Active Probes

An active probe is a useful piece of test equipment that lets you use a low input impedance piece of test equipment, such as a spectrum analyzer, to examine a high impedance circuit without loading the circuit with the test equipment's impedance.

I built and tested three different designs that I found on the internet.

YouTube episodes can be found at these links:

Episode #1

Episode #2

The three probe designs that I built and tested are:

  • The "High Input Z 'scope Probe" by Bob Pease
  • The "Poor Man's 1GHz Probe" by David Jewsbury
  • The "High Impedance Op Amp FET Probe" by Steve Hageman

None of these three are my own design creation, all that I did was re-package the information on the internet into a very small form factor.  Plus I designed and 3D printed my own probe plastic parts.  So the stuff that I created, I am making available below.  You will need to do your own internet searching to find the source materials.

Bob said you can use the 2N5485 or 5486 for the input FET's.  Don't make the mistake that I did, and use one of each!  Use two of ONE TYPE in this design.  Otherwise you won't be able to zero the DC offset.

The dual gate BF998 FET is not in production anymore.  So it's buyer beware on finding some online.  I bought some off of eBay and lucked out, mine seem to work OK.

You'll have to set the DC offset on the Bob Pease probe before stuffing the board inside the tube.

Here are links to the project files.  The 'Mechanical Parts' zip file includes the three .stl files for the end caps plus twist cap, and a drawing for cutting the tubing and probe tip.  The LTSpice zip file includes the Spice models for all three circuits plus the .lib files of custom components.  Cautionn on the Spice files - they did not simulate very closely at all to the actual performance.  Enjoy!

Bob Pease KiCad files

Poor Man KiCad files

Op Amp KiCad files

Mechanical files

LTSpice Sims






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