Patent ID: 6097244
Filing Date: 2000-08-01
Classification: H03H,H03J,H03L

Abstract:
A highly-linear ladder filter that adjusts resistance comprising:a plurality of filter stages, each filter stage having:an op amp with a pair of op-amp inputs, for amplifying a voltage difference between the pair of op-amp inputs to generate op-amp outputs;a pair of feedback capacitors, each feedback capacitor connecting an op-amp output to an op-amp input;a pair of resistor networks, each resistor network connecting a stage input to an op-amp input with an equivalent resistance, the equivalent resistance being continuously adjusted by a control voltage;wherein each resistor network includes:a first fixed resistor, connecting the stage input to an intermediate node;a fixed input resistor, connecting the intermediate node to the op-amp input;a variable resistor, connecting the intermediate node to a ground node, the variable resistor having a resistance controlled by the control voltage;wherein the variable resistor is a transistor biased in a linear region of operation by the control voltage applied to a gate of the transistor, wherein a drain of the transistor is connected to the intermediate node and wherein a source of the transistor is connected to the ground node: andan analog control loop for generating the control voltagewhereby the control voltage adjusts channel resistance through the transistor and whereby linearity is improved by using the fixed input resistor connected to the op-amp input while the equivalent resistance of the resistor network is adjusted with the control voltage by the variable resistor to the ground node.