Class AB amplifier with galvanic isolation. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers
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Figure 1 shows a class AB amplifier circuit.
The two-channel inverting amplifier provides a maximum gain of up to 1000 V in the frequency range from 0 to 20 kHz. At frequencies above 20 kHz, the gain of the amplifier decreases. The ratio of resistors R2 and R1 determines the gain of the device.
The positive and negative channels of the amplifier are built using 2N3 type photodiode opto-isolators IC6 and IC136 and therefore have no input-output galvanic coupling. Transistors Q1 -Q4 increase the output voltage and current of the opto-isolators. The positive and negative channels work in antiphase.
The ±15V supply voltage and resistors R4 and R5 provide the necessary bias to ensure that the output transistors are always on.
By fine tuning the values of resistors R4 and R5, output crosstalk is removed.
Zener diodes D1 and D2 keep the photodiodes of the opto-isolator reverse-biased at 6,2 V.
Resistors R10-R13 provide negative feedback around the output transistors. STW8N80 MOS-FET transistors must be mounted on heatsinks.
The device does not require any active short circuit protection.
On fig. 2 shows waveforms of the input square wave 10 V with a frequency of 10 kHz and the output signal of the amplifier 1 kV. On fig. Figure 3 shows oscillograms of an input sinusoidal signal with a swing of 10 V at a frequency of 20 kHz and an output inverse pure harmonic signal with an amplitude of 1 kV.
Author: D.Ting, Taiwan
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