How can this circuit work? -- LF351(Jfet op amp) drive tip127, tip122 amplifier.

Discussion in 'General Electronics Chat' started by PickyBiker, Nov 20, 2015.

  1. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    I was searching the web for a push-pull audio amp circuit that could produce 10 watts. I came across on on circuittrue.com with an op amp feeding TIP122 and TIP127 transistors. It looked like what I wanted, but as I looked at it, I couldn't see how it could possibly work. The op amp output does not feed the base of the transistors. The transistor bases are connected to the + and _ power inputs of the op amp.

    Can someone please explain how this could actually work?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    It's using the volt drop across R3 and R2 to feed the transistors,
     
    Last edited: Nov 21, 2015
  3. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    Isn't the speaker being driven by the transistor collectors?
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Yes, the transistors will provide the current in the speaker.
     
  5. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Even 47uF is too small. More like 2200uF if you want a full range amp with an 8 ohm speaker.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Do not forget about op amp internal circuit.
    12a.PNG

    No, you do not need to change C1 value. C1 has a different task to do.
     
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  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Yes. C1 is not the output coupling capacitor. This circuit does not have one because it has bipolar supplies.

    R1 is the key. Any current flowing through it in either direction must come from one of the two IC power leads because there are no other sources. That current goes through R2 or R3 depending on polarity, and when the voltage drop across the resistors exceeds Vbe, the transistors start to turn on and move current to the load. The overall feedback loop keeps the base current low enough that the transistors amplify linearly.

    There is a problem with the circuit in post #1. Negative feedback is taken from the output of the opamp, not the overall circuit output as it should be. The way the circuit is drawn, the high power output transistors are completely outside the feedback loop, and will saturate. For the correct way to do this, and see several variations, see this thread:

    http://forum.allaboutcircuits.com/threads/output-power-boost-for-op-amps.117595/

    ak
     
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  8. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    That's what I thought, the speakers get power from the transistor collectors, not C1.
    Thanks for the advice. I will adjust the circuit accordingly.
     
  9. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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  10. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    IMG_1071.JPG IMG_1072.JPG Not too happy with this circuit. There is a lot of what I think is called crossover due to no bias. I cant really see exactly how the common collector Push Pull circuit would get the bias either.

    Here's a look at the input sign wave and a really distorted output.

    Is this circuit worth pursuing?
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The power transistors in post #1 are Darlingtons! This changes significantly the circuit bias resistors. Look up the Vbe for the transistors and adjust R2 and R3 accordingly.

    ak
     
  12. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    I have to put this project aside for a couple weeks for vacation. Back to it in a couple weeks.
     
  13. Bordodynov

    Active Member

    May 20, 2015
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    The electronic circuitry is no good. Recent transistors are not covered by the feedback DC. See the results of the simulation. In the second amplifier feedback Ampl3.png is taken from the output.
     
  14. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Did you connect the right side of R4 to the output (between Q1 and Q2 collectors)?
    Find another circuit.
     
  15. PickyBiker

    Thread Starter Member

    Aug 18, 2015
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    I Think you are right Jony130. A better quality class AB audio amp should not be hard to find.

    I'll look for one when I return from vacation in a couple weeks.
     
  16. Bordodynov

    Active Member

    May 20, 2015
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