A Doherty amplifier, comprising: a first transistor configured to amplify a first signal received from a signal source and output an amplified first signal; a second transistor configured to amplify a second signal received from the signal source and output an amplified second signal; and a combining circuit configured to combine the amplified first signal output from the first transistor and the amplified second signal output from the second transistor, and output a combined signal of the amplified first signal and the amplified second signal; wherein the signal source is configured to divide a communication signal into the first and second signal, to switch an operation mode of the Doherty amplifier to a Doherty operation mode or an outphasing operation mode in accordance with a frequency of the communication signal, said frequency of the communication signal being equal to a frequency of the first signal and a frequency of the second signal, and to output the first and second signal such that each of the first and second signal have an amplitude and phase corresponding to the operation mode of the Dohery amplifier, wherein the Doherty amplifier is configured such that the operation mode is a first Doherty operation mode when the amplitude of the first signal is equal to or larger than the amplitude of the second signal, a difference between the phase of the first signal and the phase of the second signal is constant and the first and second amplified signal are combined in phase, and wherein the Doherty amplifier is configured such that the operation mode is a first outphasing operation mode when the amplitude of the first signal is the same as the amplitude of the second signal, the phase of the first signal is opposite to the phase of the second signal, the first and second amplified signal are combined out of phase and a current of a combined signal output from the combining circuit monotonically decreases with an increase in the difference between the phase of the first signal and the phase of the second signal. a first transistor configured to amplify a first signal received from a signal source and output an amplified first signal; a second transistor configured to amplify a second signal received from the signal source and output an amplified second signal; and a combining circuit configured to combine the amplified first signal output from the first transistor and the amplified second signal output from the second transistor, and output a combined signal of the amplified first signal and the amplified second signal; wherein the signal source is configured to divide a communication signal into the first and second signal, to switch an operation mode of the Doherty amplifier to a Doherty operation mode or an outphasing operation mode in accordance with a frequency of the communication signal, said frequency of the communication signal being equal to a frequency of the first signal and a frequency of the second signal, and to output the first and second signal such that each of the first and second signal have an amplitude and phase corresponding to the operation mode of the Dohery amplifier, wherein the Doherty amplifier is configured such that the operation mode is a first Doherty operation mode when the amplitude of the first signal is equal to or larger than the amplitude of the second signal, a difference between the phase of the first signal and the phase of the second signal is constant and the first and second amplified signal are combined in phase, and wherein the Doherty amplifier is configured such that the operation mode is a first outphasing operation mode when the amplitude of the first signal is the same as the amplitude of the second signal, the phase of the first signal is opposite to the phase of the second signal, the first and second amplified signal are combined out of phase and a current of a combined signal output from the combining circuit monotonically decreases with an increase in the difference between the phase of the first signal and the phase of the second signal.