Power amplifier with field effect transistor. Scheme, description

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Power amplifier with a field effect transistor as a current source for the input stage and balancing elements for the output stage. 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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The amplifier has the following main technical characteristics:

Rated output power ...... 75 W
Harmonic distortion .......... 0,06%
Operating frequency band .......... 20 ... 40 Hz
Signal-to-noise ratio .......... 86 dB
Supply voltage ........... ±40 V
Quiescent current .............. 20 mA

The improvement in quality indicators in this amplifier compared to the previous one was achieved by a number of circuit solutions. A field-effect transistor current source is included in the emitter circuit of the input differential stage. This makes it possible to increase the transmission coefficient of the first stage and improve its thermal stability. To improve the symmetry of the arms of the output stage of the amplifier and reduce non-linear distortion, a corrective circuit is introduced into the emitter circuit of one of the transistors of the final stage, consisting of a diode, resistor and capacitor.

Schematic diagram of the amplifier is shown in fig. 1. It contains a differential input stage (VT2, VT4), current amplifiers (VT3) and voltage amplifiers {VT6), an output stage (VT9-VT12) and an overload protection device (VT7, VT8). As already mentioned, the current source on the transistor VT1, included in the emitter circuits of transistors VT2, VT4, allows, without increasing temperature instability, to increase the voltage transfer coefficient of the differential stage. Transistor VT3 allows you to reduce the load on the output of the differential stage. The load-shared cascade on the VT6 transistor amplifies the voltage signal, providing the maximum output voltage swing.

Fig.1 (click to enlarge)

The quasi-complementary output stage, made on composite transistors (VT9, VT11 and VT10, VT12), is well matched with a low-resistance load. The corrective circuit, consisting of parallel-connected diode VD2, resistor R28 and capacitor C10, improves the symmetry of the arms of the amplifier, thereby reducing non-linear distortion. The initial bias at the bases of the output transistors for operation in AB mode is determined by the voltage drop in the collector-emitter section of the transistor VT5 and is regulated by resistor R16. Transistors VT7 and VT8 shunt the emitter junction of the output transistors during overload, thereby protecting them. Elements C3, R5, C4, R31, C12 prevent self-excitation of the amplifier at high frequencies. Circuit R7, Sat serves to equalize the frequency response of the amplifier at high frequencies in the audio range (up to 20 kHz).

Structurally, the amplifier is assembled on a printed circuit board. Temperature stabilization of the quiescent current of the output transistors is carried out using the transistor VT5, installed on a common radiator with VT12 or VT11; in close proximity to them. To power the amplifier, a bipolar source is required that provides a current of at least 40 A at a voltage of ± 2,5 V.

Establishing an amplifier assembled from serviceable elements consists in checking the correct installation and setting the quiescent current of the output transistors with resistor R16 within 20 ... 40 mA.

Publication: cxem.net

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