A circuit tuneable between first and second frequencies comprising gain control circuitry operable to control the gain of the circuit between the first and second frequencies, the gain control circuitry comprising a resistor network having: at least two resistor lines arranged in parallel, each resistor line comprising one or more resistors; and for each resistor line, a switch operable to select or deselect the corresponding resistor line; the resistor lines and switches being arranged such that the net resistance of the resistor network is the parallel sum of each of the selected resistor lines; and logic circuitry configured to control said switches so as to minimize the variation in gain of the circuit between the first and second frequencies.