GB1486512A - Device for protecting an audio amplifier against overload and/or short circuit - Google Patents

Abstract

1486512 Protecting transistors NOVANEX AUTOMATION NV 17 Jan 1975 [24 Jan 1974] 2210/75 Heading H3T An audio amplifier comprising a preamplifier 24, 25 controlling an output amplifier having at least one branch including an output transistor, 1 or 2, is protected against overload and/or short circuit by means limiting the current in a branch to a predetermined value, and means for uncoupling the output amplifier from the preamplifier if such a current is reached in at least one branch. The output currents of transistors 1, 2 flow through measuring resistors 7, 8 respectively. Increasing voltage across 7 or 8 causes transistors 44 or 45 respectively to turn on and to take away base current from driver transistors 3 or 4 respectively. Transistors 3 and 4 are normally driven by 17 acting as an amplifier, but on overload and/or short circuit 17 with 19 and resistors 18, 20 form a constant current source, so as to limit the output current. A transistor 35 is connected across measuring resistors 7, 8, and if the voltage across either 7 or 8 reaches a predetermined limit 35 fires thyristor 38, short circuiting transistor 17, and also 14. Transistor 14 together with 16 and resistors 15 and 34 form a constant current source providing a constant bias voltage across resistor 13. Resistor 18 may be half the value of resistor 15, so that the maximum current of 17, 19 is twice that of 14, 16, in order to equalize the gain of the amplifier for positive and negative signals. Amplifier 17 is driven via transistor 21 by a differential amplifier, formed by transistors 24, 25, which compares the input signal with a fraction of the output signal from potential divider 28, 29. A Zener diode (not shown) may be connected to the control electrode of thyristor 38. An LED (not shown) may be connected in series with the resistor 40 or 41 to make visible the firing of thyristor 38.