amdx wrote:
wrote in message
...
On Feb 20, 11:14 am, "amdx" wrote:
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.
r
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.

That's too tight a box, and the dielectric screws things up--you'll be
making shunt caps to the shield.

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?
..........................

Just the twisted gimmick itself is probably all you'll need. The
bootstrapping is lagniappe.

I tried a twisted pair of #26 wire and got way more capacitance then I
wanted.
I ended up with just about 1/4" layed side by side as about equal to 0.3pf.
When I put the cover on there was much less effect on the signal than with
the previously
used plate capacitor.
The side by side wire is not stable for long term use, so I'll find a way to
stablize it and
call this one a wrap. I'll try tying a low capacitance knot ;-)
I have two more boards and boxes, I'll build a bootstrapped version for the
next design.
I'll be back, I might try one with a gain of 1 and one with a gain of
maybe 10.
Mikek





Someone suggested the use of a chip cap for the input; much more
stab;e - and smaller than the plate setup..