This brief presents an inverter-based low-noise single-ended to differential continuous-time variable gain amplifier (VGA) for 2-6-MHz second harmonic cardiac ultrasound imaging probes. The proposed VGA is based on a transimpedance amplifier and composed of resistive arrays and three equal inverters to form the resistive feedback loop. The inverters operate in the class C mode by being biased in the subthreshold region to improve both the power and noise performances of the VGA. The 6-bit thermometer resistive arrays can control the gain range from -1 to 21 dB for a 2-D piezoelectric transducer (PZT) model, and for a 50-Ω source resistor, the gain range is from 20 to 26 dB. The second harmonic distortion (HD2) is lower than -55 dB in most cases due to the single-ended to differential conversion, and the equivalent input referred noise is 7 nV/√Hz at 5 MHz with the maximum gain. The VGA is fabricated in 65-nm CMOS technology and has the power consumption of 54 μW at a supply voltage of 0.5 V. The die size of the VGA is 154 μm × 102 μm.