A circuit comprising; a transistor having an input electrode; an input matching network having an input for being fed by an input signal and having transistor an output connected to the input electrode of the transistor; and a power level sensing circuit for being fed by the input signal, wherein the input matching network is responsive to the power level sensing circuit to configure the input matching network with a first input impedance when the power level sensing circuit senses that the input signal has a first power level, and to configure the input matching network with a second input impedance different from the first input impedance when the power level sensing circuit senses that the input signal has a second power level, the said first power level being lower than the said second power level, and characterised in that: the transistor is a gallium nitride transistor having a field plate or gamma gate, the input matching network includes: i) a pair of electrical components (L1, L2); and ii) at least one switch (20, 22), the at least one switch (20, 22) is configured to operate in response to the power level sensing circuit to: i) electrically decouple one of the pair of electrical components (L1, L2) from the input matching network at said second power level; and ii) electrically couple said one of the pair of electrical components (L1, L2) to the input matching network at said first power level, and the first input impedance is matched for a small-signal input signal and the second input impedance is rotated clockwise between about 10 degrees and about 15 degrees from the first impedance on the Smith chart; the first input impedance corresponding to the first power level at which the transistor exhibits soft compression in the power transfer characteristic and the second input impedance corresponding to the second power level at which the soft compression is eliminated or significantly reduced. a transistor having an input electrode; an input matching network having an input for being fed by an input signal and having transistor an output connected to the input electrode of the transistor; and a power level sensing circuit for being fed by the input signal, wherein the input matching network is responsive to the power level sensing circuit to configure the input matching network with a first input impedance when the power level sensing circuit senses that the input signal has a first power level, and to configure the input matching network with a second input impedance different from the first input impedance when the power level sensing circuit senses that the input signal has a second power level, the said first power level being lower than the said second power level, and characterised in that: the transistor is a gallium nitride transistor having a field plate or gamma gate, the input matching network includes: i) a pair of electrical components (L1, L2); and ii) at least one switch (20, 22), the at least one switch (20, 22) is configured to operate in response to the power level sensing circuit to: i) electrically decouple one of the pair of electrical components (L1, L2) from the input matching network at said second power level; and ii) electrically couple said one of the pair of electrical components (L1, L2) to the input matching network at said first power level, and the first input impedance is matched for a small-signal input signal and the second input impedance is rotated clockwise between about 10 degrees and about 15 degrees from the first impedance on the Smith chart; the first input impedance corresponding to the first power level at which the transistor exhibits soft compression in the power transfer characteristic and the second input impedance corresponding to the second power level at which the soft compression is eliminated or significantly reduced. the transistor is a gallium nitride transistor having a field plate or gamma gate, the input matching network includes: i) a pair of electrical components (L1, L2); and ii) at least one switch (20, 22), i) a pair of electrical components (L1, L2); and ii) at least one switch (20, 22), the at least one switch (20, 22) is configured to operate in response to the power level sensing circuit to: i) electrically decouple one of the pair of electrical components (L1, L2) from the input matching network at said second power level; and ii) electrically couple said one of the pair of electrical components (L1, L2) to the input matching network at said first power level, and i) electrically decouple one of the pair of electrical components (L1, L2) from the input matching network at said second power level; and ii) electrically couple said one of the pair of electrical components (L1, L2) to the input matching network at said first power level, and the first input impedance is matched for a small-signal input signal and the second input impedance is rotated clockwise between about 10 degrees and about 15 degrees from the first impedance on the Smith chart; the first input impedance corresponding to the first power level at which the transistor exhibits soft compression in the power transfer characteristic and the second input impedance corresponding to the second power level at which the soft compression is eliminated or significantly reduced.