Abstract: Methods and devices for improving power added efficiency in multi-stage power amplifiers. The disclosed techniques implement harmonic termination at the driver amplifier while accounting for parasitic elements. The described teachings improve power added efficiency, enhance reliability through reduced peak voltages, and maintain or improve linearity. The techniques are effective at higher operating frequencies and can be applied to both single-ended and differential amplifier architectures. In one embodiment, a multi-stage power amplifier system includes a driver amplifier (DA) stage coupled through an interstage matching network to a power amplifier stage. A DA harmonic termination circuit provides an approximate short circuit at second harmonic frequencies and an approximate open circuit at third harmonic frequencies.

Abstract: In a portable radio transceiver, a power amplifier system includes a load line switch that operates in response to a mode control signal to promote high efficiency in both a high-power mode and a low-power mode. The switching circuit is operable to couple the inductance to the shunt capacitance in response to a control signal indicating operation in a low-power mode. The switching circuit is further operable to decouple the inductance from the shunt capacitance in response to a control signal indicating operation in a high-power mode. In the low-power mode, the inductance couples with the shunt capacitance to decrease total capacitance at the output of the power amplifier. In the high-power mode, the total capacitance at the output of the power amplifier is substantially equal to the shunt capacitance alone, and the inductance essentially has no effect on total capacitance at the power amplifier output.