A 3–5GHz low power amplitude and spectrum tunable BPSK modulated impulse radio UWB transmitter is designed with SMIC 0.13 um CMOS technology. IR-UWB is a carrier-less system. It has the advantage of low power and low complexity, but it has difficulty in controlling the spectrum. In this design the amplitude and spectrum of the output impulse are both tunable to fix this problem. The simulation results indicate that the 3-step peak-to-peak voltage amplitudes are 480mV, 400mV and 270mV. The center frequency of the impulse can be tuned from 3.9GHz to 4.8GHz. A power controlled output buffer is implemented to drive the antenna. The total power consumption is only 3.7mA when transmitting a baseband signal of 100MHz.