I am looking for a circuit for a low capacitance FET circuit to act as a
Buffer/Amp for VHF signals (ie. to probe Oscillators etc). (Homebrew
Active Probe).

I have searched quite a bit on www but cannot locate a suitable design.

One starting point might be a homebrew active-FET oscilloscope probe
design shown by Robert Pease in "Troubleshooting Analog Circuits"
(page 16).

As shown, it has a bandwidth of 90 MHz, slew rate of 300 V/usec, input
Z of 10^11 ohms in parallel with 0.29 pF. Construction is "ugly"
style, with components soldered together in midair to minimize
parasitic capacitance to ground.

It uses a cascoded pair of 2N5486 or 2N5485 JFETs, driving a
2N3904/2N3906 buffer, and takes +/-15 VDC supply voltage.

I suspect that its bandwidth might be improved by using faster
bipolar-buffer transistors (maybe MPSH-something-or-other) and
possibly J310 or U310 JFETs for the front end.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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