Economical amplifier with increased thermal stability. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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The use of mode B in the final stage completely removed the problem of temperature stabilization of the quiescent current, made the amplifier economical and thermally stable. Among the advantages of such an amplifier is also its ability to operate from a power source with increased ripple. The amplifier is designed to work with a pre-amplifier whose output impedance does not exceed 200 ohms.
The main parameters:
- Rated frequency range, Hz.......20...20 000
- Rated output power at a load with a resistance of 8 ohms (with a harmonic coefficient in the nominal frequency range of not more than 0,5%), W ....... 30
- Rated input voltage, V ....... 1,5
- Phase shift in the nominal frequency range.......10°
- Temperature range of stable operation of the amplifier, °С.......-20...+60
The first stage of the amplifier is assembled on the operational amplifier A1. In order for the slew rate of the signal at the amplifier input not to exceed the allowable value, an R1C1R2 low-pass filter with a cutoff frequency of about 20 kHz is applied. The amplified signal is fed to the inverting input of the operational amplifier, the OOS signal (from the amplifier output) is fed to its non-inverting input. Capacitor C2 corrects the phase response of the amplifier in the high frequency region. The cutoff frequency of the cascade (taking into account the correction through the capacitor C3) is about 30 kHz.
The second stage is made on transistors V4-V7 according to the push-pull cascode amplifier circuit. The cutoff frequency of this stage is 4,7 MHz. In addition to inverting the signal, it performs the functions of a generator of stable bias currents for transistors of the final stage on transistors of different structures V8 and V9. Resistors R12, R13 included in their emitter circuits create local OOS for current, which, together with stable bias currents, determines the high thermal stability of the amplifier as a whole. The quiescent current of transistors V8, V9 is 30 mA (at a temperature of 60 ° C, it increases to 50 mA). The cutoff frequency of this amplifier stage is 130 kHz.
The terminal stage transistors V10, V11 are connected according to the emitter follower circuit and operate without initial bias, i.e., at a quiescent current equal to zero. To reduce the inevitable in this case distortion type "step" introduced resistor R14. Due to this, at low signal levels, when the transistors V10, V11 are closed, the pre-terminal stage works on the load. The cutoff frequency of the cascade on transistors V10, V11 is about 140 kHz.
An operational amplifier with a voltage gain of at least 2000 is suitable for operation in this amplifier. It is desirable to select the final stage transistors with the same current transfer coefficients (h21e > 50). Instead of GT321A transistors in the amplifier, you can use KT626 transistors (with letter indices A, B, C), instead of GT905A and GT806V, KT814G and KT816G, respectively. Coil L1 (30 turns) is wound in two layers with PEV-2-1,0 wire on a frame with a diameter of 7 and a length of 25 mm.
To cool transistors V8, V9, a U-shaped heat sink is used, bent from a strip (dimensions 100 x 50 mm) of a sheet aluminum alloy 2 mm thick. The dimensions of the heat sink base are 50 x 50 mm, the shelves (the transistors are fixed on them) are approximately 25 x 50 mm. The heat sink is mounted on the circuit board in such a way that the outputs of the transistors can be connected to the rest of the parts with short conductors. Transistors V10 and V11 are mounted on universal heat sinks of type 8.650.022 with an effective cooling area of 300 cm2.
The amplifier does not require tuning. Before connecting a loudspeaker, you must make sure that the DC output voltage does not exceed 0,1 V, and the quiescent current of transistors V8, V9 is 50 mA.
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