Dual supply op amp

Discussion in 'General Electronics Chat' started by P Del, Jun 23, 2014.

  1. P Del

    Thread Starter New Member

    Jun 23, 2014
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    I've attached a schematic of a couple op amp circuits. There is a sign wave input to each of the amps.

    I believe the first should center the sine wave on ground. If the supply voltages were +15.5V & -15V would the output be offset from ground.

    The 2nd amp should amplify the sine wave. If the supply voltages were +125.75V & -126.52V would the output positive swing be amplified differently than the negative swing?
     
  2. dougc314

    Member

    Dec 20, 2013
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    The thing to remember with OPAMPS is that they they are not supposed to reflect power supply imbalances in their output signal. They do, slightly, by a specified property called power supply rejection ratio.

    In general the PSRR is so high so that for most practical amplification purposes the error introduced is very small. In the two circuits presented here the gains are modest, ~-5 and ~10 so that the error introduced by power supply imbalance should be quite small.

    However, your circuits are DC coupled. So if there is a DC component to your input signal it will be amplified by the DC gain of the circuit. If you want an arbitrary signal centered around ground you should add a series capacitor (with the input R) large enough to not cause undesired frequency (amplitude or phase) response.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    Op amps are relatively insensitive to differences in their plus and minus supply voltages so these voltages normally have a negligible affect on the offset or gain of op amp circuits.
     
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  4. P Del

    Thread Starter New Member

    Jun 23, 2014
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    By "very small" & "negligible" would you mean that one swing of a +15V/-15V signal would not be 0.4V greater than the other?
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Correct. In fact, I doubt that you could measure any offset caused by unequal power rails. Very, *very* small.

    ak
     
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  6. Lestraveled

    Well-Known Member

    May 19, 2014
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    A typical op-amp, a LF351, has a power supply rejection ratio of 80 dB.

    dB= 20 log (v1/1volt), (dB volts)

    80 dB = .0001 volt per 1 volt change in PS voltage.

    Yup, pretty small change.

    Mark
     
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  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    But (and it's been a while since I had to dig into this), the PSRR is an AC effect; that's why it changes with frequency. I don't think there is anything in a typical opamp design, other than some kind of inherent non-linearity in the input stage, that would cause a DC offset in the output related solely to an inequality in the power rails relative to GND.
     
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