JPS551788A - Protection circuit for gain control circuit - Google Patents

Abstract

PURPOSE:To protect the first and second Miller transistors from being damaged, by driving the third Miller transistor with the collector current of the second Miller transistor constituting the protection circuit to control the gain of the differential amplifying circuit. CONSTITUTION:The circuit is provided wish the terminal 1 to which the input signal is fed, differential amplifying circuit 2 consisting of a pair of the same conduction type transistors 3 and 4 to amplify the input siganl, and constant current transistor 5 inserted between the common emitter and ground of the transistors. Further, it provides the output transistor 6 connecting the base to the collector of one transistor 3 of the circuit 2, and signal generator circuit 7 controlling the gain based on the emitter output signal. Further, it provides the gain control circuit 8, first and second control transistors 9 and 10, current Miller circuit 11 supplying the current equal to the collector current of 9 to the collector of 10, and diodes 12 to 15. The circuit 11 consists of the first to third transistors 17, 18, 20 and the resistor 19 is placed between the collector of 18 and ground.