A Doherty amplifier , comprising: a main amplifier and a peak amplifier ; a first microstrip with λ/4 electric length connected between the main amplifier and the peak amplifier ; a second microstrip with λ/4 electric length connected between a junction of outputs of the peak amplifier and the main amplifier , and an output terminal ; and at least one tuner connected, in series with the first microstrip , between the main amplifier and the peak amplifier , wherein the at least one tuner comprises a third microstrip connected with the first microstrip in series, wherein the at least one tuner is configured to adjust a radius of Voltage Standing Wave Ratio, VSWR, circle of the main amplifier based on at least a characteristic impedance (Z1) of the third microstrip , wherein the at least one tuner comprises a switch unit for controlling the third microstrip as a shunt, characterised in that , the characteristic impedance (Z1) and an electric length (θ) of the third microstrip are based on a distance between a load impedance of the main amplifier at a high signal condition (m1) and the best efficiency point at a back-off power level (m3), wherein the characteristic impedance (Z1) of the third microstrip is different from that of the first microstrip , wherein the VSWR radius of the main amplifier equals Zmt*Zload/ZcT 2 where Zmt represents the characteristic impedance of the first microstrip , ZcT represents the characteristic impedance of the second microstrip , and Zload represents a load impedance of the Doherty amplifier , and wherein if the distance is larger than the VSWR radius of the main amplifier , the characteristic impedance (Z1) of the third microstrip is taken a value lower than that of the first microstrip ; and if the distance is smaller than the VSWR radius of the main amplifier , the characteristic impedance (Z1) of the third microstrip is taken a value larger than that of the first microstrip . a main amplifier and a peak amplifier ; a first microstrip with λ/4 electric length connected between the main amplifier and the peak amplifier ; a second microstrip with λ/4 electric length connected between a junction of outputs of the peak amplifier and the main amplifier , and an output terminal ; and at least one tuner connected, in series with the first microstrip , between the main amplifier and the peak amplifier , wherein the at least one tuner comprises a third microstrip connected with the first microstrip in series, wherein the at least one tuner is configured to adjust a radius of Voltage Standing Wave Ratio, VSWR, circle of the main amplifier based on at least a characteristic impedance (Z1) of the third microstrip , wherein the at least one tuner comprises a switch unit for controlling the third microstrip as a shunt, characterised in that , the characteristic impedance (Z1) and an electric length (θ) of the third microstrip are based on a distance between a load impedance of the main amplifier at a high signal condition (m1) and the best efficiency point at a back-off power level (m3), wherein the characteristic impedance (Z1) of the third microstrip is different from that of the first microstrip , wherein the VSWR radius of the main amplifier equals Zmt*Zload/ZcT 2 where Zmt represents the characteristic impedance of the first microstrip , ZcT represents the characteristic impedance of the second microstrip , and Zload represents a load impedance of the Doherty amplifier , and wherein if the distance is larger than the VSWR radius of the main amplifier , the characteristic impedance (Z1) of the third microstrip is taken a value lower than that of the first microstrip ; and if the distance is smaller than the VSWR radius of the main amplifier , the characteristic impedance (Z1) of the third microstrip is taken a value larger than that of the first microstrip . <EOS>