An amplifier, in particular an RF amplifier is described having an amplifier input, the amplifier comprises: a first controllable semiconductor having a first controllable mainstream path coupled to first source means for controlling the first mainstream path, and having a first biased control input; and a second controllable semiconductor having a second controllable mainstream path coupled to second source means for controlling the second mainstream path, and having a second control input coupled to the first main stream path and to the amplifier input. Both the first and second mainstream paths are coupled to a common load, and the first and second source means are arranged for controlling input impedance and noise impedance respectively of the amplifier input. This amplifier arrangement allows independent control and optimisation of both the amplifier input impedance and the noise impedance.