Amplifier with common mode mode stabilizer. Scheme, description

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Amplifier with common mode mode stabilizer. 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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An amplifier with a common-mode mode stabilizer is designed to work with a pre-amplifier-tone block on an operational amplifier. A distinctive feature of the device is the presence of an effective stabilizer of the quiescent current of transistors of differential stages.

Common Mode Regulator Amplifier

The main parameters:

Rated frequency range, Hz.......10...25 000
Rated output power, W, at a load with a resistance of 8 Ohm.......12
Rated input voltage, V ....... 1
Harmonic coefficient, %, at rated output power at frequency, Hz: 1000.......0,01
20 000......0,02
Relative level of noise and background, dB.......-80
Input impedance, kOhm.......15

The amplifier contains two differential signal voltage amplification stages on transistors V1, V2 and V4, V5 and an output current amplification stage on transistors V6-V9.

The functions of the mode stabilizer are performed by the transistor V3, the base of which is connected to the emitter circuit of the transistors of the second differential stage, and the collector is connected to the emitter circuit of the transistors of the first stage. The OOS that occurs in this case on the common-mode component of the signal balances the differential stages and increases the resistance to ripples and fluctuations in the supply voltage.

Transistor V3 is fixed on the heat sink of transistor V8 of the output stage, so it also performs the functions of a thermal mode stabilizer. The current through the transistor V3 (40 ... 60% of the current of transistors V1, V2) must be taken into account when calculating the resistance of the resistor R3. By replacing this resistor with a current source, the amplifier can operate at a supply voltage of 12 V up to the limit value for the transistors used. If the supply voltage is greater than 36 V, it is recommended to turn on another transistor of the p-n-p structure (its base is connected to the collector of transistor V4) into the collector circuit of transistor V7 (at point a). The signal level shift that occurs in this case will ensure the thermal balance of the amplifier and the equality of voltages on the collectors of transistors V4, V5.

To balance the amplifier, the non-inverting input (the base of transistor VI) is connected to the midpoint of the voltage divider R1R2. This made it possible to simplify the node for protecting the transistors of the output stage from overcurrent. in this amplifier, it consists of a resistor R14 in the collector circuit of transistor V8 and a diode V10 connecting this circuit to the base of transistor V3.

In addition to those indicated in the diagram, transistors KT361B, KT361G, KT361E, KT209E, KT209K (V1, V2) can be used in the amplifier; KT315B, KT315G, KT315E, KT342A, KT342D (V3-V5); KT626B, (V6, V7); KT816B, KT816V, KT818B (V8, V9). The static current transfer coefficients of transistors V6, V7 and V8, V9 should be as close as possible. With such a replacement, the polarity of the power supply, electrolytic capacitors and the V10 diode must be reversed.

It is not difficult to adapt the amplifier to work on a load with a resistance of 4 ohms (output power at a supply voltage of 35 V will increase to 18 W). To do this, the resistance of the resistor R6 must be reduced to 6,8 kOhm. R7 - up to 68 kOhm, R9 - up to 90 Ohm, R10 and R11 - up to 680 Ohm. The input impedance after such changes will drop to 6,5 kOhm

Setting up the amplifier comes down to setting the quiescent current of the output stage with a tuned resistor R8. If the signal limitation does not reach at the same time (due to the unequal amplification of the output stage arms), resistors R10, R11 are selected (in the arm, where the limitation occurs earlier, the resistance of the resistor should be increased, and in the other it should be reduced by the same amount). Due to the unavoidable imbalance of the second differential stage, it is not recommended to change the resistance of the resistors by more than ± 20% (from the ratings indicated in the diagram).

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