Dave,

Thanks for the tip. I don't want to purchase an entire book for one
circuit though.

I do have some BF998 Dual Gate Mosfets, maybe I can make up a circuit
based on these.

I need around 1pF input capacitance and 1 Meg Ohm, Frequency up to a
couple of hundred MHz.

Regards

David

Dave Platt wrote:
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.