This article presents a high efficiency current-mode class-D (CMCD) amplifier using a GaN high electron mobility transistor (HEMT) for class-S system-a digitally controlled by deltasigma modulation. To enhance a drain efficiency of CMCD amplifier, the chip-on-board (COB) technique, which can reduce the external parasitic components of the packaged transistor and allow fast switching operation at high frequencies by minimizing distortion of the pulse waveform, is adopted. A LC-resonance circuit with adequate Q values between each transistor and output matching network controls harmonic signal by using the lumped elements to extract pure fundamental frequency of the output signal. For a high efficiency pushpull operation of CMCD amplifier, low-loss rat-race baluns are integrated at input and output circuits. From the measured results for a continuous wave of 3.35 GHz, the CMCD amplifier using COB technique shows a maximum output power of 35.88 dBm and a maximum power gain of about 10 dB. Also, the proposed CMCD amplifier provides a high power-added-efficiency (PAE) of 63.1% and a drain efficiency of 76.3% at maximum output power. The drain efficiency is still remained over 70% in the frequency range of 200 MHz.

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