Active bias control for improved pulse droop performance of GaN HEMT transistor

An observation on using GaN HEMT transistors for pulsed radar applications is that the devices exhibit rather high pulse droop when RF input power is backed off from saturated output power levels. A novel active control circuit has been developed to supply additional gate bias voltage to the transistor during the `on-time´ of the RF pulse to counteract this droop phenomenon. The added gate bias voltage is incrementally increased as a function of an RC time constant to provide higher added gate voltage at the end of the pulse compared to the beginning of the pulse. Implementing this active control significantly reduced the pulse droop over a wide dynamic range of RF input drive levels without significantly impacting the overall RF performance. This paper will briefly discuss the development of the active bias control circuit and its incorporation into the RF test circuit for a GaN power transistor.