This paper presents two inverter-based lownoise, low-power, single-ended to differential switchedcapacitor variable gain amplifiers (SC-VGAs) for 2-6-MHz second harmonic cardiac ultrasound imaging probes fabricated with 0.18 lm complementary metal-oxidesemiconductor technology. By employing inverters in class C mode instead of operational trans-conductance amplifiers, the power consumption of 150 lW at a supply voltage of 1 V is achieved for both VGAs; -75 and -72 dBm of integrated noise with 2-6-MHz bandwidth at maximum gain and a sampling frequency of 30 MHz are achieved for VGA1 and VGA2, respectively. Both VGAs have two stages. VGA1 uses a fully switched-capacitor approach for gain control, and the gain range is -21-21 dB with 12-bit capacitor arrays. VGA2 adopts an inverter-based SC-VGA in the first stage with a 6-bit capacitor array to tune the gain from -9 to 9 dB, and the second stage is a differential amplifier with a 4-bit thermometer-coded resistor array to tune the gain from 0 to 13 dB. Both SC-VGAs complete the single-ended to differential conversion in the second stage to suppress the second harmonic distortion (HD2). Both VGAs have HD2 less than -50 dB. The die size of VGA1 is 245 lm 9 134 lm, and the die size of VGA2 is 109 lm 9 164 lm.
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.
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