A digital-feedback pre-distortion technique for integrated high-voltage ultrasound transmitting power amplifiers

Ultrasound systems require high-voltage (HV) transmitters to excite the transducers. Compared to digital pulsers, class-AB power amplifiers have many advantages, such as lower harmonics and the capability of generating complex arbitrary waveforms for Coded-Excitation Mode and other advanced ultrasound imaging modes. However, the need to suppress cross-over distortion and achieve high slew-rate usually requires large biasing current. This significantly degrades the power efficiency of the system, limiting the number of integrated transmitter channels in a single package. This paper presents a digital feedback pre-distortion system for ultrasound HV class-AB power amplifier (PA) to reduce the harmonic distortions while at the same time improving the power efficiency. The proposed system searches the error coefficients of the output feedback signal using the least-mean-square (LMS) algorithm, and stores the error data in the look-up table (LUT) memory. On the next cycle the errors are used to equalize the ideal input waveforms with the negative harmonic components to cancel the amplifier's non-linear response. The proposed algorithm applies to both constant magnitude sine-wave and the Morlet Wavelet. The measurement results show that the proposed system reduces the second-order harmonics (HD2) in the output sine-wave signals by 20 dB and the third-order harmonics (HD3) by 15 dB, while increasing the amplifier power efficiency by 18%. The proposed technique reduces the nonlinear higher-order effects of the PA to improve the ultrasound transmitting signal fidelity, which relieving the requirement on amplifier biasing current and supply voltage headroom.