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class A push - pull amplifier ?

Discussion in 'General Electronics Discussion' started by anne ranch, Aug 2, 2021.

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  1. anne ranch

    anne ranch

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    Aug 2, 2021
    I am looking into building a simple push--pull amp using 2N2222.
    I need to verify that push-pull configuration COULD work just fine without heat sinks.

    Also - most designs use resistor in emitter and it is USUALLY 10 Ohms.
    Some designs use two or more transistors in parallel and reduce the emitter resistor down to 1 Ohm.

    What exactly is the purpose of such small resistor in first place ?

    I can supply schematic when somebody gets interested to discuss this .
     
  2. DBingaman

    DBingaman

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    Jun 27, 2021
    Post the schematic, be happy to look at it.
     
  3. Audioguru

    Audioguru

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    Sep 24, 2016
    The emitter resistor in the output transistor stage is never as high as 10 ohms, it is usually 0.1 ohms to 0.33 ohms.
    The resistors allow matching of the Vbe and current gains of the output transistors.
    Here is a simple class-AB amplifier using little transistors that are near their maximum allowed current so they work poorly producing distortion. The output power is so low that the transistors do not need heatsinks.
     

    Attached Files:

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  4. anne ranch

    anne ranch

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    Aug 2, 2021
  5. anne ranch

    anne ranch

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    Aug 2, 2021
    1. The schematic I have looked had 10 Ohms emitter resistor with a note that "the size depends on power desired " and ended up with 1 Ohm resistor.
    I am just asking for more technical explanation - this "try this and that " is little vague.
    2. I have some understanding how callas A operates , however , this operating on half of the wave does not really explains why there is no need for heat-sink.
    In my case I will be using 2N2222 (metal can - TO8 ?) and want to have maximum gain without smoking the transistor .
    3. Now I am looking to have CW HF ( 14MHz) only , so should I be (too) concerned about distortion?
     
  6. VenomBallistics

    VenomBallistics

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    Aug 30, 2018
    What I read specified a NEED for a heat sink, which makes perfect sense as the transistor is never "off"
     
    Last edited by a moderator: Aug 7, 2021
  7. crutschow

    crutschow

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    May 7, 2021
    Push-pull is not Class A, it is Class AB or B.
     
  8. WHONOES

    WHONOES

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    May 20, 2017
    What do you want to use your creation for. The link you provided seems to be for r.f. amplifiers.
     
  9. Audioguru

    Audioguru

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    Distortion is produced by harmonics. The harmonics of 14MHz are at 28MHz, 42MHz, 56Mhz, 70MHz and higher that cause interference to licensed radio communication and will make your circuit illegal.

    A Metal case 2N2222 is expensive, rare and old but is still available as a replacement. Its maximum output power without a heatsink is only 0.5W. Maximum gain is voltage gain or current gain and is different from output power since you can have maximum gain with a very low output power (and very low heating).
     
  10. VenomBallistics

    VenomBallistics

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    Aug 30, 2018
    in the example given, true. I think our subject is terminally confused on the subject matter.
    The 2N2222 is not any sort of power transistor. It wont sustain any serious duty WITH a heat sink, let alone without.
    The article even goes so far as to say a heat sink is required.
     
  11. anne ranch

    anne ranch

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    Aug 2, 2021
    I am busy trying to load / install spice circuit analysis software .
    Hopefully it will actually answer my questions.
    Cheers
     
  12. VenomBallistics

    VenomBallistics

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    Aug 30, 2018
    it might answer some of them, but it does not show you heat
     
  13. WHONOES

    WHONOES

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    Try Simetrix. It's very good.
     
  14. anne ranch

    anne ranch

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    Aug 2, 2021
    I am still operating under assumption that push-pull , in whatever class, doesn't IN theory produce heat - until proven otherwise.
    Currently using ltspice and I have the basic schematic ready to be analyzed.
     
  15. WHONOES

    WHONOES

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    May 20, 2017
    Class A can also be push / pull. Heat is generated in the majority of circuits, it just depends on the level. It is a by product of power being consumed.
     
  16. VenomBallistics

    VenomBallistics

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    Aug 30, 2018
    a quote from the page you linked to ....
    "The big disadvantage of this circuit is that TR1 must have a high standing DC current, which usually means the transistor must be fitted with a heat-sink. "
    A class A amp, by definition, works on the principal of of it's active components being biased to a half way point in it's quiescent state. It's never off, current is always flowing, thus it's making heat, even at no signal.
     
    Last edited: Aug 4, 2021
  17. anne ranch

    anne ranch

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    Aug 2, 2021
    You could read "the rest of the story " especially about "push-pull"
     
  18. VenomBallistics

    VenomBallistics

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    Aug 30, 2018
    I've built class A push pull amps, even without heat sinks.
    But they were tube fired.
    Do you men Class AB? If so, these are not a true class A in that they are not biased to midpoint, but rather, just inside the range of conductance. They are really just class B with a dirty shortcut at the bias generator.
     
  19. anne ranch

    anne ranch

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    Aug 2, 2021
    I am not that concerned what is the class - the objective is to build and analyze using spice .
    As long as the smoke stays in the TO can , I am happy camper.

    At this point I have my basic schematic in EAGLE .
    My next step is to add spice info to it - seems a little challenge.

    As far as hardware - I found suitable strip board and got the transistor sockets mounted .
    I did not find 1 Ohm resistor - so I got 2.7 Ohms in parallel and I can reduce it by adding more 2.7 in parallel.
    Little messy but will do for the purpose of this experiment.

    Now the question is - which direction to wind the load torroids?

    Should the load winding start at the collector ? hence wound OPPOSITE to each other?
    I am busy googling, but so far nothing...
     
  20. WHONOES

    WHONOES

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    Class AB transistor amplifiers are always biased to the mid point otherwise they would not work correctly. Class A transistor amps are just class B (or AB) biased to have a large current flowing in the output stage in the quiescent state. A class AB would have the same output stage topology but would be biased so that the output stage passes just a couple of mA in the quiescent state.
     
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