Schematic for Low Capacitance Fet Buffer
 

Hi,

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.

Thanks in advance

Regards

David

Schematic for Low Capacitance Fet Buffer
 

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Schematic for Low Capacitance Fet Buffer
 

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.

Schematic for Low Capacitance Fet Buffer
 

David wrote:

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

These specs are probably rather optimistic, in my opinion. I would
probably consider something like a large resistor in series with the
input of a cascode amplifier (cascode chosen to reduce effective input
capacitance due to feedback). Or perhaps a resistive voltage divider,
rather than just a series resistor, in an attempt to get flat response
over frequency. But there really is no such thing as a 1 Meg resistor
at VHF. The stray capacitance across resistors limits the maximum
achievable impedance to perhaps tens of kilohms.

However, I'm not expert on such circuits. You might try posting the
question to sci.electronics.design. There are some very clever
electronic designers there (along with a number who get their jollies
from insulting others).

Steve VE3SMA

Schematic for Low Capacitance Fet Buffer