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