This paper is aimed to investigate and resolve the odd-mode instability of a hybrid high-frequency power amplifier (PA) based on the fT-doubler technique. A description of odd-mode stability based on pole-zero identification is performed to provide insight into the causes of the odd-mode oscillation. The unwanted right half poles (RHPs) at frequencies less than the unity gain frequency, resulting from the circuit configuration, corrupt the PA operation. Reducing the length of the interconnects and tuning the values of the circuit elements is a conventional approach to get rid of the RHPs. However, in practice, in hybrid technology, there is poor control over interconnects. Besides, changing the circuit elements to prevent the odd-mode instability results in performance degradations such as a reduction in power gain, output power, power added efficiency (PAE), and unity current gain frequency (fT). A proposed fT-doubler PA using Wilkinson combiner with a calculated odd-mode resistor is exploited to overcome the odd-mode oscillation and relax the trade-off between odd-mode stability and performance of the PA. The measurement of the modified fT-doubler GaN-HEMT PA with Wilkinson combiner demonstrates a broadband PA with a stable behavior.

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