Amplifier with matching transformer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Amplifier with matching transformer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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A feature of the device is the use of a matching transformer made using a special technology to connect the pre-terminal cascade with the final one. This made it possible to obtain high stability of the amplifier operation with a small number of parts, and eliminated the need to protect the final stage from a short circuit in the load.

In the final stage of the amplifier, transistors with a relatively small (equal to the supply voltage) limit voltage between the emitter and collector can be used.

Amplifier with matching transformer

The main parameters:

Rated frequency range (with a decrease in the amplitude-frequency characteristic at the edges of no more than 3 dB), Hz.......5...100
Rated output power at a load with a resistance of 4 ohms (with a harmonic coefficient of not more than 1%), W.......20
Rated input voltage, mV.......100
Input impedance, kOhm.......45
Relative level of noise and background, dB.......-65

The amplifier contains a two-stage signal voltage amplifier on transistors V1, V3, V4 and an end stage on transistors of the same structure V5, V6. The load - loudspeaker B1 - is included in the diagonal of the bridge formed by transistors V5, V6 and capacitors C7, C8, which made it possible to improve the amplitude-frequency response (AFC) in the low-frequency region.

The amplifier is covered by a common OOS, the voltage of which is removed from the load and fed into the emitter circuit of the transistor V1. Another OOS - local - is created through the R2R6 * circuit connecting the output of the pre-terminal stage to the input of the first. The variable component of the current closes through the C2R1C1 circuit. At the same time, the capacitor C2 and the resistor R1 of this circuit, together with the resistor R2, provide compensation for ripples with a frequency of 100 Hz that penetrate the amplifier input through the R8C6 circuit when powered by an unstabilized voltage. Trimmer resistor R1 and capacitor C1 form a decoupling filter in the power supply circuit of the preamplifier. If another source is used to power the preamplifier, capacitor C1 must be shunted with a 3 kΩ resistor.

In the pre-terminal and final stages of the amplifier, it is necessary to use transistors with similar static current transfer coefficients h21e in pairs. Transistors V5 and V6 must be mounted on heat sinks with a cooling surface area of about 400 cm2. The thermistor R13 should be glued to the body of the first of them, R14 - to the body of the second.

For a matching transformer T1, any magnetic circuit with a cross section of 1,5 ... 2,5 cm2, for example, Ш12 X 16, is suitable. The winding is wound with a bundle of eight wires folded together: six - PEV-2 - 0,17 and two - PEV-2 -0,31. After winding, the ends of the wires are tinned, each of the PEV-2 - 0,17 wires is detected with an ohmmeter and, connecting them in series, winding I is obtained. The remaining two wires PEV-2 - 0,31 are used as windings II and III.

The power transformer T2 is wound on a USh20 x 30 magnetic circuit. Winding I contains 1600 turns of wire PEV-2 - 0,41, windings II and III - 185 turns of wire PEV-2 - 0,9 each (winding in two wires).

The adjustment begins with setting the transistor modes for direct current. Turning the trimmer resistor R1 to the extreme left (according to the diagram) position and breaking the OOS circuit at point L, by selecting resistor R6 *, a voltage equal to half the supply voltage is achieved at point B. Then, by selecting resistors R9* and R11*, the same voltage is set at point B. By selecting the same resistors, the required (450 ... 500 mA) quiescent current of the terminal stage transistors is achieved.

After that, a voltage of 3..10 mV with a frequency of 20 kHz is applied from the signal generator to the input of the amplifier, and the shape of the output voltage is checked using an oscilloscope. Its distortion in the form of a "step" is eliminated by increasing the quiescent current of transistors V3, V4. The easiest way to do this is to temporarily replace the resistor R3 with a variable (resistance of 1 kOhm). By increasing its resistance until distortion disappears, it is necessary to ensure that the quiescent current of transistors V3, V4 (it is convenient to control it by the voltage drop across the resistor R7) does not exceed 10 ... 12 mA. The symmetry of limiting a signal that exceeds the nominal level is achieved by selecting resistors R10 *, R12 *.

After that, the general OOS circuit is restored and, using the trimmer resistor R8, its depth is set to 20 dB. By reducing the input signal to zero, the trimming resistor R1 achieves a minimum of ripples with a frequency of 100 Hz at the load. This operation is performed with the pre-amplifier connected to the power circuit. To obtain small non-linear distortions, the output impedance of this amplifier should not go beyond 1 ... 5 kOhm. A minimum of distortion at the highest frequencies of the audio range is achieved by selecting a capacitor C5 *.

See other articles Section Transistor power amplifiers.

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