This work presents a type of low noise amplifiers (LNA) that are used for ultrasound imaging systems. To account for the attenuating nature of ultrasound echo signals, a so-called time gain compensation (TGC) circuit is required. By increasing the gain over time, the output dynamic range is decreased, while the switching artifacts are suppressed by the continuous gain control.
The proposed work combines the LNA structure with TGC functionality. This is done, such that the first component in the ultrasound receive chain does not need to be able to handle the full dynamic range of the input signal. The goal is to reduce die area and power consumption costs compared to systems that utilize separate LNAs and TGCs. The combined TGC-LNA consists of a transimpedance amplifier (TIA) with an exponentially-varying feedback resistance. As the feedback network and feed-forward path are separately designed, the design of the TGC functionality and low noise functionality can for a large part be independently designed. The TGC functionality is implemented by an exponentially-varying feedback resistance. This is achieved by means of implementing triode transistors as voltage-controlled resistors. Three branches with differently sized triode devices are required to obtain the full gain range of 40 dB. A two-stage telescopic amplifier realizes the loop amplifier. The bias current of the first stage is a linear function of the total feedback resistance to create a constant unity-gain bandwidth in a power efficient method. Realized in 0.180 μm BCDMOS technology, the combined TGC-LNA amplifies the signals from a Capacitive Micromachined Ultrasound Transducer (CMUT) with a center frequency of 7.5 MHz. The total achieved gain range is 40 dB where the gain varies during a receive period of 100 μs. During the receive period the total harmonic distortion remains below -44 dB and the noise floor is 1.12 pA/√(Hz) at the highest gain setting. Drawing 5.5 mW from a 1.8 V supply and requiring approximately 0.01 mm² die area, the proposed TGC-LNA provides a new method for reducing power consumption and area costs for miniature ultrasound applications.