As stated previously active probes are based on transistors. Due to the very high input resistance and very low capacitance active probes are very useful for high frequency applications. The output impedance of the active probe must be matched to the oscilloscope, at 50 Ω. When the active probe is connected to the oscilloscope it automatically selects the coupling to 50 Ω. In the 50MHz to 1GHz range it is important to keep capacitance low, typically the active probe is a very low 0.5pF to 1.5pF.