IC compatible variable shunt impedance for Dolby B system

Claim 1

In a signal processing system of the Dolby B noise reduction type including a variable frequency high pass filter having a control terminal, a current variable shunt impedance with a non-linear control characteristics operable as a variable impedance shunt resistor at the output terminal of said variable frequency high pass filter comprising: variable shunt means including: a source of selected input bias voltage, a first transistor possessing a first emitter, a first collector and a first base, said first collector and said first base both connected to said output terminal of said variable frequency high pass filter; a second transistor possessing a second emitter, a second collector and a second base, said second base connected to said source of selected input bias voltage; first and second diode chains each formed from equal pluralities of series connected diodes, said first diode chain possessing a first end anode connected to said first emitter and a first end cathode, said second diode chain possessing a second end anode connected to said second emitter and a second end cathode connected to said first end cathode to form said control terminal: current balancing means connected to said first and second collectors for providing currents equal one half and selected control current in each of said first and said second diode chains; and non-linear voltage-to-current converter means responsive to control voltage at a system control terminal for providing system control current in response to voltage from virable shunt means including: maximum impedance current leakage means connected to said control terminal for providing a constant initial biasing current at said control terminal for providing a constant initial biasing current at said control terminal such that said selected impedance provided by said variable shunt means does not exceed 700K ohms; low current compensated voltage-to-current converter means coupled to said control voltage for providing in initially restricted and progressively more linear response to increases in said control voltage a first partial control current such that the sum of said initial biasing current and said first partial control current equals the value of said selected control current required by said variable shunt means to provide values of said selected impedance approximating the source drain impedance of said replaced field-effect transistor over a range of control voltage values corresponding to source-drain impedances in the range of 700K to 1K ohms; and high current compensation means connected to said low current compensated voltage-to-current converter means for providing a non-linear second partial control current above a predetermined control voltage value such that the sum of said initial biasing current, said first partial control current sand said second partial control current equals the value of said selected control current required by said variable shunt means to provide values of said selected impedance approximating the source-drain impedance of said replaced field-effect transistor over a range of control voltage values corresponding to source-drain impedances in the range of 1K to 70 ohms.