An amplifier circuit comprising: a first amplifier configured to amplify a first input signal, wherein an output of the first amplifier is coupled to an output node via a first transmission line, the first transmission line comprising a first portion (13 1) having a first characteristic impedance and a first length, and a second portion (13 2) having a second characteristic impedance and a second length; a second amplifier configured to amplify a second input signal, wherein an output of the second amplifier is coupled to the output node via a second transmission line, the second transmission line comprising a first portion (23 1) having a first characteristic impedance and a first length, and a second portion (23 2) having a second characteristic impedance and a second length; and an auxiliary amplifier configured to amplify a third input signal, wherein an output of the auxiliary amplifier is coupled to an auxiliary transmission line network comprising: a first auxiliary transmission line (33 1) coupled between an auxiliary junction and a first intersection between the first and second portions (13 1, 13 2) of the first transmission line; a second auxiliary transmission line (33 2) coupled between the auxiliary junction and a second intersection between the first and second portions (23 1, 23 2) of the second transmission line; and a third auxiliary transmission line (33 3) for coupling the output of the auxiliary amplifier to the auxiliary junction; wherein for each of the first and second transmission lines (13, 23,) the first and second portions consist of a higher-impedance portion and a lower-impedance portion whose lengths are substantially equal; and wherein the lengths of the respective first portions (13 1, 23 1) of the first and second transmission lines (13, 23) sum to half a wavelength, and the lengths of the respective second portions (13 2, 23 2) of the first and second transmission lines (13, 23) sum to half a wavelength. a first amplifier configured to amplify a first input signal, wherein an output of the first amplifier is coupled to an output node via a first transmission line, the first transmission line comprising a first portion (13 1) having a first characteristic impedance and a first length, and a second portion (13 2) having a second characteristic impedance and a second length; a second amplifier configured to amplify a second input signal, wherein an output of the second amplifier is coupled to the output node via a second transmission line, the second transmission line comprising a first portion (23 1) having a first characteristic impedance and a first length, and a second portion (23 2) having a second characteristic impedance and a second length; and an auxiliary amplifier configured to amplify a third input signal, wherein an output of the auxiliary amplifier is coupled to an auxiliary transmission line network comprising: a first auxiliary transmission line (33 1) coupled between an auxiliary junction and a first intersection between the first and second portions (13 1, 13 2) of the first transmission line; a second auxiliary transmission line (33 2) coupled between the auxiliary junction and a second intersection between the first and second portions (23 1, 23 2) of the second transmission line; and a third auxiliary transmission line (33 3) for coupling the output of the auxiliary amplifier to the auxiliary junction; a first auxiliary transmission line (33 1) coupled between an auxiliary junction and a first intersection between the first and second portions (13 1, 13 2) of the first transmission line; a second auxiliary transmission line (33 2) coupled between the auxiliary junction and a second intersection between the first and second portions (23 1, 23 2) of the second transmission line; and a third auxiliary transmission line (33 3) for coupling the output of the auxiliary amplifier to the auxiliary junction; wherein for each of the first and second transmission lines (13, 23,) the first and second portions consist of a higher-impedance portion and a lower-impedance portion whose lengths are substantially equal; and wherein the lengths of the respective first portions (13 1, 23 1) of the first and second transmission lines (13, 23) sum to half a wavelength, and the lengths of the respective second portions (13 2, 23 2) of the first and second transmission lines (13, 23) sum to half a wavelength.