Patent ID: 6977541

Claim:
A variable time constant circuit using a time constant control signal, comprising: a first signal input terminal and a second signal input terminal; an inverting amplifier including a positive input terminal, a reverse phase input terminal, a negative output terminal, and a positive output terminal; a first signal output terminal and a second signal output terminal which are connected to the negative output terminal and positive output terminal, respectively; a first resistor and a second resistor which are connected in series between the first signal input terminal and the positive input terminal; a third resistor and a fourth resistor which are connected in series between the second signal input terminal and the negative input terminal; a first capacitor connected between the positive input terminal and the negative output terminal; a second capacitor connected between the negative input terminal and the positive output terminal; a first field effect transistor including a first gate terminal connected to a first junction point of the first resistor and the second resistor, a first source terminal kept at a constant potential, and a first drain terminal connected to one of the positive input terminal and negative input terminal, the first field effect transistor flowing a current corresponding to a first voltage-current conversion ratio through the first drain terminal according to the voltage between the first gate terminal and the first source terminal; a second field effect transistor including a second gate terminal connected to a second junction point of the third and the fourth resistor, a second source terminal kept at a constant potential, and a second drain terminal connected to the other of the positive input terminal and negative input terminal, the second field effect transistor flowing a current corresponding to a second voltage-current conversion ratio through the second drain terminal according to a voltage between the second gate terminal and the second source terminal; and a control circuit which controls the first voltage-current conversion ratio of the first field effect transistor and the second voltage-current conversion ratio of the second field effect transistor according to the time constant control signal.