Amplifier with low dynamic distortion. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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The so-called dynamic intermodulation distortion occurs in transistor amplifiers with sharp changes in signal level. These distortions are especially noticeable when playing music programs. To reduce such distortions to a minimum, this amplifier widely uses local current OSS, the so-called "current mirror" is used, which improves the symmetry of the amplified signal at the input of the final stage, and the frequency response correction is used in advance.
The main parameters:
- Rated frequency range, Hz 16.......100 000
- Rated output power at a load with a resistance of 8 ohms (with a harmonic coefficient of 0,35% at frequencies of 63 ... 1 and 000 Hz), W ...... 10
- Rated input voltage, V ....... 1
- Relative level of noise and background, dB......-60
The amplifier contains an input differential stage on transistors V1, V2, a balancing stage on transistors V3, V5 with a "current mirror" on transistors V4, V6, an output stage on transistors V14-V17 and a short circuit protection device in the load on transistors V9, V10.
Resistors R3, R4 in the emitter circuits of the transistors of the first stage create a local OOS for current, which increases the linearity and input resistance of the cascade, as well as improves its symmetry. Resistors R11, R14 create a local OOS in the second stage. Correction of the frequency response in advance is carried out by capacitors C2 and C6.
The output stage is made according to the traditional scheme with a phase inverter on transistors of different structures V14, V15. The quiescent current of transistors V16, V17 is set by a tuning resistor R15 and is stabilized when the temperature changes by the transistor V7, which has a thermal connection with one of them. Diodes V18, V19 protect the output stage transistors from overvoltages with an inductive load.
The amplifier is covered by the OOS, the voltage of which is removed from the load and through the circuit R10C4C5R9 enters the input of the first stage (into the base circuit of the transistor V2). The R28C10 circuit increases the resistance of the amplifier against self-excitation.
The device for protecting the output stage from a short circuit in the load is made according to the bridge circuit. For the negative half-wave of the amplified signal, the bridge is formed by the load resistance and resistors R26, R20 and R17. The emitter junction of the transistor V9 is included in the diagonal of the bridge. With a sharp decrease in load resistance, the balance of the bridge is disturbed, transistor V9 opens and, with its low resistance of the emitter-collector section, shunts (through diode V5) the input of the terminal stage on transistor V14. As a result, the output stage current is instantly limited. For a positive half-wave of the signal, the bridge is formed by the load resistance and resistors R27, R21 and R19, the emitter junction of the transistor V10 is included in the diagonal of the bridge.
For good linearity of the amplifier, a pair of transistors V1 and V2, V3 and V5 V4 and V6, V16 and V17 must be selected according to the static current transfer coefficient h21e.
Transistors V14, V15 are mounted on U-shaped heat sinks bent from a strip of sheet (2 mm thick, 20 mm wide) aluminum alloy (heat sink dimensions - 20 x 25 x 15 mm). The heat sinks of each of the transistors V16, V17 must have a cooling surface of about 250 cm2. A V7 transistor is glued to one of these heat sinks with 88-N glue.
Establishing the amplifier comes down to eliminating (by trimming resistor R7) a constant voltage at the output and setting (by trimming resistor R15) the quiescent current of the output cascade within 80 ... 100 mA.
See other articles Section Transistor power amplifiers.
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