This work presents a low-noise amplifier (LNA) for miniature 3D ultrasound probes. Time gain compensation (TGC) is required to provide continuously variable gain and compensate for the attenuated echo signal, resulting in decreased output dynamic range (DR). As TGC is embedded in the LNA, a power-hungry LNA is no longer needed to handle the full dynamic range of attenuated echo signals. Compared to the prior art where TGC is applied after the LNA, this structure reduces die area and power consumption greatly.

The LNA with built-in TGC functionality is comprised of a transimpedance amplifier (TIA) with an exponentially increasing feedback resistive network. Since a transducer with a relatively high impedance is targeted, a TIA is utilized to interface with the transducer and sense the signal current. TGC is implemented in a continuous fashion by tunable resistors so as to alleviate imaging artifacts associated with gain-switching moments. The resistive feedback network is achieved by triode transistors with exponentially decreasing gate voltages. Three parallel branches of triode transistors are varied simultaneously to obtain a 40dB gain range. Each branch consists of two back-to-back triodes to mitigate non-linearity related to the body effect.


The variable-gain loop amplifier employing a current-reuse topology enables constant closed-loop bandwidth in an energy-efficient way. The first stage is a fixed-gain stage with dynamic biasing to save power at the lowest gain setting. The next two stages are variable-gain stages with variable resistive loads. The load resistor is implemented in the same fashion as the TIA's feedback resistor to achieve intrinsic gain matching. The last stage is a buffer to provide low output impedance for stability.

The LNA has been designed in 0.18 $\mu m$ CMOS technology and occupies an estimated die area of 0.0339 $mm^2$. The effective gain range is 40 dB with $\pm 1$ dB gain error. The LNA's noise floor at the highest gain is below 1.15 $pA/\sqrt{Hz}$ and its harmonic distortion is better than -40 dB. During 100 $\mu$s receive period, the total power consumption is 6 mW from a $\pm0.9$ V supply. The LNA featuring a small area and high power efficiency is a promising circuit for miniature 3D ultrasound probes.