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Feedback in audio esp wrt op-amps.

Discussion in 'Electronic Design' started by Eeyore, Aug 19, 2007.

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  1. Eeyore

    Eeyore Guest

    There was part of a thread a while back about how adding negative feedback can
    create higher order harmonic distortion products than exist open-loop in an
    amplifier stage.

    This made me think about the application of op-amps in audio generally. Negative
    feedback is used primarily to linearise the transfer function and is used in
    huge quantites as much as 80dB @ 1 kHz for example.

    Since this amount of NFB is not required to provide an accurate gain setting, it
    struck me that it's somewhat counter productive. If instead the open-loop
    transfer characteritic was made more linear by degeneration of the open-loop
    gain for example, when NFB is applied, the overall result should be largely
    similar (i.e. no worse) but would presumably also suffer less from the creation
    of these new distortion products .

    Comments ?

    Graham
     
  2. There was part of a thread a while back about how adding negative feedback
    That negative feedback linearizes the transfer function at the expensive of
    adding higher-order harmonics has been long-known. What you say is perfectly
    logical.

    However, the presence of higher-order harmonics is not the only factor, but
    their amplitude. Below a certain percentage (I'm sure Arny will be able to
    tell us what that is), they're inaudible.

    A good op amp can be used as a buffer and be sonically transparent, its
    output indistinguishable from its input.
     
  3. MooseFET

    MooseFET Guest

    Degeneration is NFB. It is just applied locally. What you really
    want is to go with a topology that is naturally more linear,
    One huge problem with including a lot of local NFB is that it makes
    the overall system harder to close. Local feedback often creates 2
    pole systems with modest Q values within the system. When you go to
    close the loop, you have to keep a good phase margin so you are forced
    to use a lower overall loop gain.

    Try spice modeling a thing like this:


    Vcc
    ---------------+------------
    !
    \
    /
    Vbias \
    ! !
    / +--------------
    \ ! !
    ! !/ e !
    ---!!--+------! PNP ---
    !\ ---
    +--Out !
    \ !
    / !
    \ !
    ! !
    +--------------
    !
    V D1
    ---
    !
    GND

    Change D1 to be a resistor and back and you will see quite a
    difference in the amount of degeneration needed to get the same
    distortion values for a modest signal of lets say 10mV in.
     
  4. Eeyore

    Eeyore Guest

    Sorry I didn't make that clearer.

    Yes, I'm referring to the reduction of overall loop feedback.

    Graham
     
  5. Eeyore

    Eeyore Guest

    That's not my experience. Quite the reverse actually. But then I do tend to
    incorporate internal lead-lag compensation. This results in a far BETTER phase margin.

    Graham
     
  6. D from BC

    D from BC Guest

    Just speaky from some audio hobby work....

    *Like with most things in electronics, there are frequency limits. I
    think feedback decreases with frequency. The harmonic distortion
    becomes an ultrasonic problem.
    *Feedback is a correction signal.. If nothing messes up this process
    then all's well.
    *For large signals, doesn't every semiconductor naturally distort?
    Developing the best linear open loop design may not be enough.

    D from BC
     
  7. Eeyore

    Eeyore Guest

    You need to learn more.

    I appreciate your interest but your grasp of the issues is beginner level.

    Graham
     
  8. Eeyore

    Eeyore Guest

    There's more than a little discussion about what level that is, and indeed it's
    known that audibility varies according to harmonic number.

    As a buffer it has 100% NFB and I hope that's the case.. As a gain stage with
    say 40dB of voltage gain that isn't the case however.

    Really, part of what I'm saying is that the classic op-amp isn't really the
    ideal gain stage for audio circuits if you want to produce totally 'technically
    blameless' performance.

    Graham
     
  9. MooseFET

    MooseFET Guest

    This means that you have lowered the outter loop gain in the process.
    If the internal part looks kind of like this:

    --!!-/\/\--
    ! !
    ---/\/\-+--!-\ !
    ! >------+---
    !+/

    The amplification stage you are placing the NFB around must have a
    great enough bandwidth to make the feedback determine the responce.

    The local feedback has all of the problems a global feedback has with
    creating upper harmonics. Your global feedback is now at a lower gain
    and thus can't remove them. This is just a case of the lack of a free
    lunch.

    The pole and zero inside the loop is a good thing to do to improve the
    phase margin when you have other poles in the system. It allows you
    to determine where the gain crossover happens and the phase at the
    crossover. It is a method of lowering the overall loop gain. It
    doesn't however get rid of the harmonics issue. It also is something
    that you can only do a few times inside the loop. When the system
    starts to look like 3 of those in series, you are back in trouble.

    In audio stuff, you generally want to put the pole-zero thing near the
    output, ideally enclosing the output. This makes the system apply a
    low pass filter to any distortion products that the feedback can't get
    rid of.
     
  10. MooseFET

    MooseFET Guest


    Even at reasonable gains, there are many that will perform well enough
    that nobody will hear the difference. Power amplifiers are the place
    where it gets very hard to keep distortion low at reasonable
    efficiencies.
     
  11. MooseFET

    MooseFET Guest

    Yes it typically does generally decrease. It also has a phase shift.
    If you add feedforward, you can have a band in which the feedback
    increases with frequency.
    One problem is that ultasonic things can interact on any nonlinear
    part of the system. This can lead to frequencies that are things like
    7*F1 - 9*F2 in the circuit. It is like someone injected a signal at
    that frequency into that point in the circuit. How the system
    responds to it determines whether it will be heard or not.
     
  12. MooseFET

    MooseFET Guest


    For some reason my cursor went away. This makes it harder to edit
    what I'm typing.

    Making the "best linear open loop" is for all practical purposes never
    enough.
    You need a very linear open loop design with a low enough phase shift
    to make the NFB work and ideally to have a lowpass effect applied to
    any distortion that is created.

    You also have to trade off performance against water cooling. A
    simple class A power MOSFET common source stage can be used as an
    example. If you use about 10 power MOSFETs in parallel, have each one
    passing about 0.5 Amps, and run with a 50V supply, you will have a
    circuit that is darn linear for a 1mV input signal.
     
  13. whit3rd

    whit3rd Guest

    All amplifiers have characteristic curves; the gain isn't completely
    linear.
    Feedback components can be (very linear) resistors. So you get
    some combination of amplification and negative feedback in most useful
    low-distortion amplifiers.

    A single transistor can have power gain of 10,000; a single vacuum
    tube
    or MOSFET can have more. Giving up gain for linearity is a good
    trade.
    It's never perfect (even resistors are distortion sources, if you have
    signals
    at 2 Hz and the self-heating of the resistors isn't insignificant),
    but it's good
    enough. Listen. Enjoy.
     
  14. A good op amp can be used as a buffer and be sonically transparent,
    That's certainly true. But does it matter what type of circuit or components
    you use if the performance is audibly blameless?
     
  15. MooseFET

    MooseFET Guest


    It also has to work for a reasonable time, be easy to manufacture and
    look good.
     
  16. Eeyore

    Eeyore Guest

    Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop
    gain makes no sense for audio. Besides, gain is cheap these days. I have no objection to the
    introduction of another gain stage for example. I'd rather have a sensible amount of very
    linear and well defined gain than oodles of 'poor quality' gain.

    Yes and yes. Many IC op-amps used for audio have GBPs in the 10MHz region so this isn't too
    difficult even if using one of those inside the loop (which I did in a recent design ).
    Discrete stages suitably degenerated can have higher GBPs than that.

    I'm not sure I 'get that' entirely. I see where you're coming from and that would lead one
    to imagine that pursuit of linearity in individual stages was a pointless pursuit and you
    might as well have tons of non-linear gain and I know that's not the case, not least because
    the very hugh gain system has to be stable and that tends to lead to rolling off the gain
    (and the advantage of NFB) from very low frequencies.

    3 of them would be a bit much. I've not used more than 2 inside the loop in fact.

    Interesting idea.

    Graham
     
  17. Eeyore

    Eeyore Guest

    I know. That's why I said the individual stages should be degenerated to linearise
    them. This results in a lower gain but this may not be a problem in practice as
    long as GBP is maintained.

    In every audio amplifier stage I know, POWER gain is of little importance. Voltage
    gain is what's required. Cuurent gain can be readily added where needed by using
    emiiter followers.

    That was indeed my point wrt giving up some of that *open-loop* gain in a gain
    block.

    Graham
     
  18. Eeyore

    Eeyore Guest

    You can (and people do) argue forever about what is or isn't subjectively
    audible. The '990C' discrete op-amp was mentioned in another thread for example.
    With THD of 0.06% (-64dB) under some conditions it strikes me that those
    distortion products could easily be audible yet ppl leapt to its defence.

    If it can be shown that the defects must be inaudible from first principles
    (such as distortion below 100dB for example) you're on firmer ground IMHO.

    Graham
     
  19. MooseFET

    MooseFET Guest

    You do need merely "high gain" however. This high gain needs to be
    true at the frequencies of interest so the GBP does have to be at
    least some reasonable amount.

    Very high values of loop gain makes for very large amounts of
    reductions in the harmonics within the band. This can argue for much
    more gain than it would normally appear you need if you only needed
    enough gain to be sure that the feedback resistors were what was
    setting the gain.
    Adding stages adds to the phase shifts. This is another "no free
    lunch situation". When you increase the number of stages you also
    want to increase the bandwidths of most of the stages to keep the
    phase shift near the gain cross over within reason.


    Keeping this for reference later:


    [.. stuff we have covered and agree on ...
    No what I am pointing out is that local feedback is not a good
    substitute for a naturally more linear stage. Consider this sort of a
    situation:

    ---------- ------ ---------- ------
    Signal--->! Subtract !---! Gain !--->! Subtract !--->! Bad !--+-------------- ------ ---------- ! gain ! !
    ^ ^ ------ !
    ! ! !
    ------------------------+---------------------

    You can trade back and forth how much subtracting you do in the two
    subtraction circuits but you can't really fix the "bad gain" section.
    I am for not leaving out the idea that the good gain has a limited
    bandwidth and assuming all of the bandwidth limiting happens in the
    "bad gain". I think this makes the idea obvious in it simple form.
    To take a bit of a real example, consider a dreadful output stage that
    works like this:

    ----+--------+---
    ! !
    \ \
    R1 / / R2
    \ \
    ! !
    ! !/e
    +-------! PNP
    ! !\
    !/ !
    ! NPN !
    !\e ! R3
    ----------+----/\/\---
    --- mirror for other half

    R3 is providing a measure of local feedback. R2 also is doing so.
    This stage will still be a horror story. Adding a diode in series
    with R1 to match to the E-B drop of the PNP makes it much less so.
    The diode makes the PNP act much more like a linear current mirror and
    thus reduces the natural distortion.

    Since the transistors used in power stages are usually slower than the
    others in the design. The output is almost always where the pole you
    didn't design in lives.

    Trust me on this: Don't put three inside the loop. Reconsider the
    design if you find yourself going there. Two is ok. One plus a
    feedforwards is ok but three always seems to mean trouble.


    It only works up to a point. It also requires largish (mechanically)
    parts be involved. You have a capacitor and a resistor with fairly
    large swings on them and are working at lowish impedances.
     
  20. Eeyore

    Eeyore Guest

    10MHz seems to work reasonably well but 120dB gain at LF is not a requirement.

    That's sort of what I'm after.

    Needn't be a very significant phase shift. Plus, if the 'natural' phase shift of the existing
    stages is reduced through degeneration, that's all fine.

    Oh yes and degeneration will do that of course.

    Graham
     
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