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How to you guess the open-loop output impedance?

Discussion in 'Electronic Design' started by alan, Jan 23, 2005.

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  1. alan

    alan Guest

    I'd like to do some feedback analysis of op-amps using Mathematica, i.e.
    doing those bode plots of open loop gain and beta, etc. This is to
    check for stability and other stuff. Problem is that most spec sheets
    don't give the open loop output impedance. What can I do?
  2. Why? You are usually far better using an electronic simulation program
    for this sort of job. Some actually have hooks to do loop gain plots
    directly, i.e. no fiddling about:)
    Use a good spice model, guess, measure, or calculate from the known

    Typically open loop output impedances are 1-200 ohms.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  3. Jim Thompson

    Jim Thompson Guest

    Typically the open-loop output impedance would be something like
    looking into the emitter of a Darlington with its base having 10-30pF
    on it.

    I've never tried modeling it that exactly, but you certainly have
    broached an interesting question!

    ...Jim Thompson
  4. Hello all,
    this has been true for the non rail to rail amplifiers.
    The RRIO and RRO amplifiers have the drains or collectors
    connected to the output. That's the reason why they have
    an open loop output impedance of a few hundred ohms to one kOhm.

    Best Regards,

    RRIO: Rail to Rail Input and Output
    RRO: Rail to Rail Output
  5. Jim Thompson wrote...
    Or often a single-transistor emitter follower. Following your
    suggestion, that's something like Zo = beta * Xc = beta / 2pi f C,
    but at the higher frequencies beta = f / f_T, so that in the end
    Zo looks more or less resistive. E.g., we get 40 ohms for 20pF
    and an output transistor f_T = 200MHz. And even tho we're being
    rather crude, we should still add on r_e, which can be another
    20 to 100 ohms, depending on the class-AB quiescent current.

    It's when we put this resistive Zo inside the feedback loop that
    the opamp's overall Zo looks like an inductor, as Alan will see.
  6. Jim Thompson

    Jim Thompson Guest

    Open drains or collectors would give open-loop output impedances well
    in excess of 1K, UNLESS there was a local loop around the output

    Does anyone own a network analyzer? It would be interesting data to
    see open-loop output impedance versus frequency for various popular
    OpAmps... under no load, sinking current and sourcing current

    ...Jim Thompson
  7. I think the open-loop output impedance of 4558 type amplifiers at
    moderate frequencies is surprisingly high. Had trouble with this years
    ago with low-level amplfiers in extremely noisy situations

    Best regards,
    Spehro Pefhany
  8. Ken Smith

    Ken Smith Guest

    Are we defining "open loop" as having the compensation circuit disabled or
    just breaking the external feedback loop.

    With compensation enabled:

    The output devices are typically either driven by the last point that the
    compensation circuit encloses or are them selves enclosed in the loop so
    i'd expect:

    With an op-amp like an LT1498, I'd expect the output impedance to look
    somewhat capacitive at lowish frequencies. At, lets say about 100Hz, the
    impedance will start to look almost purely resistive and somewhere in the
    50 to 100 ohms range. As the frequency gets near the top of the range of
    the op-amp, I'd expect to see a slight swing into the inductive side of
    the phase curve.

    With the very high speed op-amps, I'd expect the output to not start
    looking resistive at a higher frequency. The swing into the inductive
    side to be either missing or very slight.
  9. Jim Thompson wrote...
    My HP Agilent network analyzer starts at 300kHz. I have an old HP
    network analyzer that goes from analog frequencies to 13MHz, but its
    digital interface is rather obsolete. So now I'm looking for eBay
    to present me with an HP 4192A 5Hz-to-13MHz vector impedance analyzer
    at an affordable price sometime soon. If not, I may have to consider
    the $3.5k to $8.5k asking price in the surplus-instrument markets.

    In the meantime, I may be able to employ my 10kHz to 10MHz HP 4275A
    LCR meter for some of these tasks, with an appropriate test fixture.
  10. Mike Monett

    Mike Monett Guest

    Paul Kiciak, N2PK, has an interesting VNA design using a DDS and a
    novel method of measuring phase. Here is some info

    "This is a homebrew VNA capable of both transmission and
    reflection measurements from 0.05 to 60 MHz, with about 0.035 Hz
    frequency resolution and over 110 dB of dynamic range. Its
    transmission measurement capabilities include gain/loss magnitude,
    phase, and group delay."

    "Its reflection measurement capabilities include complex impedance
    & admittance, complex reflection coefficient, VSWR, and return

    The main interest for rf work is extending the high frequency
    capability, but it might be possible to go lower in frequency.

    Mike Monett
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