This article presents a low-noise transimpedance amplifier (TIA) designed for miniature ultrasound probes. It provides continuously variable gain to compensate for the time-dependent attenuation of the received echo signal. This time-gain compensation (TGC) compresses the echo-signal dynamic range (DR) while avoiding imaging artifacts associated with discrete gain steps. Embedding the TGC function in the TIA reduces the output DR, saving power compared to prior solutions that apply TGC after the low-noise amplifier. The TIA employs a capacitive ladder feedback network and a current-steering circuit to obtain a linear-in-dB gain range of 37 dB. A variable-gain loop amplifier based on current-reuse stages maintains constant bandwidth in a power-efficient manner. The TIA has been integrated in a 64-channel ultrasound transceiver application-specific integrated circuit (ASIC) in a 180-nm BCDMOS process and occupies a die area of 0.12 mm2. It achieves a gain error below ±1 dB and a 1.7 pA/ √ Hz noise floor and consumes 5.2 mW from a ±0.9 V supply. B-mode images of a tissue-mimicking phantom are presented that show the benefits of the TGC scheme.

Miniature ultrasound probes, such as the intra-cardiac echography (ICE) probe shown in Fig. 23.6.1, increasingly employ in-probe ASICs to interface with the elements of an ultrasound transducer array to improve signal quality and reduce cable count [1]-[4]. For each transducer element, such an ASIC contains a pulser that drives the element to generate a pressure wave, a low-noise amplifier (LNA) that amplifies the resulting echo signal, and a time-gain compensation (TGC) circuit that compensates for the time-varying echo-signal amplitude due to propagation attenuation of the acoustic wave. Without TGC, the first echoes, from shallow tissue, are much larger than later echoes from deeper tissue. The TGC circuit corrects for this, ideally by providing a gain that increases exponentially with time, thus reducing the dynamic range (DR) by as much as 40dB and strongly relaxing the requirements of subsequent blocks. In conventional ultrasound systems, TGC is typically performed after the LNA, implying that a power-hungry LNA is required that can handle the full DR of the echo signal [5], [6]. In recent in-probe ASICs, programmable-gain LNAs have been employed that provide a step-wise TGC approximation [1]-[3]. While this saves power, the associated gain-switching transients lead to imaging artefacts. In this paper, we present an LNA with a built-in continuous TGC function that mitigates this problem. The LNA is a transimpedance amplifier (TIA) optimized to amplify the signal current of a capacitive micro-machined ultrasound transducer (CMUT). We demonstrate its integration into a 64-channel ASIC for a CMUT-based ICE probe.