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Abstract

A tuning amplifier 1 is provided with two cascade connected phase shifting circuits 10C and 30C and an adding circuit composed of a feedback resistor 70 and input resistor 74. The circuit 10C is constituted of an operational amplifier 12, a resistor 18 one end of which is connected to the inverting input terminal of the amplifier 18, a resistor 20 connected between the inverting input terminal and the output terminal of the amplifier 12, a series circuit which comprises a capacitor 14 and a variable resistor 16 and one end of which is connected to the other end of the resistor 18. The fluctuation of the output amplitude of the circuit 10C is suppressed when the tuning frequency of the circuit 10C is changed because the resistance of the resistors 18 and 20 are equal to each other. When the signal inputted to a voltage dividing circuit composed of resistors 21 and 23 is taken out as an output, the tuning amplifier 1 itself has a gain and the amplitude of the signal is amplified simultaneously with the tuning operation. The phase shifting circuit 30C has the same structure as that of the circuit 10C, and total phase shift by the circuits 10C and 30C is 360.degree. at a prescribed frequency. Therefore, the tuning amplifier 1 outputs only a signal having a prescribed frequency as a tuned signal.

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