0.5 V inverter‐based ultra‐low‐power, low‐noise VGA for medical ultrasound probes

An inverter‐based ultra‐low‐power, low‐noise, single‐ended to differential continuous‐time variable gain amplifier is presented for 2–6 MHz second harmonic cardiac imaging ultrasound probes in a 65 nm CMOS technology. The proposed variable gain amplifier (VGA) consists of three equal inverters and resistor arrays which form a feedback loop. To improve both the power and noise performances, the inverters operate in the sub‐threshold region by adopting a 0.5 V supply voltage. By doubling the input transconductance of the VGA, the noise figure (NF) is enhanced further. The 6‐b thermometer resistor arrays achieve a gain range of the VGA from 0 to 22 dB. The total power consumption is 55 μW, NF is 2.7 dB referred to an 8 kΩ source resistor at a centre frequency of 4 MHz. 8‐b thermometer calibration codes are added into the inverters to force the static operating point of the inverters to half of the supply voltage, which could increase the immunity of the second harmonic distortion (HD2) to the process variation. The HD2 of the proposed VGA is −60.6 dB at the 330 mV peak‐to‐peak output swing. The active size is 154 × 102 μm.