US Patent 7570112 Ultrasound imaging or other use variable input impedance preamplifier

A preamplifier has a variable input impedance. The input impedance is tailored with the signal level to maintain a more optimal performance. The input impedance is varied by at least two amplifiers connected in parallel. Each amplifier has a different input impedance. By controlling the bias currents to the amplifiers, the contribution to the input impedance of the parallel amplifiers is controlled. Gradual variation in input impedance may be obtained by gradual variation in relative contribution by the different amplifiers.