Gain control circuits for audio amplifiers

A gain control circuit for an audio-frequency amplifier apparatus which has an audio signal input terminal, an audio signal output terminal, an audio amplifier circuit for amplifying the audio signal supplied to said input terminal and feeding the amplified audio signal to said output terminal, and a basis circuit for supplying biasing voltage to said amplifier circuit to control the gain thereof, said gain control circuit comprising:

• a. a detection circuit having an input terminal connected to said audio signal input terminal for deriving and detecting a portion of the audio signal supplied to said audio signal input terminal thereby to control potential at an output terminal of said detection circuit through the detected signal,
• b. a control circuit having an input terminal connected to said output terminal of said detection circuit and an output terminal connected to said bias circuit for said audio amplifier circuit, the potential of said output terminal of said control circuit being controlled in dependence on the potential at said output terminal of said detection circuit thereby to correspondingly control said biasing voltage of said bias circuit for the gain control of said audio amplifier circuit,
• c. a first capacitor connected to the connection between said output terminal of said detection circuit and the input terminal of said control circuit, said first capacitor comprising a portion of a first discharging circuit in cooperation with said detection circuit which controls the potential at said output terminal of said detection circuit in accordance with said audio input signal when said detection circuit is made conductive by said audio signal applied to said audio input terminal,
• d. a first resistor connected between an electric power source and said connection between the output terminal of said detection circuit and the input terminal of said control circuit and comprising a portion of a first charging circuit in cooperation with said first capacitor with said first resistor and said source arranged for charging said first capacitor and for restoring the potential at the output terminal of said detection circuit with a longer time duration than the discharging time duration of said first discharging circuit, when said detection circuit is nonconductive,
• e. a second discharging circuit comprising the parallel combination of a second capacitor and a second resistor coupled with said first discharging circuit for discharging with a larger time constant than that of said first discharging circuit during the detecting operation and for delaying the restoration of the potential at the output terminal of said detection circuit in accordance with the input time duration of said audio signal,
• f. a switching circuit connected between said second discharging circuit and the connection between the output terminal of said detection circuit and the input terminal of said controlling circuit, said switching circuit being nonconductive during the detecting operation of said detection circuit and the discharging operation of said second discharging circuit and being conductive when the detecting operation of said detection circuit has ceased and said first capacitor is charged thereby to be restored with a predetermined potential at the output terminal of said detection circuit, and
• g. said second charging circuit comprising said second capacitor, said switching circuit, said first resistor and said power source, for charging said second capacitor upon the cessation of the detecting operation and the conduction of said switching circuit, and for restoring the potential at the output terminal of said detection circuit in cooperation with said first charging circuit.

A gain control circuit for an audio-frenquency amplifier apparatus which has an audio signal input terminal, an audio signal output terminal, an audio amplifier circuit for amplifying the audio signal supplied to said input terminal and feeding the amplified audio signal to said output terminal, and a bais circuit for supplying biasing voltage to said amplifier circuit to control the gain thereof, said gain control circuit comprising:

• a. a detection transistor for detecting a portion of said audio signal applied to said audio signal input terminal and producing a potential at an output electrode thereof which is controlled by said detected signal,
• b. a gain control transistor having an input electrode connected to the output electrode of said detection transistor and an output electrode, the potentials of which are controlled in dependence upon said controlled potential as said output electrode of said detection transistor, the potential available at said output electrode of said gain control transistor is adapted to be utilized for controlling the biasing voltage of said bias circuit to thereby control the gain of said audio amplifier circuit,
• c. a first capacitor having a relatively small capacitance and connected to the connection between said output electrode of said detection transistor and the input electrode of said gain control transistor, said first capacitor comprising a portion of a first discharging circuit in cooperation with said detection transistor which controls the potential at said output electrode of said detection transistor in accordance with said audio input signal when said detection transistor is made conductive by said audio signal applied to said audio input terminal,
• d. a first resistor connected between an electric power source and said connection between the output electrode of said detection transistor and the input electrode of said gain control transistor in series with said first capacitor, and comprising a portion of a first charging circuit in cooperation with said first capacitor and said source for charging said first capacitor and for restoring the potential at the output electrode of said detection transistor with a longer time duration than the discharging time duration of said first discharging circuit, when said detection transistor is nonconductive,
• e. a second discharging circuit comprising the parallel circuit of a second capacitor having a comparatively small capacitance and a second resistor coupled with said first discharge circuit for discharging with a larger time constant than that of said first discharging circuit during the detecting operation and for delaying the restoration of the potential at the output terminal of said detection transistor in accordance with the input time duration of said audio signal,
• f. a switching diode connected between said second discharging circuit and the connection between the output electrode of said detection transistor and the input terminal of said controlling circuit, said switching diode being nonconductive during the detecting operation of said detection transistor and the discharging operation of said second discharging circuit and being conductive when the detecting operation of said detection transistor has ceased and said first capacitor is charged thereby to be restored with a predetermined potential at the output terminal of said detection transistor, and
• g. said second charging circuit comprising said second capacitor, said switching diode, said first resistor and said power source, for charging said second capacitor upon the cessation of the detecting operation and the conduction of said switching diode, and for restoring the potential at the output terminal of said detection transistor in cooperation with said first charging circuit.

A gain control circuit for an audio-frequency amplifier apparatus which have an audio signal input terminal, an audio signal output terminal, an audio amplifier circuit for amplifying the audio signal supplied to said input terminal and feeding the amplified audio signal to said output terminal, and a bias circuit for supplying biasing voltage to said amplifier circuit to control the gain thereof, said gain control circuit comprising;

• a. a detection transistor having an input electrode connected to said audio signal input terminal and two output electrodes connected, respectively, to a current source side and the ground side, said detection transistor serving to detect a portion of said audio signal supplied to said audio signal input terminal and produce at the first output electrode of the current source side a potential controlled by said detected signal,
• b. a gain control transistor having an input electrode connected to said first output terminal of said detecting transistor and having two output electrodes connected, respectively, to said current source side and the ground side, said first grounded said output electrode being connected to said bias circuit for said audio amplifier circuit, wherein the potentials at said input electrode and said first output electrode being controlled in dependence upon the controlled potential at said first output electrode of said detection transistor, thereby to control said biasing voltage of said bias circuit through said potential at said first output electrode of said gain control transistor for the gain control of said audio signal amplifier circuit,
• c. a first time constant circuit comprising a first resistor connected between the junction of said first output electrode of said detection transistor and said input electrode of said gain control transistor and said current source, and a first capacitor connected between the above junction and the ground,
• d. a switching diode having one electrode connected to a junction between said first output electrode of said detection transistor and said input electrode of said gain control transistor, and
• e. a parallel connection of a second resistor and a second capacitor, said parallel connection constituting a second time constant circuit connected between the other electrode of said change-over or switching diode and the ground,
• wherein said first time constant circuit controls the potential at said first output electrode of said detection transistor for a short duration when said audio signal is applied to said audio signal input terminal regardless of magnitude of said audio signal and restores rapidly the controlled potential said output electrode of said detection transistor to the starting potential when said application of audio signal is interrupted, while said second time constant circuit is operative to regulate the restoration of the first output electrode potential of said detection transistor in cooperation with said first time constant circuit so that the restoration of said emitter potential is rapidly effected when said audio signal has a short input duration, while the restoration is decelerated upon a long input duration of said applied audio signal, and said switching diode is so arranged that the discharging from said second time constant circuit toward said first output electrode of said detection transistor is inhibited.