Revisit amps input impedance

[amdx]

I got this high input impedance amp out again and redid my input
capacitor. I have it at about 0.3pf.
http://www.crystal-radio.eu/enfetamp.htm
The writer says the input impedance is near 5 gigaohms but,
I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal.
Then I put a ten Meg in series with the input and my output
dropped to about 0.5Vpp. That certainly doesn't seem like
it is anywhere close to 5 Gigaohm.
So can someone calculate the input impedance, I want to figure out if
I have something wrong or if the writer got it wrong.

Thanks, Mikek

 

[Phil Allison]

"amdx"

I got this high input impedance amp out again and redid my input
capacitor. I have it at about 0.3pf.
http://www.crystal-radio.eu/enfetamp.htm
The writer says the input impedance is near 5 gigaohms

** He does not say that at all.

I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal.
Then I put a ten Meg in series with the input and my output
dropped to about 0.5Vpp. That certainly doesn't seem like
it is anywhere close to 5 Gigaohm.

** The writes says there is 1.4pF of capacitance at the input.

THAT alone has an impedance of just over 100 kohms at 1MHz.

Radio frequencies and impedances in the megohms do not go together.

FFS learn some math.



.... Phil

 

[amdx]

"amdx"

** He does not say that at all.



** The writes says there is 1.4pF of capacitance at the input.

Ok, I missed that, he's figuring additional capacitance from his input
wires and input connection. Just the 1.4pf has an impedance of about
115K ohms. Then the 0.3pf in series with 20 Megaohm has an impedance not
much more than 20 Megaohms, so we have 115k parallel 20 meg, so we are
still about 115K. Then throw in the gate impedance and we are slightly
less than 115k.
So where did the writer come up with:

"But the input resistance of the amplifier is much higher then 20 M.Ohm,
in theorie even 17² times higher (so, 5780 M.Ohm), this is because over
the 20 M.Ohm resistor is only 1/17th part of the input voltage.
In practice the input resistance will be lower then 5780 M.Ohm because
of dielectric losses e.g. in the gate of the FET."
145k is certainly lower than 5780 Megaohms, but I would not call it
high impedance.

???

I'm not sure how the 1.4 pf should be treated, the circuit is used to
measure Q of LC circuits, so the 1.4pf could just lower the resonant
frequency slightly and the Q would be affected only by the quality of
the 1.4pf of capacitance.
So maybe this isn't as bad as the 115 kohm impedance suggests, but
I'm still stuck with my 10 megaohm experiment that started my question.

THAT alone has an impedance of just over 100 kohms at 1MHz.

Radio frequencies and impedances in the megohms do not go together.

FFS learn some math.

Yes, that would be very helpful, maybe when I retire.
If I was starting over I would have studied math and physics.
Thanks, Mikek

 

[Phil Allison]

"amdx"

So where did the writer come up with:

"But the input resistance of the amplifier is much higher then 20 M.Ohm,
in theorie even 17² times higher (so, 5780 M.Ohm),

** Ask him - you fucking tenth wit.

Looks like pure goobledegook to me.

Yes, that would be very helpful, maybe when I retire.

** So what is your profession now - cab driver maybe ?

If I was starting over I would have studied math and physics.

** Instead of what ?

Pure and Applied Bullshit ?



.... Phil

 

[John S]

I got this high input impedance amp out again and redid my input
capacitor. I have it at about 0.3pf.
http://www.crystal-radio.eu/enfetamp.htm
The writer says the input impedance is near 5 gigaohms but,
I put in a 1 Mhz 4Vpp signal and got out a 4Vpp signal.
Then I put a ten Meg in series with the input and my output
dropped to about 0.5Vpp. That certainly doesn't seem like
it is anywhere close to 5 Gigaohm.
So can someone calculate the input impedance, I want to figure out if I
have something wrong or if the writer got it wrong.

Thanks, Mikek

The *magnitude* of the input impedance is 94.6k.

 

[amdx]

"amdx"


** Ask him - you fucking tenth wit.

Looks like pure goobledegook to me.

Hmm.. his test of connecting a second unit and noting no loading effect
on the measurement was evidence of a very high input impedance.
But how high is the input impedance? and how to treat the 1.4pf.
I'm thinking the 1.4pf is just part of the resonating capacitance and
can be ignored as a load (except for the loss resistance)

** So what is your profession now - cab driver maybe ?

Fish Monger.
And I'm nice for about it.

** Instead of what ?

Pure and Applied Bullshit ?



... Phil

High school Auto body repair.
Thanks, Mike

 

[amdx]

It's a silly circuit. A bootstrapped jfet or phemt could have unity
gain (no need for all that other junk) and a fraction of a pF of
actual terminal capacitance; he has 1.4 pF, mostly parasitic.

John

Hi John,
How would you treat the 1.4pf? I'm thinking it is just additional
resonating capacitance with maybe a little more loss than the capacitor
in the LC circuit.
You mentioned bootstrap last time I brought this up. I'd be happy to
look at a bootstrapped fet circuit if you can put one together. I
have LTspice if you want to post a netlist. I have BF256C fets in
my little stock.
Here's a photo of the layout I have, except I have changed the input
0.3pf cap to a very small piece of Duroid 5880 hung between the fet gate
and the input with #32 wire. I also removed the white piece of
polypropylene.
http://i395.photobucket.com/albums/pp37/Qmavam/Kleijerampinbox.jpg
Thanks, Mikek

 

[John S]

Hi John,
How would you treat the 1.4pf? I'm thinking it is just additional
resonating capacitance with maybe a little more loss than the capacitor
in the LC circuit.
You mentioned bootstrap last time I brought this up. I'd be happy to
look at a bootstrapped fet circuit if you can put one together. I
have LTspice if you want to post a netlist. I have BF256C fets in
my little stock.
Here's a photo of the layout I have, except I have changed the input
0.3pf cap to a very small piece of Duroid 5880 hung between the fet gate
and the input with #32 wire. I also removed the white piece of
polypropylene.
http://i395.photobucket.com/albums/pp37/Qmavam/Kleijerampinbox.jpg
Thanks, Mikek

Maybe this will help you understand the input circuit:

Version 4
SHEET 1 880 680
WIRE -192 112 -288 112
WIRE -32 112 -112 112
WIRE 128 112 -32 112
WIRE 256 112 192 112
WIRE 352 112 256 112
WIRE -288 128 -288 112
WIRE -32 144 -32 112
WIRE 256 144 256 112
WIRE 352 144 352 112
WIRE -288 224 -288 208
WIRE -32 224 -32 208
WIRE 352 224 352 208
WIRE 256 240 256 224
FLAG -288 224 0
FLAG -32 224 0
FLAG 352 224 0
FLAG 256 240 0
SYMBOL voltage -288 112 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value SINE(0 4 1e6)
SYMATTR Value2 AC 4
SYMBOL cap -48 144 R0
SYMATTR InstName C1
SYMATTR Value 1.4p
SYMBOL cap 336 144 R0
SYMATTR InstName C2
SYMATTR Value 4.8p
SYMBOL cap 128 96 M90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value .3p
SYMBOL res 240 128 R0
SYMATTR InstName R1
SYMATTR Value 20e6
SYMBOL res -208 96 M90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10e6
TEXT -322 264 Left 2 !.ac dec 1e6 999999 1000001

 

[amdx]

Maybe this will help you understand the input circuit:

Version 4

Thanks for the netlist John,
I made physical measurements with 8 volts input at 1 Mhz.
The FET output is .48 volts.
When I put the 10 Meg in series with the input, the FET output
drops to 0.065 volts. by juggling the stray capacitance from 1.4pf
to 0.7 pf I can get LTspice to read 0.065 volts. This is because I
tried a couple of things to reduce the stray capacitance. (smaller wire,
more distance, polystyrene and no long leads)
Next I installed about 12 inches of lead wire spaced about 1 inch
apart and measured 0.015 volts at the FET output. I had to adjust the
stray capacitance to 4pf to get 0.015 volts.
So good for all that.
0.7pf of input capacitance isn't bad, however it is hard to make a
measurement without leads. So my thought is to use teflon wire for the
low loss and use spacing to keep capacitance low and consistent.
Then with a bootstrapped circuit cancel out the stray capacitance.
Thoughts,
Thanks, Mikek

 

[John S]

Thanks for the netlist John,
I made physical measurements with 8 volts input at 1 Mhz.
The FET output is .48 volts.

I thought you made the measurements at 4Vp-p.

When I put the 10 Meg in series with the input, the FET output
drops to 0.065 volts.

That's not what you said in your OP ("0.5Vpp") and; what FET output? I
thought you were measuring your *circuit* output which includes the x17
gain and driver.

I'd be glad to help, but your reply doesn't agree with your OP. I can't
work with that.

 

[amdx]

I thought you made the measurements at 4Vp-p.

Yes, I redid the test and in order to get voltages I could see on
my scope I raised the drive to 8 volts.

That's not what you said in your OP ("0.5Vpp") and; what FET output? I
thought you were measuring your *circuit* output which includes the x17
gain and driver.

Yes I eliminated x17 the part of the circuit.

I'd be glad to help, but your reply doesn't agree with your OP. I can't
work with that.

Sorry, these numbers are correct but before the x17 amp.
Mikek

 

[Fred Abse]

The writer says the input impedance is near 5 gigaohms

I wonder what he measured that with.

That's verging on electrometer-type input resistance.

 

[John S]

Yes, I redid the test and in order to get voltages I could see on
my scope I raised the drive to 8 volts.



Yes I eliminated x17 the part of the circuit.


Sorry, these numbers are correct but before the x17 amp.
Mikek

Well, in spite of the moving target, the model is good. Good luck.

 

[amdx]

I wonder what he measured that with.

That's verging on electrometer-type input resistance.

He didn't, that's a calculation. It's also been called goobledegook.

I'm thinking the capacitance just adds to the resonating capacitor of
the LC being measured and should not be called part of the input
impedance when measuring the Q of an LC circuit. The loss in the
capacitor will show as a large parallel resistor.


Mikek

 

[amdx]

Maybe this will help you understand the input circuit:

Version 4
SHEET 1 880 680
WIRE -192 112 -288 112
WIRE -32 112 -112 112
WIRE 128 112 -32 112
WIRE 256 112 192 112
WIRE 352 112 256 112
WIRE -288 128 -288 112
WIRE -32 144 -32 112
WIRE 256 144 256 112
WIRE 352 144 352 112
WIRE -288 224 -288 208
WIRE -32 224 -32 208
WIRE 352 224 352 208
WIRE 256 240 256 224
FLAG -288 224 0
FLAG -32 224 0
FLAG 352 224 0
FLAG 256 240 0
SYMBOL voltage -288 112 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value SINE(0 4 1e6)
SYMATTR Value2 AC 4
SYMBOL cap -48 144 R0
SYMATTR InstName C1
SYMATTR Value 1.4p
SYMBOL cap 336 144 R0
SYMATTR InstName C2
SYMATTR Value 4.8p
SYMBOL cap 128 96 M90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value .3p
SYMBOL res 240 128 R0
SYMATTR InstName R1
SYMATTR Value 20e6
SYMBOL res -208 96 M90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10e6
TEXT -322 264 Left 2 !.ac dec 1e6 999999 1000001

John, I saved, put an extension on the file and successfully ran
your netlist, Thanks.
Now I want to post a netlist, I can't figure out how to generate it.
I've tried Tools-export netlist, but the file doesn't look like those
I've seen.
Can you help?
Thanks Mikek

 

[ehsjr]

Here's a photo of the layout I have, except I have changed the input
0.3pf cap to a very small piece of Duroid 5880 hung between the fet gate
and the input with #32 wire. I also removed the white piece of
polypropylene.
http://i395.photobucket.com/albums/pp37/Qmavam/Kleijerampinbox.jpg
Thanks, Mikek

Nice construction.

Ed

 

[amdx]

First, the author is claiming that the circuit looks like 0.3pF in
parallel with 5Gohm, not 5Gohm bare. That 0.3pF has a reactance of
around 500kOhm, which is almost sorta-kinda consistent with what you're
seeing.

hmm... His writing is not real clear on that, (as I read it).

Second, the author is using theory that doesn't match the part he's
using. He needs to sit down, make his little FET all nice and comfy, and
explain to the little critter how it is supposed to work and hope that it
changes its ways. Or, he needs to understand what the hell he's talking
about.

I'm not inclined to go through the math right now, but the effective
input impedance at the gate of the FET is going to look like the FET gate
capacitance in series with some resistance that's going to be in the kilo-
ohms. I'm not sure _how many_ kilo-ohms, but it's going to be 100k or
less.

I have modified John S's LTspice file to include an LC driven through a
100Meg ohm. I then connect the (input circuit) .3pf in series with
(paralleled) 20meg and 4.8pf to ground. This looks like an input
impedance of 950K according to LTspice. If I add 0.7pf as my input stray
capacitance the input impedance drops to 275k.

Here's where I want to get some understanding.

I think the .7pf becomes part of the resonating LC, so it should not
be considered as part of the load on the LC. It is just additional
capacitance that will lower the resonant frequency. Yes there will be
losses in the stray capacitance.

Keep in mind, though, that the circuit ought to be doing pretty much what
the author claims for it: making measurements on LC circuits without
significantly affecting their Q. I do _not_ see him claiming that the
resonant frequency won't be affected. So when you clip that thing onto a
tank circuit you should expect that the resonant frequency _will_ change
(probably more from the parasitic capacitance of the leads than the 0.3pF
cap), but it shouldn't get loaded down to any significant degree.

Whether the rest of the amplifier maintains calibration close enough for
even remotely useful measurements is left as an exercise for the reader
-- if you're using it for narrowband ratiometric stuff, like finding the
bandwidth of the tank, then it'll probably work fine.

That's what he designed it for.

I cannot find info on how to generate an LTspice netlist that I can post.
Will someone walk me through it?
Thanks, Mikek

 

[Jamie]

First, the author is claiming that the circuit looks like 0.3pF in
parallel with 5Gohm, not 5Gohm bare. That 0.3pF has a reactance of
around 500kOhm, which is almost sorta-kinda consistent with what you're
seeing.

Second, the author is using theory that doesn't match the part he's
using. He needs to sit down, make his little FET all nice and comfy, and
explain to the little critter how it is supposed to work and hope that it
changes its ways. Or, he needs to understand what the hell he's talking
about.

I'm not inclined to go through the math right now, but the effective
input impedance at the gate of the FET is going to look like the FET gate
capacitance in series with some resistance that's going to be in the kilo-
ohms. I'm not sure _how many_ kilo-ohms, but it's going to be 100k or
less.

Keep in mind, though, that the circuit ought to be doing pretty much what
the author claims for it: making measurements on LC circuits without
significantly affecting their Q. I do _not_ see him claiming that the
resonant frequency won't be affected. So when you clip that thing onto a
tank circuit you should expect that the resonant frequency _will_ change
(probably more from the parasitic capacitance of the leads than the 0.3pF
cap), but it shouldn't get loaded down to any significant degree.

Whether the rest of the amplifier maintains calibration close enough for
even remotely useful measurements is left as an exercise for the reader
-- if you're using it for narrowband ratiometric stuff, like finding the
bandwidth of the tank, then it'll probably work fine.

Something tells me he has much more parallel capacitance at the input
than he thinks.

Jamie

 

[amdx]

Something tells me he has much more parallel capacitance at the input
than he thinks.

Jamie

It could be, here's what I did.
I measured the different in the output voltage of my fet with and
without a 10meg resistor in series with the input. I plugged the
circuit into LTspice and juggled the stray capacitance number until it
matched my measurements. .7pf is the number that matched.
I will post the netlist when I find out how to.
Thanks, Mikek

 

[John S]

John, I saved, put an extension on the file and successfully ran
your netlist, Thanks.
Now I want to post a netlist, I can't figure out how to generate it.
I've tried Tools-export netlist, but the file doesn't look like those
I've seen.
Can you help?
Thanks Mikek

Using Notepad, open your .asc file. Highlight all. Copy. Paste into your
message.