UZCH for the receiver with low-voltage power supply. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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UZCH for the receiver with low-voltage power supply. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Receivers with low-voltage power supply are gaining the attention of radio listeners with high efficiency and convenience, they are small in size and weight, power costs are minimized (especially if small batteries are used). But the downside of these advantages was the limitation of the number of listeners to one due to the difficulty of implementing "loud-speaking" playback. However, this drawback is completely overcome - get acquainted with the solution that the author of the article proposes.

Over the previous years, many descriptions of radio receivers operating at a supply voltage of 1,2 ... 1,5 Viv were published in the Radio magazine and other publications, mainly for head phones. At present, with the development of semiconductor manufacturing technology, it is possible to make these receivers loud-speaking. The literature describes UCH microcircuits operating at a supply voltage of 1,0 ... Unfortunately, such a microcircuit could not be found for sale. However, the use of discrete elements has some advantages in the form of the possibility of selecting parts and adjusting to optimize the operation of the device.

The operation of ultrasonic frequency converters, the supply voltage of which is 1,0 ... 1,5 V, has its own characteristics: a small dynamic range of voltage gain, a high level of non-linear distortion, the difficulty of stabilizing the quiescent current and the voltage of the midpoint of the output stage, reducing the amplifying properties of bipolar transistors at low collector voltages.

The diagram of a three-stage full-wave ultrasonic frequency converter on seven transistors is shown in the figure. It is operational at a supply voltage of 0,7 to 3,2 V at a quiescent current of 7 ... 10 mA. With a supply voltage of 2,8, 1,5 and 1,0 V, the maximum output power is 110, 40 and 12 mW, respectively, when operating on an 8 ohm sound head.

UHF for low voltage receiver

The input stage is made on the transistor VT2, the signal is fed to its base through the capacitor C1. The load of this cascade is the resistor R3 and the emitter junctions of the transistors VT3 and VT4 before a little cascade. From the transistor VT3, the amplified signal is fed to the base of the transistor VT6 - the upper arm of the output stage. The load VT4 is resistor R6. Through the coupling capacitor C3, the amplified signal is fed to the gate of the transistor VT5 and then to the base VT7. These two transistors form the lower arm of the output stage. Capacitors C2, C4, C5 prevent self-excitation of U3CH at high frequency.

Field effect transistor VT1 and resistors R1, R2, R7, R9 form a negative feedback circuit that regulates the base current VT2. A tuned resistor R9 sets the voltage of the midpoint of the output stage by changing the voltage at the gate VT1. Resistor R1 reduces the depth of the OOS in order to avoid "overshooting" the voltage of the midpoint of the output stage, its resistance depends on the steepness of the characteristic VT1 and is selected during tuning.

The ultrasonic frequency converter uses separate adjustment of the quiescent current of the upper and lower arms of the output stage. The quiescent current of the transistor VT7 is stabilized by the field effect transistor VT5. The VT6 quiescent current is set automatically when the midpoint voltage is held by the action of the direct current feedback, which covers all stages of the ultrasonic frequency converter, with the exception of VT5 and VT7.

Details and settings. Fixed resistors - MLT-0,125, R9 - SP4-3 Capacitors C6 and C7 - K50-38, the rest - ceramic KM6 or imported. Transistors VT1, VT5 - KP303A, KP303Zh, VT2 - VT4 - KT3102A, KT3102B with a base current transfer coefficient of 150 ... 200; VT6 - KT681 A; VT7 - KT680A with a base current transfer ratio of 150 ... 200. It is permissible if the current gain of the transistor VT3 is greater than that of VT4, and VT6 is greater than y VT7.

The assembly of the UZCH must begin with the selection of a pair of transistors VT5 and VT7 - at a voltage of 1 V, the collector current VT7 must be within 6 ... 10 mA. Then solder all the parts except R1.

The UZCH is tuned at a supply voltage of 1,0 ... 1,5 V. A trimming resistor R9 is used to set a voltage equal to half the supply voltage on the collectors VT6 and VT7. Then the supply voltage is reduced or increased, while the midpoint voltage relative to the supply minus will change in the same direction, but to a greater extent than necessary. The resistance of the resistor R1 is selected so that at a supply voltage of 0,8 ... 1,6 V, the voltage on the collectors VT6 and VT7, equal to half the supply voltage, would be maintained with an accuracy of 0,05 V - the level of nonlinear distortion of the ultrasonic frequency depends on this, especially at its low value (0,8 ... 1,2 V).

Then an audio frequency signal is fed to the input of the amplifier, and capacitance C2 is selected during self-excitation. If step-type distortions are found and it is not possible to replace transistors VT5 or VT7, you need to connect resistor R5, as shown in the diagram. At the same time, the quiescent current of VT7 and the entire UZCH will increase. The resistance of the resistor R5 must be chosen the largest at which distortion will stop.

If the ultrasonic frequency converter is supposed to operate at a supply voltage of 1,8 ... 3,2 V, the adjustment is made in this voltage range. With an increase in the supply voltage from 2,4 to 3,2 V due to a change in the modes of transistors VT1 - VT3, the midpoint voltage begins to lag by 0,15 ... 0,2 V, which is not of great importance. It is enough to accurately set the voltage on the collectors VT6 and VT7 at the lowest supply voltage.

Author: A. Panshin, Moscow

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