JPS6451724A - Amplifying circuit - Google Patents

Abstract

PURPOSE:To prevent the oscillation by using a control signal generated from an echo detecting circuit so as to change the amplification factor of an amplifier in a closed loop. CONSTITUTION:A driving circuit 61 of an echo detecting circuit 6 divides two inputs to form a ratio of input and output levels at points a, b. The output of the result of division is fed to a sampling circuit 62, where the output is sampled at an interval of a prescribed time. The sampled value is fed to a detecting circuit 63 and compared with a reference level. When the sampled value is a level in excess of the reference value, a detection signal is sent to a counter 64. The counter 64 counts number of tires where a sampled value in excess of the reference value appears for a prescribed time. When the count of the counter reaches a prescribed value or over, a control signal changing the amplification factor of the amplifier 14 is sent from the counter. The amplification factor of the amplifier at the circuit network input is reduced by the signal to prevent the occurrence of oscillation by a closed circuit.