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How can I measure the output impedance of an operational amplifier ?

Discussion in 'Electronic Basics' started by Martin Sigwald, Aug 16, 2007.

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  1. I need to physically measure the output impedance of an operational
    amplifier (LM741), working as an inverter and with a capacitor as
    load. Any ideas on how to do this? Impedance analyzer is out of the
    Thanx in advance.
  2. Phil Allison

    Phil Allison Guest

    "Martin Sigwald"

    ** Drive the output from a sine wave generator ( 600 ohms source Z or
    thereabouts) and measure the resulting voltage imposed across the output
    with a scope. Calculate Z from the current and voltage.

    You may be able to measure phase angle as well, if you synch the scope to
    the generator .

    The signal voltage at the 741's output may be very small if the test
    frequency and closed loop gain are low.

    Maybe use another op-amp ( TL071 etc) as a fixed ( 100 times) gain pre-amp
    to up the level to the scope.

    ........ Phil
  3. Ross Herbert

    Ross Herbert Guest

    Phil has given a short description of the method but it won't hurt to
    expand it a little.

    The output impedance can be measured by connecting the input to ground
    with a 1K resistor across pins 2 and 3, and then injecting a sine-wave
    (eg. 1KHz) into the output through a resistor R (eg.1K). Measure with
    an oscilloscope the sine-wave voltage Vo between the output and ground
    (will be quite small). The arrangement makes a voltage divider out of
    R and Zout, so Vo/Vgen = Zout/(R + Zout), and since Zout is quite
    small Zout ~ R(Vo/Vgen).
  4. It is my understanding that you can`t inject a signal through the
    output, since this is an active component. Furthermore, I already
    tried and since my input was 0, my output was also 0 so I couldn`t
    measure anything. Have you tried doing this yourself? Because if you
    did please explain further, so I can see what I'm doing wrong.

    Phil: Where you referring to the same method in your post? Cause I
    didn`t understand what you meant by "drive the output" (sorry, english
    is not my native language).

    Thanx for your answers.
  5. Phil Allison

    Phil Allison Guest

    "Martin Sigwald"

    ** Then you simply have no damn " understanding ".

    ** Only demonstrates your inability - at everything.

    ** Has been fully explained to you already.

    ** Wot - so you speak good pigeon boss ?

    Or just another dodgy code scribbling, pig ignorant useless turd ?

    ......... Phil
  6. Guest

    1) With no load connected measure the output level with scope.
    2) Connect either a pot or a (decade resistance box) and adjust until
    the level is half the original level.
    3) Measure the pot's resistance.

  7. John Larkin

    John Larkin Guest

    Can't? Who says so? Just do it!

    Furthermore, I already
    It wasn't zero, it was just very, very small. That's because the
    output impedance is very low.

    Note that closed-loop Zout will increase with frequency as the opamp
    gain drops. So try it again at a higher frequency. Zout of a 741
    follower will approach the open-loop impedance as frequency approaches
    1 MHz, in the rough turf of 100 ohms. Zout will be about inverse on
    frequency below that, down to 10 Hz maybe.

  8. John Larkin

    John Larkin Guest

    That will be driving the opamp into current limit, not the same thing
    at all.

  9. I think I was more than polite when asking my questions, so I really
    think the tone of your answer was uncalled for. Never the less, you
    are free to answer questions the way you want, so so be it. Lets get
    back to the topic in discussion.
    First of all, my professor (which is a PhD in EE and chairman of IEEE
    in Argentina, so I reckon she knows what she is talking about) told me
    you can`t use the common aproach of impedance measurements used in
    passive components to measure an active one. When I tried injecting a
    signal into the output with a grounded input, a friend of mine
    suggested that since the impedance was to small the voltage was
    equally small, far beyond the oscilloscope's range and suggested
    amplifying it. Before doing that I asked my proffesor who, as I told
    you already, said that wasn't a valid approach.
    I haven`t found anything either on the internet or in books, since
    they always refer to the ideal opamp, so I am at a loss here. Any help
    will be welcomed.
  10. Phil Allison

    Phil Allison Guest

    "Martin Sigwald = PITA IDIOT "

    ** You are an utter ass.

    You " friend " is a fool.

    Your prof is a dumb bitch ASS !!

    The topic has NOTHING to do with anyone's opinion.

    Do the test, just as described and find out just how WRONG you all are.

    ........ Phil
  11. Guest

    Yes that's true.

  12. Chris

    Chris Guest

    Hi, Martin. In fact, Mr. Allison gave you the correct answer -- your
    teacher is wrong. Look at the bottom of page one of National
    Semiconductor's Applications Brief 108 -- that's exactly the way
    output impedance in op amps is measured.

    I'd assume you'll regard the manufacturer of opamps as a sufficient

    By the way, it only took a minute or two of poking around National's
    website to find this. Try using the manufacturers' websites as a
    source of practical information -- they'll tell you things that'd make
    your teachers blush.

    Good luck in your studies
  13. Phil Allison

    Phil Allison Guest


    ** Figure 1 (page 1 ) shows a HP network analyser being used, a HP 4195A .

    Though I much appreciate your backing my comments up - the OP clearly
    specified no high tech equipment and obediently complied with his request.

    Till the Argentinean shithead abused me, that is.

    ........ Phil
  14. John Larkin

    John Larkin Guest

    Phil lives in a constant state of profane rage, sort of a textual
    Tourette's. He's an audio tech or something. Ignore him.

  15. Guest

    Only one comment.... the measurement must be done at a level
    that does not overload the amp and at a frequency within the
    opamp's range.

    Feed a signal into the opamp that is well below the clipping level
    at the output measured with the scope.
    With no load connected measure the output level with the scope.
    Connect either a pot or a (decade resistance box) and adjust until
    the level is half the original level (The resistance will be small
    the source impedance is low).
    Measure the pot's resistance.This value is the output source
  16. The thin is I need to measure the output impedance of the whole
    circuit, opamp+capacitor, in terms of frequency. Doing what you
    propose would take an enourmous amount of time.
    What I did and think it works, since it produced something very
    similar to my analitical calculations, is measure the output voltage
    and phase of the opamp+capacitor, then add a parallel resistor and
    measure again. Using Thevenin the equivalent output resistance of the
    circuit will be Zth= (V1/V2-1) R. As I said, it seems to work.
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