# Revisit amps input impedance

Discussion in 'Electronic Design' started by amdx, Nov 26, 2011.

1. ### amdxGuest

I got this high input impedance amp out again and redid my input
capacitor. I have it at about 0.3pf.
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

2. ### Phil AllisonGuest

"amdx"
** He does not say that at all.

** 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

3. ### amdxGuest

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.

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

4. ### Phil AllisonGuest

"amdx"
** Ask him - you fucking tenth wit.

Looks like pure goobledegook to me.

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

Pure and Applied Bullshit ?

.... Phil

5. ### John SGuest

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

6. ### amdxGuest

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)
Fish Monger.
And I'm nice for about it.
High school Auto body repair.
Thanks, Mike

7. ### amdxGuest

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

8. ### John SGuest

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

9. ### amdxGuest

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)
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

10. ### John SGuest

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

work with that.

11. ### amdxGuest

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

12. ### Fred AbseGuest

I wonder what he measured that with.

That's verging on electrometer-type input resistance.

13. ### John SGuest

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

14. ### amdxGuest

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

15. ### amdxGuest

John, I saved, put an extension on the file and successfully ran
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

16. ### ehsjrGuest

Nice construction.

Ed

17. ### amdxGuest

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

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.

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

18. ### JamieGuest

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

Jamie

19. ### amdxGuest

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

20. ### John SGuest

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