Prospects and limitations of stacked-FET approaches for enhanced output power in voltage-controlled oscillators

This paper reports on the emerging potential of a stacked field-effect transistor (FET) approach with respect to maxium achievable RF output power with special remarks on high electron-mobility transistor (HEMT) based series feed-back oscillator monolithic microwave integrated circuits (MMICs). A stacked-FET oscillator can provide benefit in several ways - in the form of an improved RF output power, a high drain efficiency, or a stabilized oscillation behavior, even for changing load impedances. The limitation of the maximum number of transistors is analyzed with reference to the maximum achievable output power of stacked devices. This is done by describing the compromise between an increased voltage swing and a decreased RF output current. The output current is decreased since the current gain of a common gate device is smaller than unity. Based on the proposed theory a W-band stacked-FET voltage-controlled oscillator MMIC with an output power of 15 dBm and a drain efficiency of 23.3% was realized. The MMIC is based on the Fraunhofer IAF 50-nm gate-length metamorphic HEMT process. For an advanced evaluation of the power capability and the optimum target load of the utilized technology, an active W-band on-wafer load-pull system was implemented.