Shown on this slide is a Bodi plot, covering the frequency range of DC to 10GHz, comparing frequency response and frequency versus impedance plots. The red trace is a passive probe, can be seen to have a very steep roll off at the higher frequencies. The blue and dark blue traces are active probe technologies which do not decrease in impedance as fast as the passive probe. This shows that passive probes provide the highest impedance below 20KHz while the transistors are good at high frequencies, with good input resistance and circuit loading.