Patent ID: 7586380

Claim:
A bias circuit to stabilize oscillation in a ring oscillator comprising a plurality of differential delay cells having respective pairs of input nodes and output nodes and connected in series to form a ring, each of the differential delay cells comprising a pair of input transistors having gates connected to respective ones of the input nodes, sources coupled with each other, and drains connected to respective ones of the output nodes, a pair of voltage-controlled resistors connected between a power-supply line and respective ones of the output nodes, each of the voltage-controlled resistors having an effective resistance dependent on a first bias voltage, and a common current source that supplies a bias current controlled by a second bias voltage to the pair of input transistors, the bias circuit comprising: a replica arm including a replica of one of the voltage-controlled resistors connected between the power-supply line and a first control node, and a first replica of the common current source that supplies a first replica bias current controlled by the second bias voltage to the replica of one of the voltage-controlled resistors; a resistor arm including a fixed resistor connected between the power-supply line and a second control node, and a second replica of the common current source that supplies a second replica bias current controlled by the second bias voltage to the fixed resistor; and a control circuit that adjusts the second bias voltage such that voltages at the first and the second control nodes are the same, wherein the bias circuit supplies to the differential delay cells the first bias voltage from one of the first and the second control nodes and the second bias voltage from the control circuit.