A circuit stage such as an input stage is provided for a radio frequency tuner having an input 1 which typically receives a broadband signal comprising many channels. The input stage comprises a low noise amplifier 2 in the form of a long tail pair of transistors 4 and 5 and a controllable tail current source 10, 11. A level detector 3 detects the signal level at the input of the amplifier 2 and controls the tail current source 10, 11 so as to increase the tail current as the signal amplitude rises above a threshold level and so as to keep the tail current fixed when the signal amplitude is below the threshold level.