Output Power Boost for Op Amps

Discussion in 'General Electronics Chat' started by Wendy, Nov 18, 2015.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    or

    An old dog learned a new trick.

    I was going through the text book on experiments and I came across this...

    http://www.allaboutcircuits.com/textbook/experiments/chpt-6/class-b-audio-amplifier/

    [​IMG]

    It solved a problem I've been mulling, mostly due to reduced parts could. I've been over complicating this circuit from Creating a Virtual Power Supply Ground as follows.

    [​IMG] instead [​IMG]
    .....................Figure 11

    Anyone who has been around for a while on this site knows I like to doodle with schematics. So I have redrawn the experiments schematics.

    Pwr Op Amp 1.png

    Pwr Op Amp 2.png

    Figure 4 is iffy, the LED drop must be under 2.3V (a red LED in other words) for it to work, otherwise the transistors will likely smoke.

    I have not built these circuits, just thinking about it. The application as shown is for a speaker (say around 8Ω). Gain could be provided via another op amp, this is just a driver.

    It would also make a decent virtual ground for power applications, like a home brew power supply, to create something that approximates a dual tracking power supply.

    What do you think?
     
    Last edited: Nov 18, 2015
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  2. RichardO

    Well-Known Member

    May 4, 2013
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    Or do the scheme in this data sheet.

    View attachment 76074

    edit: See the High Power Audio Amplifier example.
     
    Last edited: Nov 18, 2015
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    Some of the NS appnotes show the transistor drives derived across resistors in series with the op-amp supply pins, the op amp output is tied to the node between the 2 transistors which is also the point that boosted output comes from.

    As I don't remember the exact configuration, I kept my description a bit vague - better to leave something out than include something that's wrong.
     
  4. Veracohr

    Well-Known Member

    Jan 3, 2011
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    I'm curious about Figure 2. I assume Q2 & Q4 allow for higher current than Figure 1, but what's the benefit of this arrangement over just doubling Q1 & Q3?
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    There are various examples online that don't include any bias diodes for the transistors - but the transistors are enclosed in the nfb loop.

    I've never tried it myself, and even if that worked - trying it with Darlington pairs might be a bit optimistic.
     
  6. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    The advantage is two fold. Major current drive with no loading on the op amp, where as a single transistor arrangement is going to pass some of the loading on to the op amp.

    If the beta of a transistor is 50 or so, my preferred typical value, then the 1KΩ will drop down to 1KΩ/50, or 20. The second transistor makes it a super gain transistor, a Sziklai pair. The advantage to this configuration is that it is similar to a Darlington transistor, but only has a 0.7V B-E drop. This means the 1KΩ resistor now looks like 0.4Ω seen through the transistors.

    They are both aspects of the same thing.
     
  7. RichardO

    Well-Known Member

    May 4, 2013
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    The idea preceded the existence of NS, I think... ;)

    See the High Slew Rate Power amplifier in the attached uA741 data sheet (page 113). :D
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I've used several of these circuits, and always got lower crossover distortion with the common emitters in the power supply leads than with the emitter followers on the output. OTOH, the former needs more attention paid to compensation compared to the latter.

    ak
     
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