DIY Active Oscilloscope Probe

A passive 10x probe attenuates signals before the scope's input impedance — this works but costs signal amplitude. An active probe buffers the signal with a high-impedance JFET amplifier at the probe tip, giving true 10M-ohm+ input impedance right at the measurement point.

The circuit is simple: an N-channel JFET source follower (2N5486 works well) at the probe tip, powered by a small ±5V supply derived from the scope's power rail through the BNC connector. The JFET gate goes to the probe tip; the source drives the coax to the scope with a 50-ohm series resistor.

This is particularly useful for measuring high-impedance nodes like MOSFET gates during switching transitions — a passive probe's 10pF cable capacitance loads the gate and alters the switching behavior you're trying to observe.