AF amplifier for battery powered receivers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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The article describes a simple economical AF amplifier for receivers powered by two galvanic cells. The amplifier uses non-deficient elements, it is easy to manufacture and set up.

Schematic diagram of the AF amplifier is shown in the figure. The input signal from the volume control R1 is fed to the gate of the field-effect transistor VT1, which has a very high input impedance, which allows the amplifier to be used with high-impedance signal sources. The operation of a diode amplitude detector, for example, improves markedly with a high load resistance: the transmission coefficient and sensitivity increase, and distortion decreases. A very small (30 μA) drain current of the first transistor creates a voltage drop of about 2 V across the load resistance R0,5, sufficient to open the second transistor VT2, which “swings” the final stage of the amplifier.

The collector current VT2 is approximately 140 μA, and the amplitude of the amplified voltage of the AF can reach 1,5 V. This voltage is applied to the input of a composite emitter follower [1], [2], assembled on two complementary pairs of germanium transistors VT3-VT6. They amplify only the current, the amplitude of which, when the amplifier is operating on an eight-ohm load, can reach 100 mA. The output stage mode is close to the class B mode, which means that with a positive half-wave of the signal, only the upper (VT3 and VT5) shoulder of the cascade opens, and with a negative half-wave, only the lower (VT4 and VT6) opens. A small initial offset of about 0,15 V, necessary to reduce step-type distortion, is obtained due to the direct voltage drop across the diodes VD1, VD2.

The amplifier mode, and, as can be seen from the circuit, it has a direct connection between the stages, is stabilized as follows: a constant voltage of 1,5 V from the output of the amplifier, supplied through the resistor R4 to the source of the transistor of the first stage, is its bias voltage, since the gate connected for direct current with a common wire through the regulator R1, has a potential of -1,5 V with respect to the source. An accidental increase, for example, in the output voltage leads to a decrease in the drain current of the transistor VT1. Following it, the collector current VT2 decreases, the voltage on its collector decreases and causes the output voltage to return to its previous level. Thus, 100% DC FOS is obtained. The OOS coefficient for alternating current is much less due to the R3C1 circuit, which reduces the alternating component of the voltage at the source of transistor VT1 by about eight times. The voltage gain of the entire amplifier has the same value. It can be set by changing the value of the resistor R3.

The amplifier also has a positive feedback circuit (PFC). It is formed by connecting the right (according to the diagram) output of the resistor R5 not with a common wire, but with the "hot" output of the BA1 loudspeaker head. The PIC coefficient is somewhat less than one, since the voltage transfer coefficient of the output stage is less than one, so the amplifier does not self-excite. POS significantly improves the symmetry of the output voltage, i.e., reduces non-linear distortion.

The fact is that with a positive half-wave of the output voltage, the upper-side transistors of the output stage VT3 and VT5 open well, since the base current VT3 sets the opening transistor VT2. With a negative half-wave, this transistor closes, and the base current of the transistor VT4 is determined by the resistor R5, which is unprofitable to reduce the resistance due to a decrease in the efficiency of the amplifier. By connecting a resistor to the upper (according to the diagram) output of the BA1 loudspeaker head, we increase the voltage on it, and, consequently, the opening current of the VT4 transistor. Such an inclusion is sometimes called a "voltage boost" circuit.

In the AF amplifier, a field-effect transistor with a cut-off voltage of 1,5 ... 2 V is applicable. It can be selected from transistors of the KP303 series (preferably indexes A, B, I) and KP307 (A, E). It is desirable that the current gain of bipolar transistors be at least 50 ... 70.

It will be very good if the output stage transistors are selected with approximately the same gains. A pair with a lower coefficient is better to use as VT5, VT6. There are no special requirements for the rest of the details.

Dynamic head VA1 - 2GD-38 or similar with a voice coil resistance of 8 ohms. It is advisable to use heads with a high return, regardless of their size and power. It is highly recommended to place the head in a large wooden case - and the return (loudness) and sound quality will improve significantly.

Setting up the amplifier begins with checking the mode: the voltage at the junction point of the collectors of transistors VT5, VT6 should be equal to half the supply voltage, i.e. 1,5 V. It can be corrected by selecting the resistance of the resistor R2. If this cannot be done when the resistance changes within reasonable limits (say, from 10 to 27 kOhm), then you need to take the VT1 transistor with a high cutoff voltage, this will allow you to get a higher output voltage. Then, by including a milliammeter in the power circuit and selecting the number and type of diodes VD1, VD2 connected in parallel, the quiescent current of the amplifier is set equal to 1 ... 1,5 mA. You can not turn off all the diodes at once, because the amplifier current will increase to an unacceptably large value. Any low-power germanium diodes are suitable, for example, D2, D9, D18, D311, GD507, etc. In conclusion, by giving an audio signal to the input of the amplifier and observing the voltage on the dynamic head with an oscilloscope, they make sure that there are no "step" type distortions and the symmetry of limiting the half-waves of the output voltage at its large amplitudes.

The amplifier made by the author had the following parameters: supply voltage - 3 V, quiescent current - 1,3 mA, current at maximum signal - 30 mA, maximum undistorted signal power at a load of 8 ohms - 25 mW, reproducible frequency band - 70 .. .10 Hz.

If it is necessary to expand the band towards lower frequencies, it is necessary to increase the capacitance of capacitors C1 and C3. You can limit the band from the high frequencies by connecting a capacitor with a capacitance of 2 ... 150 pF between the collector and the base of the transistor VT300.

Literature

1. Polyakov V. Autodyne synchronous receiver. - Radio, 1994, No. 3, p. 11-13.
2. Polyakov V. Transistor loudspeaker. - Radio, 1994, No. 8, p. 23-26.

Author: V. Timofeev, Moscow

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