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OpAmp Gain Values Revisited

Discussion in 'Electronic Basics' started by [email protected], Oct 28, 2005.

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  1. Guest

    Okay, I will see if I can overstay my welcome in a hurry here :)

    Folks helped me with the first stage of a three stage audio circuit, and I
    get the idea of what each one does, thanks to the NS app notes I was
    referred to, but now what I am not getting is, what might be the purpose
    of R7. I have no notes or access to any designer, just the PCB. Section
    two appears to be a 2X amp, [ (r1+r2/r1)/Vin ] but it does not give me
    2x, and I am guessing it is due to R7. Can anyone speculate on this?

    The circuit is out of a one to four distribution switch. Of course I am
    only showing portions of it, and I ran out of space on the right to show
    the 47 ohm resistor in series with the final output stage. :)

    I have miscontrued the original intent, and thought that each portion was
    to be unity gain, so I asked about gain on stage one. It seems to me that
    the second portion is actually a 2X gain, so I need to adjust the first
    stage down to .5, using 10K resistors? I thought that would give me unity
    gain througout, but I am seeing slightly less than that, so I can only
    suspect the one component I do not fully understand, and that would be R7.
    I don't want to adjust anything, or remove components that may have other
    purposes for being included, but in the end, I just want to tweak this to
    create unity gain without affecting performance doing something I am not
    fully understanding.

    I have tweaked the values, and tried to observe the results on a scope,
    but I prefer concrete understanding, and solid reasoning, over just doing
    a 'plug and play' kind of design.

    Any comments or advice would be appreciated.


    R8
    20K 10K ___
    ___ R4 R6 ___ -|___|-|
    |---|___|--| |--|___|-------| | 270 |
    R1 | | | | | -12V |
    20K | -12V | | |-12V | | |\| |
    In- ___ | | | | |\| | --|-\ |
    -----|___|-+ | |\| | +---|-\ | | >-+---
    |--+----|-\ | | | >-------+------|+/
    | >--+-----------|---|+/ | |/|LM833
    In+ 20K +-----|+/ | |/| |
    -----|___|---+ |/| LM33078 | | | +12V
    R2 | | .-. +12V .-.
    | | R5| | | | R7
    .-. +12V 10K | | | |10K
    | | '-' '-'
    R3 | |20K | |
    '-' | |
    | GND GND
    |
    ===
    GND
    (created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
     
  2. (snip)

    I haven't followed the story very well, up till now, so I don't know
    what NS app notes you refer to and what the middle amplifier device is.
     
  3. Chris

    Chris Guest

    Hi, Mark. For Mr. Popelish's reference, the earlier thread was

    http://tinyurl.com/ey5ch

    and the National Semiconductor appnote was AN-31, "Op Amp Circuit
    Collection". That's the AN consisting of basic circuit schematics for
    most of the basic op amp configurations, along with a relevant equation
    or two for each.

    As shown in your ASCII art above, the first op amp you've drawn is set
    up as a diff amp with a gain of 1 * (V2 - V1). The second op amp is
    set up as a non-invering amplifier with gain of 1 + (R6 / R5), or 2.

    R7 just seems to be a plain old 10K load resistor, which should not be
    anywhere near enough to bog down the op amp output. Even with the
    second op amp railed at V+, it will only draw about 1mA. But R7
    doesn't change the basic gain equation of the op amp. I believe it's
    there because the NPN transistor internal to the LM833 output is a lot
    better at sourcing current than the PNP is at sinking current. See the
    data sheet. Keeping the output in a current sourcing mode will tend to
    improve slew rate, and reduce crossover distortion issues.

    http://www.national.com/ds/LM/LM833.pdf#page=1

    The third op amp is a voltage follower (a non-inverting amplifier with
    a gain of 1). The 50 ohm output resistor at the third op amp output
    may be part of a voltage divider, or it may just be made so that the
    amplifier output impedance is about 50 ohms (51 ohms is the closest 5%
    value). The thing is, the 51 ohm resistor may be acting with the input
    impedance of the signal destination to create a voltage divider, which
    may be slightly reducing your output. Either way, you're right -- if
    you want a gain of 1 for the whole block, you'll have to adjsut values
    on the diff amp resistors.

    By the way, I would also check the exact values and particularly the
    matching on the resistors in the diff amp. Small changes here can make
    a proportionately larger change in the output (see the equations). Use
    1% resistors, and use an ohmmeter to get a match in the input resistors
    and the feedback resistors. Diff amps don't work very well without
    adequate resistor matching.

    Moral of the story -- even techs who just work with the circuits should
    take the time to look at the data sheets. They have good information
    for everybody, from design engineers to hobbyists.

    Good luck
    Chris
     
  4. I see they used a 2k pull down load resistor for the distortion and
    slew rate graphs. I wonder how much worse the result would be without it.

    I still don't see this 3 stage amplifier in any reference. Why not
    leave the middle amplifier out, entirely? Can't get less distortion
    from it than that way.
     
  5. Chris

    Chris Guest

    Good morning, Mr. Popelish. From what the OP has said, I'm tending to
    think he's working on somebody else's homebrew stuff (poor guy -- no
    schematics, no help at all other than muscling out the circuit
    schematic from board inspection). I had mentioned in the prior post
    that someone who has no design experience, but just wanted to have a
    quick "cheat sheet" for elementary op amp circuits on the bench could
    do worse than printing out AN-31, which is just sketches of basic op
    amp circuits with gain equations added for clarity. You would look at
    the individual op amps, then figure out what each one does (diff amp,
    non-inverting amp, voltage follower). That might be some help for the
    newbie.

    You're right of course, the middle amp of the three does look like an
    advanced case of "clean your plate" syndrome. It's actually a good
    indication the original circuit was done by a hobbyist. The LM833 _is_
    a dual, after all. ;-)

    Cheers
    Chris
     
  6. Guest

    Good morning, Mr. Popelish. From what the OP has said, I'm tending to
    Chris, you hit that one dead on :) I just went in and created a schematic
    by reverse engineering the layout with a meter. Man, those SM resistors
    are hard to read :)
    That has come in really handy.
    Well, I was going to ask about that, then I figured I would not ask for
    too much 'free' help, :) but I wondered about the middle stage, and
    figured it was for loading purposes in order to attain four outputs from
    the single MC33708 input.

    Can the one MC33708 drive four LM833's? I can do digital loading analysis,
    but never have gone into the concept using opamps, and am not sure what
    parameters I would have to deal with.


    Thanks very much, again.

    John



    |\| LM833
    -|-\
    ----| >-------
    | -|+/
    | |/|
    |
    LM833 |
    |
    |\| | |\| LM833
    -|-\ | -|-\
    - - | >----- | >-------
    | -|+/ -|+/
    |\| | |/| |/|
    -|-\ |
    | >--|
    -|+/ | LM833
    |/| |
    | |\| |\| LM833
    | -|-\ -|-\
    MC33708 |-- | >-- -- --| >-------
    -|+/ | -|+/
    |/| | |/|
    |
    |
    |
    | |\| LM833
    | -|-\
    --- | >------
    -|+/
    |/|
    (created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)
    (created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)


    It is a 'one in' to 'four out' box, so I assumed it was a loading issue
    that required the middle stage of the circuit.

    Can the MC33708 drive four inputs of the LM833?
     
  7. Chris

    Chris Guest

    An ideal op amp has infinite input impedance. Of course, none are
    perfect, but nearly all standard op amps have very light loading
    inputs. The datasheet for the LM833 says the input bias current is
    typically 1/2 microamp. I would guess you could actually drive dozens
    of just about any op amp inputs from a single op amp output, or
    hundreds if the signal was slow enough that input capacitance and
    lumped transmission impedances weren't a problem. In general, "unit
    loading" just isn't a problem.

    Do yourself a favor -- take a good look at the manufacturer data
    sheets. They provide a lot of really good information that, once you
    decode the terms, is very useful in using the little beasties.

    Don't worry too much about wearing out your welcome -- honest questions
    are always welcome here, and are frequently answered elegantly and well
    by several individuals. (Homework questions unaccompanied by offers of
    payment usually aren't. I believe the unofficial going rate is 20
    zloty per clue. But even just giving them teh answers to their
    homework questions really doesn't help them. Darwin would have
    postulated that the potential techs and engineers who are capable of
    surviving their classes are the ones who will be able to do the job.
    The ones who find their way around Darwin usually aren't a good fit in
    electronics, and even if they don't electrocute themselves, would have
    been better off transferring majors to journalism. In that profession,
    actual facts and analytical thought are irrelevant.)

    Also, you may wish to use google groups with their search engine to
    skim through questions in this group over the last several years
    relating to your question. Typically, if the question was answered
    well 10 years ago, the answer is still good today. You'll be able to
    learn a lot more that way.

    Mr. Popelish in particular provides a lot of attention to detail, is
    almost always able to check the newsgroup daily, and his answers are
    always worth reading.

    Good luck
    Chris
     
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