An efficiency-enhanced multiband power amplifier (PA) driver is proposed. It introduces a self-biasing circuit and improved switch-capacitor banks to enhance the efficiency. The presented PA driver has been implemented in 65 nm CMOS technology. Measurement results demonstrate a broad frequency coverage of 0.7-5.6 GHz and a flat gain within 2 dB variation. The PA driver has an output power above 6 dBm over the broadband, and it consumes a comparatively low power of 38 mA from a 2.5 V supply.