PURPOSE:To prevent the generation of harmful noise in a high impedance circuit and use this circuit in a current sypply circuit of a telephone exchange to improve the precision of the state monitor in the telephone side by suppressing the transient oscillation of a current waveform at the time when turning-on/off of the high impedance circuit consisting of an electronic circuit is controlled. CONSTITUTION:A DC amplifier 1 of the high impedance circuit consists of an operational amplifier OP1 and a transistor TRQ1, and a current IL is supplied to a load RL by the amplifier 1. This amplifier 1 is provided with a low pass fikter consisting of a series circuit of a resistance Ri, a switch SW2, and a capacitor CDC, and a feedback loop to the amplifier 1 is formed by this filter to eliminate AC components. When a switch SW1 of this circuit is closed, the switch SW2 is turned off, and the output energy of the filter is stored temporarily in the capacitor CDC to suppress the transient oscillation of the current waveform at the circuit turning-on/off control time, thus preventing the generation of harmful noise.
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