Control circuitry, comprising: first control means operable to generate a first control signal, the first control signal being indicative of a relationship between an output signal and a first reference signal, and to generate said output signal in dependence upon said first control signal, the first control means being configured to tend to maintain a first desired relationship between the output signal and the first reference signal in response to said first control signal; and second control means configured to influence operation of said first control means in response to said first control signal by way of a second control signal so as to tend to maintain a second desired relationship between said first control signal and a second reference signal.