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Old June 11th 06, 12:45 PM posted to rec.radio.amateur.homebrew
David
 
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Default 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
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Old June 11th 06, 07:23 PM posted to rec.radio.amateur.homebrew
Dave Platt
 
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Default 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
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Old June 12th 06, 02:09 AM posted to rec.radio.amateur.homebrew
David
 
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Default 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.

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Old June 12th 06, 05:21 PM posted to rec.radio.amateur.homebrew
 
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Default 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

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Old June 12th 06, 09:11 PM posted to rec.radio.amateur.homebrew
Roy Lewallen
 
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Default Schematic for Low Capacitance Fet Buffer

wrote:
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.


The "totem pole" follower (a FET source follower with a matched FET as
the source "resistor") was widely used by Tektronix. It provides flat,
near unity gain over a very wide bandwidth, with high input resistance
and low input capacitance. It would be very difficult to make a cascode
amplifier with response that flat.

The 1 meg ohm resistor is to establish the DC and low frequency
impedance. The unavoidable shunt C from the FETs and other sources makes
it immaterial at high frequencies. I agree that 1 pF is optimistic -- it
would be very difficult to achieve. You would probably have to do some
bootstrapping to reduce the C to that level, if you can do it at all,
and bootstrapping becomes more difficult as frequency increases.

. . .


Roy Lewallen, W7EL
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