wrote in message
...
On Feb 20, 10:20 am, "amdx" wrote:
Hi all,
I finished the amp that had the 5 Ghz transistor, I changed it to a slower
one.
The objective of this amp is to cause minimal loading of the circuit it is
measuring.
When I install the box cover the voltage gain drops by 7%, so I think the
input capacitor
plate is being loaded by the cover.
The input capacitor plates can be seen here;

http://i395.photobucket.com/albums/p...mspaced5mm.jpg

The plates are 1 cm x 1 cm spaced 5 mm apart.

I have thoughts about rectangular plates 0.25 cm x 4 cm to get more
distance from the top cover, (and the bottom.)
Or a real gimmick cap where I twist a couple of 39 Gauge wires together
and
attach opposite ends to input and output.

Any ideas to minimize input capacitance to the box?

Here's the amp in
box.http://i395.photobucket.com/albums/p...erampinbox.jpg

This is the original circuit page with
schematic;http://www.crystal-radio.eu/enfetamp.htm
Thanks, Mike

PS, I was having trouble getting some close-up pictures, I grabbed a
magnifying glass and took some
pictures through that, works good.
........................................
The old-tyme gimmick was two wires, twisted together. That would
contain the field better than your open plates.

Another alternative: you could surround your cap with a shield and
drive the shield from the FET source, bootstrapping it.

--
Cheers,
James Arthur

Hi James,
The twisted pair may be the easiest, but as a thought, if I took a piece
of coax and
removed the center conductor leaving just the shield, then inserted a
twisted pair as my capacitor.
I then connect the shield the FET source, Ok, I have never bootstrapped
before, but wouldn't I
then have a higher potential than desired, even a possibility of
oscillation?
Mikek
PS. Do I have any concern about inductive coupling with a twisted pair?