A digitally controlled sonar power amplifier for underwater transmitter is explained in this paper. It proposes a method of generating unipolar Sine Pulse Width Modulation (SPWM) signals for echo ranging sonar. This method can be effectively used in all kinds of inverters including high frequency inverters. A low cost digital signal controller was used to generate SPWM signals and used as gating pulses for a Controlled Full Bridge Circuit (CFBC). The power required, frequency, pulse-width and pulse repetition interval are given as the inputs to the digital controller and gating pulses for the CFBC are generated as output from the controller. A power transformer connected across CFBC boosts output voltage followed by power filter which removes unwanted harmonics present in the output. The output after the power filter is used to excite electro-acoustic transducer elements underwater. The voltage and current of the load were given as feedback to the same controller for voltage mode control loop implementation of the system. A prototype of 1kW Switch Mode Power Amplifier (SMPA) for echo ranging sonar has been designed and digital control has been implemented. Salient results are presented.