High-quality class B amplifier. Diagram, description

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High quality class B amplifier. 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 the so-called direct connection to reduce non-linear distortion.

The main parameters:

Rated frequency range, Hz.......20...20 000
Rated output power (in the rated frequency range), W: at a load with a resistance of 8 ohms with a harmonic coefficient of not more than ....... 0,02% 30
on a load with a resistance of 4 ohms.......40
Sensitivity, mV......500
Input impedance, kOhm.......20
Relative level of noise and background, dB.......-75

The amplifier contains a four-stage preamplifier operating in mode A (microcircuit A1 and transistors V3-V6, V9), an output stage operating in mode B (transistors V12, V15, V16), and an output stage protection device against overloads and short circuits in the load (V7, V13).

The first stage of the pre-amplifier is assembled on the op-amp A1, the second - on the transistor V4 with a dynamic load (V3) in the collector circuit, the third - on the transistor V9. A double emitter follower on transistors V5, V6 matches the input impedance of the cascade on transistor V9 with the output impedance of the cascade on transistor V4. The gain of the first stage at frequencies above 20 Hz is approximately equal to 15. Due to the deep DC feedback, a zero potential is maintained through the resistor R3 at the output of amplifier 34. The use of mode B in the output stage made it possible to increase its efficiency and completely eliminate the need for thermal stabilization of the quiescent current.

High quality class B amplifier

To reduce the nonlinear distortions inherent in mode B, elements C6 *, R15, R29, L1 are introduced into the amplifier, forming a balanced bridge, one of the diagonals of which is connected to the output stage, and the other - the load (loudspeaker). Simplified, the distortion compensation mechanism is that the distortion voltage that occurs in the output stage cannot cause a distortion signal if the bridge is balanced. Thanks to resistor R29, non-linear products in the output current spectrum are compensated by a "correcting" current flowing through this resistor directly into the load. In this case, higher harmonics are most effectively compensated (the resistance of the resistor R29 is chosen so that the indicated currents at these frequencies are the same in value and opposite in direction).

Due to the presence of an active component in the impedance of the L1 coil, the balance of the bridge at low frequencies is disturbed. A low level of non-linear distortion at these frequencies is provided mainly by a deep (50 ... 70 dB) OOS, the voltage of which is supplied from the amplifier output to the emitter circuit of the V4 transistor (through the divider R15R12).

The protection device limits the current through the transistors V12, V16 at 3,5 A in case of overload by the amplified signal and at 1,5 A in case of a short circuit in the load. In the upper (according to the circuit) arm of the amplifier, transistor V7 performs these functions. In the absence of a signal, a constant current flows through resistors R26, R20, R18 and R21, creating a voltage drop of about 0,45 V across the first two of them.

When a signal appears that grows in a positive direction, the voltage drop across resistor R20 decreases, and across resistor R26 (it is mainly due to the collector current of transistor V12) it increases. At the moment when the total voltage across the resistors R26 and R20 reaches a value of 0,65-0,7 V, the transistor V7 opens and the emitter-collector section shunts the resistor R16, thereby limiting the increase in the collector current of the transistor V12. Diode V8 prevents the protection device from tripping due to an increase in the voltage drop across resistor R20 at times when the output voltage changes in the negative direction.

Similarly (when the polarity of the amplified signal is changed), the protection device for the lower (according to the circuit) arm of the amplifier also works. Elements R30, L2, C11, R31, C10, C12 prevent self-excitation of the amplifier.

Instead of those indicated in the diagram in the amplifier, you can use operational amplifiers K140UD8B, K140UD7, K153UD6, transistors KT342G, KT315 with indices B, G, D, E (V4); KT361V (V3, V5, V6), KT361D (V7, V13); KT626A, KT626B (V9, V15); any silicon diodes designed for forward current 10 A (V8, V14) and 50 mA (V10, V11).

Coils L1 and L2 are wound turn to turn in two layers with wire PEV-2 - 1,0 on frames with a diameter of 7 and a length of 28 mm and contain 30 and 46 turns, respectively.

Transistors V9, V12, V15 and V16 are mounted on a heat sink with a cooling surface of about 900 cm2 and insulated from it with mica spacers 0,1 mm thick.

The amplifier is powered by a full-wave (bridge) rectifier with filtering capacitors with a capacity of 10 microfarads. Diodes 000 ^ 1, -1V are used in the rectifier. The power transformer is made on a toroidal magnetic circuit with an outer diameter of 203, an inner diameter of 100 and a height of 64 mm. The primary winding (for 32 V) contains 220 turns of PEV 1130 wire with a diameter of 2, the secondary - 0,51 X 2 turns of PEV-104 wire - 2. An electrostatic screen is placed between the windings in the form of a single layer of wire PEV-1,3 - 2. The middle point of the secondary winding and each of the filter capacitors connected to the positive and negative terminals of the rectifier are connected to the electrostatic shield terminal and the common wire of the amplifier.

Setting up the amplifier comes down to balancing the C6R29L1R15 bridge by selecting a capacitor C6 * at a frequency of 50 ... 100 kHz. At the same time, minimal distortion of the sinusoidal signal on the oscilloscope screen is achieved.

See other articles Section Transistor power amplifiers.

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