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single rail opamp

Discussion in 'Electronic Basics' started by Neil Bernard, Nov 15, 2003.

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  1. Neil Bernard

    Neil Bernard Guest

    Hi I want a balanced output using two opamps (for audio) using a single rail
    power supply +9v any ideas?
     
  2. Use two resistors (or a pair of 5.1 volt zeners) in series across the
    9 volts with a pair of .1 uf caps paralleling each of those parts to
    produce an elevated zero signal reference. Then invert the out put of
    the first signal to produce the second one.

    If you can spare a third opamp, you can use high values for the
    divider resistors and use that opamp as a follower to produce a
    stiffer zero signal reference voltage.

    If you want to keep output loading on the first output from altering
    the second one, build an inverting and noninverting pair pf stages to
    amplify the signal in parallel to produce the two outputs.

    Keep in mind that many rail to rail opamps do not handle load
    capacitance (cables) without going unstable. You may need to increase
    their stability with either series resistance (say, 100 to 620 ohms)
    between the opamp and the cable, or an RC series load between the
    outputs and the supply rails (a pair of small caps, say, 100 to
    1000pf, in series across the supply rails, with a resistor. say. 1k to
    2k, from the center point to the output).
     
  3. Bob Masta

    Bob Masta Guest

    If you are *only* going to use the outputs as a balanced pair,
    and don't mind a big common mode voltage (4.5 V in your case)
    then this should be pretty straightforward. Just use a normal
    balanced output design, with a pseudo-ground at half the
    supply. The problem with this approach is that in the real world
    of audio, it's not unusual to try to use one side of a balanced
    output relative to ground. If you build this for a special purpose,
    don't use a 3-wire connector or someone may think they can
    do that here.


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  4. dB

    dB Guest

    "Neil Bernard" wrote

    If your required gain is modest enough to be achieved with just one
    opamp, drive one side of the output from this and invert it in another
    to drive the other side, capacitively coupling both to get rid of the
    d.c. offset.
     
  5. Luhan Monat

    Luhan Monat Guest

    John,

    That pair of zeners in series is a "no-no" (at least with out some
    resistor in there somewhere).
     
  6. A pair of 5.1 volt zeners across a 9 volt battery conducts almost no
    current, while limiting the reference voltage to within a volt of the
    center of the battery rails. 4.7 volt zeners would hold the reference
    even closer to the center but would leak a bit more, especially if
    they were both at the low voltage side of their tolerance.
     
  7. Luhan Monat

    Luhan Monat Guest

    Yo,

    'limiting the reference voltage' is insufficient. All of the stages
    using this for center line voltage would have an extra 'signal' injected
    into them. It would be better to use just one zener with an appropriate
    resistor on the other leg.
     
  8. Not better from a battery life viewpoint. The capacitors connected
    across the zeners keep the voltage swing at the reference from
    happening fast enough to effect the amplified signals much.

    I suggest you try it... both ways. The two zener approach also makes
    lots less high frequency noise, which does get through the opamps.
     

  9. But what if the supply rises to 10+ Volts? Often people use cheap
    unregulated wall supplies as battery substitutes, and these can supply a few
    volts above the rated voltage. The Zeners will then conduct hard and the
    result won't be pretty... I think the Zener idea can be a hidden danger in
    this case, unless you are sure the supply will never exceed 10V. Still a
    good idea for maximisisng battery life though...

    Costas
     
  10. It will keep a load on such a supply. ;)
     
  11. Neil Bernard

    Neil Bernard Guest

    Great thanks for your help guys :)
     
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