Patent ID: 7521980

Claim:
A circuit comprising: (a) a first variable resistor having a first terminal and a second terminal, and also having a resistance control terminal, a resistance of the first variable resistor between the first terminal and the second terminal being variable in response to a first resistance control signal on the resistance control terminal; and (b) a resistance control circuit including i. a first current source circuit for supplying a first current through a first conductor to a first terminal of a reference resistor having a second terminal connected to a reference voltage conductor, ii. a second current source circuit for supplying a second current through a second conductor to the first terminal of the first variable resistor, iii. a first operational amplifier having a first input coupled to the first conductor, a second input coupled to the second conductor, and an output conducting the first resistance control signal and coupled by a third conductor to the resistance control terminal of the first variable resistor, wherein the first operational amplifier acts to set the resistance of the first variable resistor to be equal to a resistance of the reference resistor; and wherein the first current source circuit includes a first exponential current circuit which produces the first current as a bias current proportional to a collector current of a first bipolar transistor, and wherein the second current source circuit includes a second exponential current circuit which produces the second current as a control current proportional to a collector current of a second bipolar transistor in response to a second resistance control signal.