Waveform Evidence of Gate Harmonic Short Circuit Benefits for High Efficiency X-Band Power Amplifiers

An active load pull waveform measurement system has been utilized to investigate, by drain current waveform analysis, the effect of short circuits at the second and higher harmonics on the gate of a 750 μm Gallium Arsenide transistor. It is confirmed that the half wave rectified sinusoidal current waveform necessary for modes of operation such as class B, J and F is not optimally generated at X-Band frequencies for transistors controlled purely by bias. The disrupting effect of the nonlinear device gate capacitance must be accounted for to take full advantage of these high efficiency modes. The addition of a simple wide band filter circuit at the input of a MMIC test cell to “short” the gate higher harmonics is shown to force the drain current waveform towards the ideal case and hence yield a significant increase in drain efficiency of up to 10 percent.