Inverter-based low-noise, 150 AμW single-ended to differential SC-VGAs for second harmonic cardiac ultrasound imaging probes

This paper presents two inverter-based low-noise, low-power, single-ended to differential switched-capacitor variable gain amplifiers (SC-VGAs) for 2-6-MHz second harmonic cardiac ultrasound imaging probes fabricated with 0.18 A mu m complementary metal-oxide-semiconductor technology. By employing inverters in class C mode instead of operational trans-conductance amplifiers, the power consumption of 150 A mu W 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 A mu m x 134 A mu m, and the die size of VGA2 is 109 A mu m x 164 mu m.