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Directly couple singly supply op amp buffer

Discussion in 'Electronic Design' started by Stretto, Jun 4, 2011.

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  1. Stretto

    Stretto Guest

    When directly coupling a single supply non-inverting op amp buffer there are
    issues that seem to be impossible to fix.

    If you simply hook up the source to the op amp without any coupling then any
    signal swings below V- of the op amp will be clipped. One can remedy this by
    "lifting" the source by biasing it's ground. This requires a resistor in
    series, at least in some cases(not sure if all). But doing this attenuates
    the signal.

    Is there any way to direct couple an AC signal to a single supply op amp
    without having these issues(either clipping or attenuation)? I need to
    maximize headroom(hence bias it half way between the rails) and have no
    attenuation(as to reduce noise reduction. Simply amplifying the signal after
    the attenuation won't work).

    Maybe there is some configuration of negative feedback that can compensate
    for the attenuation resistor in a way that doesn't increase noise? Or maybe
    there is another way to directly couple the source that avoids these issues?
  2. John S

    John S Guest

    So, a winter amplifier won't work?
  3. Jamie

    Jamie Guest

    lift your common with a Divider for the AC input? This will work fine
    if your circuit has some kind of isolated supply.

  4. Stretto

    Stretto Guest

    "Fred Bloggs" wrote in message

    The usual way is to use a summer amplifier:
    Please view in a fixed-width font such as Courier.

    .. V+
    .. |
    .. --[2R]-+-----
    .. | |
    .. | |
    .. | |
    .. | | \
    .. IN>-----[R]--+-----------|+ \
    .. | | >--+-> OUT=IN + V+/2
    .. | ---|- / |
    .. [2R] | | / |
    .. | | | |
    .. | | --- |
    .. | | |
    .. +--[R]--+---[R]----
    .. |
    .. |
    .. ---

    This would work. The only thing I'm worried about is the noise/nonlinearity
    that now exists. The gain's depend on the source impedance. Using a large R
    will reduce it but then will add noise. My source impedance can vary a great
    deal from 10k to < 10M. I guess it's a trade off that is potentially better
    than the other methods but I'm not sure how much.
  5. Stretto

    Stretto Guest

    "Dave Platt" wrote in message
    Or worse. Some op amps (mostly older ones) suffer from an abrupt
    "jump all the way to the opposite rail!" problem if you push the
    inputs more than a short distance outside of the voltage rails... this
    can lead to a nasty latchup.
    One way is to work back through the circuit and force the previous
    stage's common-mode reference point to be near the middle of the op
    amp's common-mode range. However, doing so simply pushes the whole
    problem/question back one stage in the circuit... it doesn't eliminate

    You could stick a stack of diodes between the previous stage output
    (cathode end) and the op amp input (anode end) and use a pull-up
    resistor to V+ at this point. This will probably introduce some
    significant distortion, though.
    So, just how much noise are you concerned about, and why? Can you
    quantify it for us?

    If you stick (e.g.) a 1k resistor between the previous stage and the
    op amp input, and hang a 1k resistor from this input to V+ to bias it,
    you'd suffer a 50% (6 dB) loss of gain due to this biasing
    arrangement. Simply double the gain in the op amp stage to compensate.
    With modern low- noise low-distortion audio amplifiers, I really doubt
    that the added thermal noise from these resistors is going to be even
    vaguely significant... your incoming signal will have been through
    *far* worse, earlier in its chain-of-processing.

    The other (obvious) approach is simple - buy one high-quality
    audiophile-grade coupling capacitor, and AC couple the signal. For
    not a lot of money you should be able to buy a cap which doesn't
    introduce distortion anywhere near the threshold of audibility... once
    again, the signal you're reproducing will have suffered from much
    greater degradation before it ever saw your circuit!


    I am trying to get away from using a capacitor. This is a VLF app(around
    1Hz). The goal is to add as little distortion and noise as possible within
    reason and constraints. Since this is the first amplifier in the chain it is
    much more important to reduce the noise than further down the line. The
    source impedance has significant variation which can cause significant
    problems in biasing. Of course all this is expected to run on a battery with
    significant lifetime(not minutes or days but weeks to months).

    The only problem I am having is the input biasing issue. Fred's idea of a
    summing amplifier works but I'm a bit concerned about some of the
    noise/non-linear distortion introduced which are inversely related(I can
    trade one off for the other). The Johnson noise will almost surely not be a
    problem but I've not done any noise analysis on the circuit to know for
  6. Jamie

    Jamie Guest

    if you're using a battery for this device, why can't you use a virtual
    ground ? it's simple and would give you a basic +/- rail from a single
    battery, this is assuming that this battery is only operating your
    device here?

    Just an example.

  7. Stretto

    Stretto Guest

    "Jamie" wrote in message

    if you're using a battery for this device, why can't you use a virtual
    ground ? it's simple and would give you a basic +/- rail from a single
    battery, this is assuming that this battery is only operating your
    device here?

    Just an example.


    Battery or not it doesn't seem to work. I've tried using an op amp version
    to split the rails but it doesn't work. When connecting the source it draws
    a ton of current. I've also tried using a resistive divider to split and it
    doesn't work.

    I don't know why I can just split the rails and treat the virtual ground as
    ground and not bias anything but it seems it's more complex than that? That
    is, V - 0 - -V is not the same as 2V - VV - 0 where VV is virtual ground?

    Then again I may have had something hooked up wrong. If simply splitting
    the rails would let me treat the circuit as if I had a dual supply then life
    would be so much easier. I'll try again and see if I can get it to work. I
    may have left something connected as if it were single supply.
  8. Stretto

    Stretto Guest

    .. V+
    .. |
    .. .-------+-----.
    .. | |
    .. [220K] |
    .. | |
    .. 1.5u | | \
    .. IN>--||------+-------------------|+ \
    .. - + | | | >--+-> OUT=IN + V+/2
    .. [100K] | ---|- / |
    .. | | | | / |
    .. | | | | |
    .. | | | --- |
    .. | | | |
    .. | | '--[200K]--+
    .. | | |
    .. | | |
    .. | | 0.33u |
    .. '-------+----------||------'
    .. |
    .. |
    .. [220K]
    .. |
    .. |
    .. ---
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