Automatic gain control circuit

Claim 2

An automatic gain control circuit comprising: a field effect transistor connected between the output of a signal source and the input of an output amplifier circuit; a feedback circuit means coupling an output terminal of said amplifier to a control terminal of said field effect transistor, the impedance of said field effect transistor being varied as a function of a feedback signal via said feedback circuit means from said output terminal of said amplifier circuit; and a resistor circuit connected to said field effect transistor in parallel with the signal source; said automatic gain control circuit having a noise characteristic curve which is a function of the signal source impedance thereof, said noise characteristic curve having a knee point corresponding to a given signal source impedance at which knee point the noise level begins to rapidly rise, said resistor circuit including a resistor having a resistance less than that of the signal source impedance corresponding to said knee point of said noise characteristic curve.

Claim 3

An automatic gain control circuit according to claim 1 in which said resistor of said resistor circuit is connected to the output terminal of said field effect transistor.

Claim 4

An automatic gain control circuit according to claim 2 wherein said resistor circuit further includes an input resistor connected to the input terminal of said field effect transistor.

Claim 5

An automatic gain control circuit comprising: a field effect transistor connected between the output of a signal source and the input of an output amplifier circuit, said field effect transistor being further coupled to an output terminal of said amplifier, the impedance of said field effect transistor being varied as a function of a feedback signal from said output terminal of said amplifier circuit; and at least one resistor connected between the input and output terminals of said field effect transistor; said automatic gain control circuit having an even order harmonic distortion characteristic curve which is a function of the impedance of said field effect transistor, said even order harmonic distortion characteristic having a knee point corresponding to a given field effect transistor impedance at which knee point the even order harmonic distortion begins to rapidly rise, said at least one resistor having a resistance substantially equal to the impedance of the field effect transistor obtained just before said knee point of said even order harmonic distortion characteristic curve.

Claim 6

An automatic gain control circuit comprising: a field effect transistor connected between the output of a signal source and the input of an output amplifier circuit, said field effect transistor being further coupled to an output terminal of said amplifier, the impedance of said field effect transistor being varied as a function of a feedback signal from said output terminal of said amplifier circuit; and at least one resistor connected between the input and output terminals of said field effect transistor; said automatic gain control circuit having an even order harmonic distortion characteristic curve which is a function of the impedance of said field effect transistor, said even order harmonic distortion characteristic having a knee point corresponding to a given field effect transistor impedance at which knee point the even order harmonic distortion begins to rapidly rise, said at least one resistor having a resistance substantially equal to the impedance of the field effect transistor obtained just before said knee point of said even order harmonic distortion characteristic curve.

Claim 7

An automatic gain control circuit comprising: a plurality of field effect transistors sequentially and serially connected between the output of a signal source and the input of an output amplifier circuit; a feedback bias circuit coupling said field effect transistors to an output terminal of said amplifier for supplying respective feedback signals to the respective field effect transistors, the impedances of said field effect transistors being varied as a function of said respective feedback signals; and a plurality of resistors connected between the source and drain of the respective field effect transistors; said automatic gain control circuit having an even order harmonic distortion characteristic curve which is a function of the impedances of said field effect transistors, said even order harmonic distortion characteristic curve having a knee point corresponding to a given field effect transistor impedance at which knee point the eveN order harmonic distortion begins to rapidly rise, each of said plurality of resistors having a resistance substantially equal to the impedance of the field effect transistor obtained just before said knee point of said even order harmonic distortion characteristic curve.

Claim 8

An automatic gain control circuit comprising: a plurality of field effect transistors sequentially and serially connected between the output of a signal source and the input of an output amplifier circuit; a feedback bias circuit coupling said field effect transistors to an output terminal of said amplifier for supplying respective feedback signals to the respective field effect transistors, the impedances of said field effect transistors being varied as a function of said respective feedback signals; and a plurality of resistors connected between the source and drain of the respective field effect transistors; said automatic gain control circuit having an even order harmonic distortion characteristic curve which is a function of the impedances of said field effect transistors, said even order harmonic distortion characteristic curve having a knee point corresponding to a given field effect transistor impedance at which knee point the eveN order harmonic distortion begins to rapidly rise, each of said plurality of resistors having a resistance substantially equal to the impedance of the field effect transistor obtained just before said knee point of said even order harmonic distortion characteristic curve.

Claim 9

An automatic gain control circuit comprising: a first variable impedance circuit including a field effect transistor connected between the output of a signal source and the input of an output amplifier circuit, said field effect transistor having input, output and control electrodes; a resistor connected between the input and output terminals of said field effect transistor; a first feedback circuit coupled between the control electrode of said field effect transistor and an output of said amplifier circuit for supplying a feedback signal to said control electrode of said field effect transistor to control the impedance of said field effect transistor; a second variable impedance circuit including a bipolar transistor connected to the output terminal of said field effect transistor and in parallel with the signal source, said bipolar transistor having a control electrode; and a second feedback circuit connected between said control electrode of said bipolar transistor and an output amplifier circuit for supplying a feedback signal to said control electrode of said bipolar transistor.

Claim 10

An automatic gain control circuit comprising: a first variable impedance circuit including a field effect transistor connected between the output of a signal source and the input of an output amplifier circuit, said field effect transistor having input, output and control electrodes; a resistor connected between the input and output terminals of said field effect transistor; a first feedback circuit coupled between the control electrode of said field effect transistor and an output of said amplifier circuit for supplying a feedback signal to said control electrode of said field effect transistor to control the impedance of said field effect transistor; a second variable impedance circuit including a bipolar transistor connected to the output terminal of said field effect transistor and in parallel with the signal source, said bipolar transistor having a control electrode; and a second feedback circuit connected between said control electrode of said bipolar transistor and an output amplifier circuit for supplying a feedback signal to said control electrode of said bipolar transistor.

Claim 11

An automatic gain control circuit according to claim 6 in which: said field effect transistor of said first variable impedance circuit is an n-channel field effect transistor the gate of which is connected to said first feedback circuit; said first feedback circuit comprises a negative feedback circuit supplying a negative feedback signal to said gate of said field effect transistor; said bipolar transistor of said second variable impedance circuit is an npn transistor the base of which is connected to said second feedback circuit; and said second feedback circuit comprises a positive feedback circuit supplying a positive feedback signal to said base of said bipolar transistor.

Claim 12

An automatic gain control circuit according to claim 6 in which: said transistor of said first variable impedance circuit is an n-channel field effect transistor; said first feedback circuit comprises a negative feedback circuit for supplying a negative feedback signal to the gate of said field effect transistor; said transistor of said second variable impedance circuit is a pnp transistor; and said second feedback circuit comprises means coupled to said negative feedback circuit for supplying a negative feedback signal from said negative feedback circuit to the gate of said pnp transistor.

Claim 13

An automatic gain control circuit comprising: a field effect transistor connected between the output of a signal source and the input of an output amplifier circuit, said field effect transistor being further coupled to an output terminal of said amplifier, the impedance of said field effect transistor being varied as a function of a feedback signal from said output terminal of said amplifier circuit; and a resistor circuit connected to said field effect transistor in parallel with the signal source; said automatic gain control circuit having a noise characteristic curve which is a function of the signal source impedance on the signal source side as seen from the amplifier circuit side, said noise characteristic curve having a knee point corresponding to a given signal source impedance, as seen from the amplifier circuit side, at which knee point the noise level begins to rapidly increase; said resistor circuit including a resistor having a resistance substantially equal to the impedance on the signal source side as seen from the amplifier circuit side corresponding to a point just before said knee point of said noise characteristic curve.

Claim 14

An automatic gain control circuit comprising: a field effect transistor connected between the output of a signal source and the input of an output amplifier circuit, said field effect transistor being further coupled to an output terminal of said amplifier, the impedance of said field effect transistor being varied as a function of a feedback signal from said output terminal of said amplifier circuit; and a resistor circuit connected to said field effect transistor in parallel with the signal source; said automatic gain control circuit having a noise characteristic curve which is a function of the signal source impedance on the signal source side as seen from the amplifier circuit side, said noise characteristic curve having a knee point corresponding to a given signal source impedance, as seen from the amplifier circuit side, at which knee point the noise level begins to rapidly increase; said resistor circuit including a resistor having a resistance substantially equal to the impedance on the signal source side as seen from the amplifier circuit side corresponding to a point just before said knee point of said noise characteristic curve.

Claim 15

An automatic gain control circuit comprising: a recording and reproducing change-over switch for alternatively supplying an output of a Recording signal source and an output of a reproducing source to an input of an output amplifier circuit; a variable impedance circuit including a field effect transistor connected between said change-over switch and the recording signal source and a resistor connected to the output terminal of said field effect transistor; a feedback circuit connected between the gate of said field effect transistor and an output of said amplifier circuit to supply a feedback signal to the gate of said field effect transistor, said feedback circuit comprising a rectifying element having one terminal connected to the output of said amplifier circuit, smoothing capacitor having one terminal thereof connected to the other terminal of said rectifying element and a smoothing resistor connected in parallel to said smoothing capacitor and having a neutral point connected to the gate of said field effect transistor; and a ground switch interlocked with said change-over switch to alternatively ground the two terminals of said smoothing capacitor.

Claim 16

An automatic gain control circuit comprising: a recording and reproducing change-over switch for alternatively supplying an output of a Recording signal source and an output of a reproducing source to an input of an output amplifier circuit; a variable impedance circuit including a field effect transistor connected between said change-over switch and the recording signal source and a resistor connected to the output terminal of said field effect transistor; a feedback circuit connected between the gate of said field effect transistor and an output of said amplifier circuit to supply a feedback signal to the gate of said field effect transistor, said feedback circuit comprising a rectifying element having one terminal connected to the output of said amplifier circuit, smoothing capacitor having one terminal thereof connected to the other terminal of said rectifying element and a smoothing resistor connected in parallel to said smoothing capacitor and having a neutral point connected to the gate of said field effect transistor; and a ground switch interlocked with said change-over switch to alternatively ground the two terminals of said smoothing capacitor.