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A novel pulsed class-B load-pull measurement system is developed to characterize GaN HEMTs targeting the design of high-efficiency class-B or class-C power amplifiers operating under a pulsed-bias and pulsed-RF (pulsed-IV/RF) condition. Based on a large-signal network analyzer, the test system uses an active load-pull method to provide stable open-loop pulsed-RF loads into the drain at omega0 and 2omega0 while bypassing slow-memory effects. The load-pull measurement data obtained from AlGaN/GaN HEMTs under the class-B operation reveal that there exist optimal loads for pulsed-IV/RF condition, which differ from the ones found for a dc-IV and continuous wave condition. This is due to the avoidance of slow-memory effects in the pulsed-IV/RF load-pull measurements, which are known to degrade the device RF performance: a 2-dB increase in output power is obtained for a GaN HEMT on sapphire. The optimized pulsed-RF active load for a GaN HEMT on SiC demonstrates a power-added efficiency of 82% with 17.8-dBm output power under quasi class-B pulsed operation at 2 GHz.