| 摘要 |
PROBLEM TO BE SOLVED: To reduce the time required for a DC pull-in without generating a DC step by putting a time constant to a minimum at the instant when a switching is made from a low speed AC coupling to a high speed AC coupling and then, gradually increasing the time constant so that it becomes the time constant of the low speed AC coupling. SOLUTION: A control signal processing circuit 2 differentiates control signals to be supplied to a control terminal Tc with a high pass filter(HPF) 21. The differentiated signals are half wave rectified by a half wave rectifying circuit 22 to produce second control signals. Then, a transconductance value gm1 is changed in proportion to the second control signals so as to control the time constant of an HPF1. Thus, the time constant of the HPF1 becomes small at the instant of a DC fluctuation and then, the time constant is gradually increased. Therefore, there exists practically no change in the time constant while a switching is made from a high speed AC coupling to a low speed AC coupling even though the time constant of the HPF1 is made small during a high speed AC coupling for a high speed pull-in.
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