Class B power amplifier with distortion correction due to the use of direct coupling. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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It has the following main specifications:
Rated output power......60W
Harmonic distortion .......... 0,04%
Operating frequency band ...... 20 ... 100000 Hz
Signal-to-noise ratio ...... 90 dB
Supply voltage......±40 V
Quiescent current......0 mA
The main disadvantage of an amplifier operating in mode B is a rather high level of non-linear distortion, especially at low input signal levels. However, the disadvantage can be eliminated, even if we use the economical mode B in the output stage. This principle of constructing amplifiers is called feed forward error correction (distortion correction using direct connection). The operation of the amplifier can be seen in the example of Fig. one.
Rice. 1. Diagram explaining the principle of correction of distortions resulting from the use of feed-forward
The amplifier consists of an amplifier A1, an output stage (based on transistors VT1, VT2) operating in mode B, and bridge elements R1, C1, R2 and L1. The non-linear distortion compensation condition in such a device coincides with the bridge balance condition: L1=RIR2C2. If we exclude the resistor R2, then the device in fig. 1 can be considered as a conventional bass amplifier, where R1 provides OOS, C1 corrects the frequency response, L1 prevents high-frequency generation. In such an amplifier, the stability requirement makes it necessary to reduce the feedback value with increasing signal frequency, which naturally causes an increase in the non-linear distortion of the output current i1. When the resistor R2 is connected, a compensating current i2 appears and effective compensation occurs at medium and high signal frequencies. At low frequencies, the balance of the bridge can be disturbed due to the active component in the impedance of the inductor L1.
A similar method was first used in the English amplifier "Quad 405" and made it possible to obtain a harmonic coefficient at medium frequencies of about 0,01%.
A schematic diagram of an amplifier based on a domestic element base, using a similar method for compensating for non-linear distortions, is shown in fig. 2. Work in the output stage in mode B made it possible to increase the efficiency and solve the problem of thermal stabilization of the quiescent current. The amplifier consists of a four-stage pre-amplifier (based on DA1, VT1-VT4, VT7 elements) operating in mode A, an output stage (VT8-VT10) operating in mode B, and an output stage overload protection unit (VT6, VT5). The entire amplifier is covered by a deep DC OOS (through resistor R31), which maintains zero voltage at the amplifier output.
Rice. 2 (click to enlarge)
The imbalance of the bridge at low frequencies is compensated by a deep OOS, the voltage of which is supplied to the emitter circuit of the transistor VT2 through the divider R12R11. To prevent self-excitation of the amplifier at high frequencies, elements L1, L3, R25, R29, R30, C10 are used.
Coils LI - L3 are wound with PEV-2 1,0 wire on frames with a diameter of 7 mm, turn to turn in two layers. Coil L2-30 turns, LI, L3-46 turns. Transistors VT7, VT8, VT9, VT10 are installed on a common heat sink through mica gaskets.
An amplifier, properly assembled from serviceable elements, requires practically no tuning. To obtain minimal non-linear distortion, it is necessary to adjust the bridge by selecting capacitor C8. The amplitude-frequency, phase-frequency and transient characteristics of the amplifier are shown in fig. 3. To power it, a bipolar source is required, providing a current of at least 40 A at a voltage of ± 2 V.
Fig. 3
Literature
D.I. Ataev, V.A. Bolotnikov. Practical schemes for high-quality sound reproduction. M. Radio and communication. 1986
Publication: cxem.net
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