A series-type Doherty amplifier circuit includes a first amplifier, a second amplifier, and a directional coupler. The first amplifier is of a first type and has an input for receiving a radio frequency input signal, and an output. The second amplifier is of a second type and has an input and an output. The directional coupler has a first terminal coupled to the output of the first amplifier, a second terminal coupled to the input of the second amplifier, and a third terminal coupled to the output of the second amplifier for providing a radio frequency output signal. The series-type Doherty amplifier circuit may also include variable phase and attenuation circuits for adjusting the phase and attenuation of input signal for the second amplifier. The ability to adjust phase and attenuation allows high operating efficiency for any saturation power level.