Variable capacitance circuit

Circuit for providing a variable and controlled impedance across a pair of input terminals thereof, comprising:

• first and second capacitive means each having a first end coupled to a respective one of the pair of input terminals of the circuit;
• variable impedance means coupled in series between second ends of said first and second capacitive means, said variable impedance means receiving a control signal such that the value of impedance thereof is varied in response to said control signal wherein the total capacitance developed across the pair of input terminals is varied;
• said variable impedance means including first and second power supply conductors;
• first and second diode means each having first and second electrodes, said first electrode of said first and second diode means being coupled to said respective second end of said first and second capacitive means, said second electrodes each being coupled to said first power supply conductor;
• first and second serially connected resistors coupled between said first electrodes of said first and second diode means;
• and
• switch means coupled between the interconnection of said first and second serially connected resistors and said second power supply conductor, said switch means being rendered responsive to said control signal for causing said first and second diode means to become conductive to vary said variable impedance.

In combination with an amplifier section having a first and a second inputs coupled to a balanced tuned input circuit, a circuit for varying the bandpass characteristics of the amplifier section, comprising:

• first capacitive means having first and second leads, said first lead being coupled to said first balanced tuned input;
• second capacitive means having first and second leads, said first lead being coupled to said second balanced tuned input;
• impedance means having a variable impedance value which is varied in response to a control signal such that said first and second capacitive means are coupled in series with each other to vary the bandpass characteristics in response to said control signal, said impedance means being coupled between said second leads of said first and second capacitive means;
• said impedance means including first diode means having first and second electrodes, said first electrode being coupled at a first node to said second lead of said first capacitive means, said second lead being coupled to a first terminal at which is supplied a source of operating potential, second diode means having first and second electrodes, said first electrode being coupled at a second node to said second lead of said second capacitive means, said second electrode being coupled to said first terminal, first and second resistive means coupled in series between said first and second nodes, and switch means being responsive to said control signal for rendering said first and second diode means conductive to connect said first and second capacitive means in series through a low impedance path such that the capacitiance of the balanced tuned input is varied.

In a large scale integrated television receiver circuit including an intermediate frequency amplifier section differentially coupled to a balanced resonance tuned input stage and an automatic gain control circuit for providing a gain control signal, a circuit for varying the bandpass characteristics and center frequency of the intermediate frequency amplifier section in response to the gain control signal, comprising:

• first capacitive means having first and second plates, said first plate being coupled to a first differential input of the intermediate frequency amplifier section;
• second capacitive means having first and second plates, said first plate being coupled to the second differential input of the intermediate amplifier section;
• variable impedance means coupled at first and second nodes to respective second plates of said first and second capacitive means and receiving the gain control signal for varying the impedance between said second plates from a high impedance value to a low impedance value such that said first and second capacitive means are serially connected through a low impedance path in response to the gain control signal which cause the total capacitance of the resonance tuned stage to be varied such that the bandpass characteristic as will as the center frequency are varied accordingly, said first and second capacitive means being connected through a high impedance path relative to said low impedance path in the absence of the gain control signal.

In a large scale integrated television receiver circuit including an intermediate frequency amplifier section differentially coupled to a balanced tuned input stage and an automatic gain control circuit for providing a gain control signal, a circuit for varying the bandpass characteristics of the intermediate frequency amplifier section in response to the gain control signal, comprising:

• first capacitive means having first and second plates, said first plate being coupled to a first differential input of the intermediate frequency amplifier section;
• second capacitive means having first and second plates, said first plate being coupled to the second differential input of the intermediate amplifier section;
• impedance means coupled at first and second nodes to respective second plates of said first and second capacitive means and receiving the gain control signal such that said first and second capacitive means are serially connected through a low impedance path in response to the gain control signal such that the bandpass characteristic is varied accordingly, said first and second capacitive means being connected through a high impedance path relative to said low impedance path in the absence of the gain control signal;
• and
• said impedance means including first diode means having first and second electrodes, said first electrode being coupled to said first node, said second electrode being coupled to a first terminal at which is supplied a source of operating potential;
• second diode means having first and second electrodes, said first electrode being coupled to said second node, said second electrode being coupled to said first terminal;
• switching means being responsive to said control signal for rendering said first and second diode means conductive.