"FigureItOut" wrote in message
...
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?

You have any 0.3pf air variables.
Why is this one improper?
Mikek

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

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.

Oh, I didn't calculate the biasing, so R4's probably wrong.

--James

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

amdx Inscribed thus:

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.

Change to a co-axial cap.

--
Best Regards:
Baron.

On Feb 20, 3:56*pm, "amdx" wrote:
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.

The input gain of the original circuit depends on the voltage divider
comprising your gimmick cap, on the one hand, and the FET's
capacitance on the other.

No two FETs will have the same capacitance, so you can't know in
advance exactly what the circuit's gain will be. That's why you have
to tweak it via the input capacitor right now.


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

Not really. At 1MHz, 0.3pf has a reactance of 530K, so it just adds
530K in series with 20M. It also attenuates your signal by a factor
of 17, if the writeup is accurate.

The author says the amp's input capacitance is 1.4pF. That means
either there's a lot of stray capacitance, or the input coupling cap
is actually a lot larger than 0.3pF.

--James

wrote in message
...
On Feb 20, 3:56 pm, "amdx" wrote:
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.
........................................
The input gain of the original circuit depends on the voltage divider
comprising your gimmick cap, on the one hand, and the FET's
capacitance on the other.

No two FETs will have the same capacitance, so you can't know in
advance exactly what the circuit's gain will be. That's why you have
to tweak it via the input capacitor right now.

Hmm. I might have got a hot FET as my cap is spaced 5 mm vs his 3mm.
I calculated 0.177pf for my cap.
Had adjusted the circuit voltage gain after the FET circuit for 17. Then
adjusted the cap for a total gain of 1.

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

Not really. At 1MHz, 0.3pf has a reactance of 530K, so it just adds
530K in series with 20M. It also attenuates your signal by a factor
of 17, if the writeup is accurate.

Oh, I had assumed a much higher impedance for that small cap.
Best not to assume :-)

The author says the amp's input capacitance is 1.4pF. That means
either there's a lot of stray capacitance, or the input coupling cap
is actually a lot larger than 0.3pF.

I'll be trying the twisted pair to reduce that stray capacitance.
Mikek

On Sun, 20 Feb 2011 13:44:37 -0600, "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)


If the source of the fet follower drives an opamp with a gain of, say,
+2, you could take the amp's output, run it through a pot, and AC
couple that into the drain.

With the pot at zero gain, there's no bootstrapping, so the fet's Cg-d
loads the input 100%.

With the pot at mid-rotation, the drain is forced to swing up and down
just about as much as the input signal does. Since both ends of Cg-d
are at the same signal level, there's almost no current in that
capacitance, so it sort of disappears, as far as the gate is
concerned.

Turn the pot a little more, and a small excursion at the gate results
in a *bigger* excursion at the drain. So the current in Cg-d flows in
the opposite direction from a real capacitor, and Cg-d becomes a
negative capacitor. It can then be used to cancel all other
capacitance at the gate node, making the box have a net nearly-zero
input capacitance.

Or just add a small variable cap from the opamp output back into the
gate, and adjust for near zero input capacitance. When it oscillates,
you've gone too far.

John

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

On Sun, 20 Feb 2011 21:31:42 -0800, 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

Save up and buy yourself some scroll bars.

John