help designing gimmick capacitor
 

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.


Response before looking at others:
1. use one 10Meg and connect it to source OR cap bootstrap the 10meg
input divider to the source.
2. "wrap" a shield around the input wire & plate and drive the shield
from the FET source or T2 emitter.
3 What the heck is that HUGE block attached to the input plate?
Ditech it; you will see a significant decrease in input capacitance.

help designing gimmick capacitor
 

On Feb 21, 12:31*am, RST Engineering wrote:
On Sun, 20 Feb 2011 07:52:18 -0800 (PST),
wrote:

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.

Does the term SNIP have any meaning to you?

Jim

I quoted the original post. Did you think the original was
irrelevant? I don't agree.

I try to quote enough so that a newcomer can, in one post, understand
both the full question I'm responding to, and my reply.

--
Cheers,
James Arthur

help designing gimmick capacitor
 

On Feb 20, 3:30*pm, wrote:
On Feb 20, 2:44*pm, "amdx" wrote:





"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

We're talking about something roughly like this:

* * +12V -o-----o-----------.
* * * * * *| * * | * * * * * |
* * * * * *| * * | * * * * * |
* * * * * *| * * | * * * R5 3.3M
* * * * * *| * * |Q2 * * * * |
* * * * * *| * * *\| * * * * | * * * *V1
* * * * * *| * * * |---o-----o * ~~ ~=+3V
* * * * * *| * * .| * | * * |
* * * * * *| * * | * * | *R6 1M
* * * * * R3 * * | * * | * * |
* * C1 * * | * * | * * |C3 *===
* * 10pF * | g |-'d * ---
in--||--o--|--| * * *---
* * * * | *| * |-.s * *|100n
* * R1 10M | * * |Q1 * |
* * * * | *| * * | * * |
* * * * o--o-||--o-----o-----------out
* * * * | * *C2 *|
* * R2 10M *100nF|
* * * * | * * * *|
* * * *=== * R4 470 R
* * * * * * * * *|
* * * * * * * * *|
* * * * * * * * ===

C2 drives the center of the input bias resistor, which cancels the
loading caused by R1-R2. *This is called "bootstrapping".

Q2 does the same thing for the drain terminal--it causes the drain
terminal to go up and down with the input signal. *That saves the
input signal from having to charge Q1's gate-to-drain capacitance,
effectively making that capacitance disappear.

This front-end has *much* higher impedance than the original, and a
predictable gain that's close to 1.

R3 is to bias the FET output to (V1)/2, for maximum dynamic range.
Higher V1 would give bigger dynamic range.

A cheapskate could put a resistor in Q2's collector and use it as a
voltage-gain stage too.

--
Cheers,
James Arthur- Hide quoted text -

- Show quoted text -

Thanks for the explanation James. The bootstrapping of the base does
nothing for the input capacitance.(?) Is that correct?

George H.

help designing gimmick capacitor
 

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

help designing gimmick capacitor
 

On Feb 21, 11:18*am, "amdx" wrote:
wrote:
On Feb 20, 11:14 am, "amdx" wrote:

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.

Just half a twist is likely enough--regular twisted pair is way too
tightly coupled.

[...]


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

--
Cheers,
James Arthur

help designing gimmick capacitor
 

On Feb 21, 11:16*am, George Herold wrote:
On Feb 20, 3:30*pm, wrote:



On Feb 20, 2:44*pm, "amdx" wrote:

"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

We're talking about something roughly like this:

* * +12V -------+-----------.
* * * * * * * * | * * * * * |
* * * * * * * * | * * * * * |
* * * * * * * * | * * * R5 220K
* * * * * * * * |Q2 * * * * |
* * * * * * * * *\| * * * * | * * * *V1
* * * * * * * * * |---+-----+ * ~~ ~=7V
* * * * * * * * .| * | * * |
* * * * * * * * | * * | *R6 330K
* * * * * * * | * * | * * |
* * C1 * * * * | * * |C3 *===
* * 10pF * g |-'d * ---
in--||--+-----| * * *---
* * * * | * * |-.s * *|100n
* * R1 10M * * |Q1 * |
* * * * | * * * | * * |
* * * * +----||--+-----+------ out
* * * * | * *C2 *|
* * R2 10M *100nF|
* * * * | * * * *|
* * * *=== * R4 2.2K
* * * * * * * * *|
* * * * * * * * *|
* * * * * * * * ===

edited, simplified, R4-6 values corrected

C2 drives the center of the input bias resistor, which cancels the
loading caused by R1-R2. *This is called "bootstrapping".

Q2 does the same thing for the drain terminal--it causes the drain
terminal to go up and down with the input signal. *That saves the
input signal from having to charge Q1's gate-to-drain capacitance,
effectively making that capacitance disappear.

This front-end has *much* higher impedance than the original, and a
predictable gain that's close to 1.



Thanks for the explanation James. *The bootstrapping of the base does
nothing for the input capacitance.(?) Is that correct?

If you mean "gate" and Q1, that's inherently bootstrapped by the
voltage-follower configuration: the source already rises and falls
with the input, leaving the Miller capacitance as by far the biggest
nasty. The best, easy bootstrap for the FET input capacitance is
getting the FET follower gain up closer to unity. That means using a
current source for the source load instead of R4.

The input bootstrap above--C2 to the junction of R1-R2--serves to
reduce the a) stray capacitive loading posed by R1, and b) raise the
a.c. input impedance to much higher than the 20M input resistor in
parallel with the FET gate impedance.

--
Cheers,
James Arthur

help designing gimmick capacitor
 

amdx Inscribed thus:


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

Try a glass cased diode with one end cut off so the case is empty. Use
the wire at one end for one connection, use a piece of thick wire
inside the tube for the other connection. Adjust for "C". Secure with
a drop of adhesive.

--
Best Regards:
Baron.

help designing gimmick capacitor
 

amdx wrote:

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 ;-)

Heat shrink?

Good Luck!
Rich

help designing gimmick capacitor
 

On Feb 21, 12:03*pm, wrote:
On Feb 21, 11:16*am, George Herold wrote:





On Feb 20, 3:30*pm, wrote:

On Feb 20, 2:44*pm, "amdx" wrote:

"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

We're talking about something roughly like this:

* * +12V -------+-----------.
* * * * * * ** * | * * * * * |
* * * * * * ** * | * * * * * |
* * * * * * ** * | * * * R5 220K
* * * * * * ** * |Q2 * * * * |
* * * * * * ** * *\| * * * * | * * * *V1
* * * * * * ** * * |---+-----+ * ~~ ~=7V
* * * * * * ** * .| * | * * |
* * * * * * ** * | * * | *R6 330K
* * * * * * ** * | * * | * * |
* * C1 * * * * * | * * |C3 *===
* * 10pF * * g |-'d * ---
in--||--+-----| * * *---
* * * * | * ** |-.s * *|100n
* * R1 10M * * * |Q1 * |
* * * * | * ** * | * * |
* * * * +----||--+-----+------ out
* * * * | * *C2 *|
* * R2 10M *100nF|
* * * * | * * * *|
* * * *=== * R4 2.2K
* * * * * * * * *|
* * * * * * * * *|
* * * * * * * * ===

edited, simplified, R4-6 values corrected

C2 drives the center of the input bias resistor, which cancels the
loading caused by R1-R2. *This is called "bootstrapping".

Q2 does the same thing for the drain terminal--it causes the drain
terminal to go up and down with the input signal. *That saves the
input signal from having to charge Q1's gate-to-drain capacitance,
effectively making that capacitance disappear.

This front-end has *much* higher impedance than the original, and a
predictable gain that's close to 1.

Thanks for the explanation James. *The bootstrapping of the base does
nothing for the input capacitance.(?) Is that correct?

If you mean "gate" and Q1, that's inherently bootstrapped by the
voltage-follower configuration: the source already rises and falls
with the input, leaving the Miller capacitance as by far the biggest
nasty. *The best, easy bootstrap for the FET input capacitance is
getting the FET follower gain up closer to unity. *That means using a
current source for the source load instead of R4.

Uh yes... Gate of Q1. (thanks for reading my mind.)

George H.

The input bootstrap above--C2 to the junction of R1-R2--serves to
reduce the a) stray capacitive loading posed by R1, and b) raise the
a.c. input impedance to much higher than the 20M input resistor in
parallel with the FET gate impedance.

--
Cheers,
James Arthur- Hide quoted text -

- Show quoted text -

help designing gimmick capacitor
 

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..