Conditions of half-frequency oscillation in high-power amplifier with FET and its suppression method using quarter-wavelength open-circuited stub

With regard to the half-frequency oscillation observed in the nonlinear operation of a microwave power amplifier based on FETs, the voltage and current of the FET in a large-signal mode at an excitation frequency of 2f₀ are expanded as Fourier series in half this frequency, that is, f₀. Then, the condition that the Fourier coefficient for the f₀ component is nonzero, namely, the condition of half-frequency oscillation, is derived. It is shown that the half-frequency oscillation takes place when the determinant is zero for the matrix defined by the fundamental excitation components and time averages of the gate-to-source capacitance, mutual conductance, and drain conductance, as well as the load impedance condition at frequency f₀. Also, a method for suppression of the half-frequency oscillation by connecting an open stub one-quarter wavelength long at the half-frequency to the output side of the FET is proposed, and its effectiveness is confirmed by experiment.