is not excited, the RF cascades and the detector must be closed with a metal screen made of thin brass or tin connected to a common wire. After that, it is permissible to reduce the capacitance of the capacitor C2 to 30 pF. Initially, it is taken larger for shorting to a common wire through L1 interference from the network. Resistor R12 sets the voltage at the emitters VT4, VT5, equal to half the supply voltage. Resistor R9 regulates the quiescent current of these transistors (10...12 mA).

FET radio receiver

The boundaries of the receiver ranges are set by selecting the number of turns of the L1 coil and moving it along the rod. By selecting resistors R1 and R3, the voltage at the drain VT1 and the collector VT2 is set to about 4 V with the coil L1 closed.

The receiver can be made dual-band (LW and SV) if you wind two L1 coils and switch them, for example, with the P2K button.

The receiver is powered by a stabilized network unit with a voltage of 9 V. In the evening, on the MW band, it very loudly receives many foreign radio stations that are several hundred and even thousands of kilometers away from the reception point.

Author: Zh.Mikheeva, village Ivanishchevo, Yaroslavl region

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