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 (31; 31 1, 31 2) configured to amplify a third input signal, wherein an output of the auxiliary amplifier (31; 31 1, 31 2) is coupled via an auxiliary transmission line network (33 1, 33 2; 33 3, 33 4) to a first intersection between the first and second portions (13 1, 13 2) of the first transmission line, and to a second intersection between the first and second portions (23 1, 23 2) of the second transmission line; wherein for each of the first and second transmission lines (13, 23), the first and second portions (13 1, 13 2; 23 1, 23 2) consist of a higher-impedance portion and a lower-impedance portion whose combined length is a half wavelength; 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 (31; 31 1, 31 2) configured to amplify a third input signal, wherein an output of the auxiliary amplifier (31; 31 1, 31 2) is coupled via an auxiliary transmission line network (33 1, 33 2; 33 3, 33 4) to a first intersection between the first and second portions (13 1, 13 2) of the first transmission line, and to a second intersection between the first and second portions (23 1, 23 2) of the second transmission line; wherein for each of the first and second transmission lines (13, 23), the first and second portions (13 1, 13 2; 23 1, 23 2) consist of a higher-impedance portion and a lower-impedance portion whose combined length is a half wavelength; 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.