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Revisit amps input impedance

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

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  1. amdx

    amdx Guest

    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 Allison

    Phil Allison Guest

    ** 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. amdx

    amdx Guest

    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 Allison

    Phil Allison Guest

    ** Ask him - you fucking tenth wit.

    Looks like pure goobledegook to me.

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

    ** Instead of what ?

    Pure and Applied Bullshit ?

    .... Phil
  5. John S

    John S Guest

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

    amdx Guest

    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. amdx

    amdx Guest

    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
    Thanks, Mikek
  8. John S

    John S Guest

    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
  9. amdx

    amdx Guest

    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.
    Thanks, Mikek
  10. John S

    John S Guest

    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.

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

    amdx Guest

    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.
  12. Fred Abse

    Fred Abse Guest

    I wonder what he measured that with.

    That's verging on electrometer-type input resistance.
  13. John S

    John S Guest

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

    amdx Guest

    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.

  15. amdx

    amdx Guest

    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
  16. ehsjr

    ehsjr Guest

    Nice construction. :)

  17. amdx

    amdx Guest

    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. Jamie

    Jamie Guest

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

  19. amdx

    amdx Guest

    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 S

    John S Guest

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