Power Amplifier Linearizer for High Frequency Medical Ultrasound Applications.

Power amplifiers (PAs) are used to produce high-voltage excitation signals to drive ultrasonic transducers. A larger dynamic range of linear PAs allows higher contrast resolution, a highly desirable characteristic in ultrasonic imaging. The linearity of PAs can be improved by reducing the nonlinear harmonic distortion components of high-voltage output signals. In this paper, a linearizer circuit is proposed to reduce output signal harmonics when working in conjunction with a PA. The PA performance with and without the linearizer was measured by comparing the output power 1-dB compression point (OP1dB), and the second- and third-order harmonic distortions (HD2 and HD3, respectively). The results show that the PA with the linearizer circuit had higher OP1dB (31.7 dB) and lower HD2 (-61.0 dB) and HD3 (-42.7 dB) compared to those of the PA alone (OP1dB (27.1 dB), HD2 (-38.2 dB), and HD3 (-36.8 dB)) at 140 MHz. A pulse-echo measurement was also performed to further evaluate the capability of the linearizer circuit. The HD2 of the echo signal received by the transducer using a PA with the linearizer (-24.8 dB) was lower than that obtained for the PA alone (-16.6 dB). The linearizer circuit is capable of improving the linearity performance of PA by lowering harmonic distortions.