A first capacitor has a capacitance adjustable to a set point value by application of a bias voltage. A second capacitor also has a capacitance adjustable to a set point value by application of a bias voltage. The first and second capacitors are arranged to receive the same bias voltage generated by a control circuit. The control circuit receiving the set point value as an input and generates that bias voltage in response to a quantity representative of a capacitance of the second capacitor.