wrote in message
...
On Feb 20, 12:39 pm, John Larkin
wrote:
On Sun, 20 Feb 2011 09:20:12 -0600, "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.

Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1
cm square plates are too big and have their own capacitance to the
world.
.................................................. ............
For that matter the tiny input cap in Mike's circuit is
counterproductive--it divides the signal down and makes the gain
unpredictable.

It's not so inpredictable, I set the amplifier gain at 17 and then
adjusted
the capacitor spacing for a total amplifier gain of 1.
I'm not being argumentive, just trying understand.



Better: use 10pF coupling, lose less at the input, and use less gain
later. Bootstrap the FET so the input sees very low C. Do those and
you don't even need a gimmick.

I need to know more about bootstraping.
Also doesn't the 0.3pf cap reduce the loading effect of the 20 Meg resistor?
Thanks, Mikek