| Capacitive Micromachined Ultrasound Transducers (CMUTs) promise high transducer performance for several ultrasound applications. When using the CMUT array for medical applications where a focused ultrasound image with a 90 degree image sector is needed, we need a large number of individual elements. In off-axis beam steering, neighbor elements operate at different phase. This leads to unwanted acoustic effects caused by the interaction with the fluid medium outside the array. We see high-Q resonances close to the center frequency of the array at off-axis angles, which we want to reduce. We propose to use Transimpedance Amplifiers (TIAs) and Charge Sampling Amplifiers (CSAs) where we can easily adjust the input impedance, which opens up the possibility to design amplifiers that are optimized for an ultrasound system with CMUTs. Simulations show that a low impedance path results in suppression of the effects of resonances for both CSAs and TIAs and that co-optimization is important since the frequency of the CMUT array affects the Q-factor of the unwanted resonances. Even though we introduce an impedance mismatch the noise figure is still at an acceptable level. We present simulations in water, blood plasma with estimated data, and olive oil and show that the viscosity of the medium greatly influences the presence of resonances. This indicates that effects that might be present in human tissue may be much reduced in olive oil or other vegetable oils. |