An active transformerless common mode filter is designed for use with switching mode power supplies with a switching frequency greater than 1MHz. The filter utilizes an amplifier in a voltage sensing current adjusting architecture to cancel out common mode noise generated by the switching power supply. The filter is analyzed using a transfer function, simulation, and measurements. Several possible feedback configurations are examined and benefits of different configurations are explained. The active filter is shown to have superior performance to only passive components at frequencies up to 20MHz. Spectral domain, time domain, and S21 measurements are given to show the filter’s effectiveness.