Patent ID: 8022744

Claim:
A signal generator circuit for generating a time-varying signal, comprising: a capacitance; a first field effect transistor having a gate, a drain and a source, arranged to generate current through the drain and such that a current matched to said current through the drain is supplied to or from the capacitance; and a bias voltage generator arranged to provide a bias voltage to the gate of the first field effect transistor, wherein: the capacitance per unit area of the capacitance and the capacitance per unit area of the gate of the first field effect transistor are matched; the bias voltage generator comprises a first voltage generator and a second voltage generator and is arranged to generate a bias voltage at least approximately equal to a sum of the first voltage and the second voltage; said first voltage is arranged to be at least approximately proportional to a reference voltage; said second voltage is arranged to be at least approximately proportional to temperature; and the voltage between the source and gate of the first field effect transistor is arranged to be at least approximately equal to the sum of the bias voltage and a gate threshold voltage of the first field effect transistor, the signal generator circuit arranged to sum the bias voltage and a further voltage at least approximately equal to the gate threshold voltage to determine the voltage between the source and the gate of the first field effect transistor, wherein the signal generator circuit is arranged to control a time period of the time-varying signal dependent on the current supply to or from the capacitance.