help designing gimmick capacitor
 

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.

help designing gimmick capacitor
 

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

help designing gimmick capacitor
 

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?

help designing gimmick capacitor
 

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?

Higher potential? No. Possibility of oscillation? Yes, but not
likely. Keep the shield well clear of the gimmick to minimize that.

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

* * * * * * * * * * * *Mikek
*PS. Do I have any concern about inductive coupling with a twisted pair?

Nope.

--
Cheers,
James Arthur

help designing gimmick capacitor
 

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.

Bootstrap the drain of Q1.

"T" means transformer, which shows that this circuit was done by an
amateur. All that tricky stuff could be replaced by one opamp.

It could have close to zero Cin with a little positive feedback.

John

help designing gimmick capacitor
 

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.

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.


Bootstrap the drain of Q1.

"T" means transformer, which shows that this circuit was done by an
amateur. All that tricky stuff could be replaced by one opamp.

It could have close to zero Cin with a little positive feedback.

John

I like the one at the top of pg. 2:
http://www.national.com/an/AN/AN-32.pdf

1970--a classic.


--
Cheers,
James Arthur

help designing gimmick capacitor
 

On 2/20/2011 12:13 PM, wrote:
On Feb 20, 12:39 pm, John Larkin
wrote:
On Sun, 20 Feb 2011 09:20:12 -0600, 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.

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.


Bootstrap the drain of Q1.

"T" means transformer, which shows that this circuit was done by an
amateur. All that tricky stuff could be replaced by one opamp.

It could have close to zero Cin with a little positive feedback.

John

I like the one at the top of pg. 2:
http://www.national.com/an/AN/AN-32.pdf

1970--a classic.


--
Cheers,
James Arthur


Warning!

The 2N3644 is a PNP. The app note probably meant some other type number
similar to that in an NPN.

Cheers,
John

help designing gimmick capacitor
 

On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote:

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

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


Ever consider simply using a proper air core variable cap?

help designing gimmick capacitor
 

On Sun, 20 Feb 2011 10:13:12 -0800 (PST),
wrote:

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



One gotcha: with +12 on the drain of the jfet, expect a lot of gate
current. Hot carriers or some such.



* * * * * * * 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.

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.


Bootstrap the drain of Q1.

"T" means transformer, which shows that this circuit was done by an
amateur. All that tricky stuff could be replaced by one opamp.

It could have close to zero Cin with a little positive feedback.

John

I like the one at the top of pg. 2:
http://www.national.com/an/AN/AN-32.pdf

1970--a classic.

If the jfet drives a non-inverting opamp with gain, one could
over-bootstrap the drain to hit zero or even negative input
capacitance; tweak that with a pot or a small variable cap. Then, as
you say, dump the 0.3 pF input cap and have predictable gain.


Phil Hobbs likes this:

+-----------+-----Vcc
| |
| |
| R
| |
c |
b----+-----+ ~~ +3V
e | |
| | |
| | |
d C R
in----------g | |
s | |
| | gnd
| |
+-----+-----------out
|
|
|
R or current sink
|
|
|
Vee

John

help designing gimmick capacitor
 

"John Larkin" wrote in message
...
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.

Bootstrap the drain of Q1.

"T" means transformer, which shows that this circuit was done by an
amateur. All that tricky stuff could be replaced by one opamp.

It could have close to zero Cin with a little positive feedback.

John


Bootstrap the drain of Q1.
You need to walk me through that, (I'm an amateur)

Ah, he's done some nice work on the subject of crystal radios and high Q
inductors.
http://www.crystal-radio.eu/index.html
Page down to experiment with LC circuits.

It could have close to zero Cin with a little positive feedback.

How much closer? If the input cap is 0.3pf what do you the input impedance
is?
Input is 0.3pf, 20 Meg to ground driving FET gate.
Thanks, Mikek