Stacked Transistor Bias Circuit of Class-B Amplifier for Portable Ultrasound Systems

The performance of portable ultrasound systems is affected by the excessive heat generated by amplifiers, thereby reducing the sensitivity and resolution of the transducer devices used in ultrasound systems. Therefore, the amplifier needs to generate low amounts of heat to stabilize portable ultrasound systems. To properly control the amplifier, the related bias circuit must provide proper DC bias voltages for long time periods in ultrasound systems. To this end, a stacked transistor bias circuit was proposed to achieve a relatively constant amplifier performance irrespective of temperature variance without any cooling systems as the portable ultrasound system structure is limited. To prove the proposed concept, the performance of the gain and DC current consumption at different experimental times was measured and compared to a developed class-B amplifier with different bias circuits. The amplifier with the stacked transistor bias circuit outperformed with regard to the gain and DC current variance versus time (-0.72 dB and 0.065 A, respectively) compared to the amplifier with a typical resistor divider bias circuit (-5.27 dB and 0.237 A, respectively) after a certain time (5 min). Consequently, the proposed stacked transistor bias circuit is a useful electronic device for portable ultrasound systems with limited structure sizes because of its relatively low gain and DC current variance with respect to time.