In this paper, a very low power and low complexity all digital impulse radio ultra wideband (IR-UWB) transmitter is presented. The pulse repeating frequency (PRF) of the transmitted signal is controlled digitally, which complies with federal commission committee (FCC) regulations. In order to control the transmitted power the amplitude of transmitted signals is reduced by increasing the PRF. The transmitter can be used with both binary phase shift keying (BPSK) and pulse position modulation (PPM) schemes for modulation. We also propose a new fourth order derivative of Gaussian pulse generator circuit. The proposed signal used as impulse transmitted signal. This transmitter was designed in a 0.18 μm CMOS technology, the core chip size was only 0.025 mm . Simulated results showed that the transmitter consumed 7.2~18 pJ/pulse to achieved 2 50~400 MHz PRF. The output amplitude pulse yielded 60~350 mV peak to peak under a supply voltage of 1.8 V.