Op-amp + transistor = output reaching the upper rail?

Discussion in 'General Electronics Chat' started by amattis, Jul 2, 2012.

  1. amattis

    Thread Starter New Member

    Jul 2, 2012
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    Hi,

    I use an ordinary (non rail-to-rail) op-amp but I need it to reach the upper power rail, or at least a bit closer to it. The lower rail is not as important.

    Can this be done with a simple transistor output stage? Any suggestions?

    I've tried searching the web for similar descriptions, but surprisingly without much luck. Thanks!
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    rail to rail opamps are ordinary too. why bother when something costs $0.10?
     
  3. amattis

    Thread Starter New Member

    Jul 2, 2012
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    Apart for the much more expensive ones, rail-to-rail op-amps seem to work on very low supplies. I need at least +/- 12 V supplies. Any hints? :)
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Find a +13/-12V supply?

    Seriously, can you goose up a small voltage for just this amp?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,229
    The ADA4084-2 and LM7332 are fairly low cost 30V RR op amps.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It used to be common to use a pullup resistor on the opamp output, this runs the output stage of the opamp in "class A" mode for lack of a better term.

    It was also used with some poor quality opamps to remove class AB switching distortion and make them sound better.
     
  7. amattis

    Thread Starter New Member

    Jul 2, 2012
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    Sorry, I should have expressed it more clearly: I already have a +/- 12 V supply, and I need the op-amp to handle this.

    The LM7332 and ADA4084-2 are not low-cost, as they cost 20 and 60 times more than the op-amp I use at the moment.

    If I would use a transistor (or two) on the output of the op-amp in order to reach closer to the upper supply, how would I connect them?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    How much output current do you need? If it's a few milliamps, then THE_RB's suggestion should work.

    If it's more than that, then you could use a PNP transistor with the emitter connected to the +12V and a collector resistor to the -12V. But this will add gain and a signal inversion to the circuit. Thus you will need to compensate for this if you use negative feedback, and the feedback would go from the collector to the op amp positive input due to the signal inversion. This compensation could likely be provided by a series resistor and capacitor to ground between the op amp output and the PNP base.
     
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