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PNP transistor in CLass B push pull amplifier

Discussion in 'General Electronics Discussion' started by Damien, Jun 2, 2015.

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  1. Damien

    Damien

    55
    1
    Jan 28, 2015
    Hey guys


    I'm having a little trouble understanding how the PNP transistor in this circuit conducts half of the audio wave form as I believe that it must source it's current from the NPN transistor & if the PNP is to conduct on the negative half of the wave form (As this is what it's job is) It would need to have current available at it's emitter to pass through it's collector to ground? I presume that the load (speaker) goes in between the two transistors & then goes to ground? My main point of confusion is that if the NPN transistor is off & the audio signal at the bases of the transistors is in it's negative cycle, then where does the PNP get it's power from ? I presume that in the negative half of the cycle that the NPN is off and therefore not passing current and since the PNP gets it's current from the NPN then wouldn't the total current come to a stand still? Any advice would be appreciated as I have been racking my brain for a couple of days over this little thing. I presume that there is some kind of loophole that allows a little current to flow by biasing the bases but would that mean that the PNP can't fully switch on and therefore not conduct as strongly as the NPN? I can kind of see how this circuit would work if it were to be used with a couple of transformers at either end of the circuit but just a little confused as to how to go about the design with a transformerless circuit.
     

    Attached Files:

  2. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Perhaps it would help if you provided a reference to where this circuit came from. I don't see how it could possibly work either.
     
  3. Damien

    Damien

    55
    1
    Jan 28, 2015
    Just found it on google images.
    I think part of the circuit must be missing.

    I hear that there are two ways of building a class B?

    One involving a couple of center tapped transformers with two NPN's & the other using two separate power supplies with an NPN and PNP with the power supplies being + and the other being a - type power supply?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Instead of using separate "+" and "-" supplies you can build a class B amplifier with a single supply by adding an offset to the AC signal.
    The circuit you have is obviously a schematic showing the most elementary principle of an amplifier and lacks all details.

    See here under "class B transformerless output stage".
     
    Damien likes this.
  5. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Here's one answer.
    The NPN drives a load and generally this load is a speaker and electrolytic in series.
    When the NPN transistor turns ON, it delivers current to the speaker via the electrolytic and during this time the electrolytic charges a small amount.
    When the PNP transistor turns ON, it effectively shorts the top of the electrolytic to the 0v rail.
    The electrolytic will have a small charge on it (in it) and it will deliver this energy to the speaker.
    If you look at the positive and negative ends of the electrolytic you will see it actually supplies a voltage in the reverse direction to when the NPN was turned ON and thus the cone of the speaker initially moves into the speaker and then moves out of the speaker.
    This is how the full excursion of the cone is achieved.
     
    Arouse1973 and hevans1944 like this.
  6. BobK

    BobK

    7,671
    1,681
    Jan 5, 2010
    That is a classic push-pull output stage, though, as noted by others, a capacitor is needed in the output if it is to be powered from a single supply.

    Here is an actual low-power (1/4W into 8 Ohms) amp that I have built:

    upload_2015-6-2_11-7-19.png

    The Red trace is the output. The green trace is the emitter current in the PNP and the blue trace is the emiiter current in the NPN. You can see that the NPN (Q4) provides the current for the positive part of the waveform and the PNP (Q3) provides the current for the negative part of the waveform.

    It is capable of swinging ±2.4V or about 350mW before clipping when running from a 9V battery.

    Bob
     
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Moral of this story: you need to have all of the schematic and all of the parts to understand what is going on. The original image was probably part of a tutorial (maybe this one), and the associated text probably provided an explanation as @Colin Mitchell and @BobK did here. Can't argue with Colin's description: concise and to the point. You should try to build Bob's version to get a good "feel" for what is going on. The parts are commonly available and very inexpensive. I keep two plastic 35mm film cans on hand filled with 2N3904 and 2N3906 transistors just to "play" with circuits like this.

    I learned something here today. The complementary-symmetry directly-coupled (transformer-less) output stage using NPN-PNP transistor pairs is ancient technology, very popular in the 1960s for construction of moderate-power stereo "hi-fi" amplifiers by both hobbyists and commercial manufacturers. A great amount of effort was expended to reduce harmonic distortion or cross-over distortion by biasing the output to class AB, instead of pure class B as Bob's circuit does, and using negative feedback to make the output "stiffer" for driving speakers. What I just learned (or maybe forgotten and was reminded by Colin) is that a simple capacitor is all you need to eliminate a negative rail for the PNP. Of course today you can purchase integrated circuit amplifiers with all the details hidden inside the package. But it is fun to sometimes "roll your own" as Bob did.

    Here is a Google search with links for further reading.

    Hop
     
    Arouse1973 likes this.
  8. Damien

    Damien

    55
    1
    Jan 28, 2015
    Thanks so much everyone I think this will help alot and yep I will have a go at Bobs circuit thanks for sharing that Bob.

    Yeah it's weird I guess as I have access to any sort of amplifier off the shelf but choose to make my own to get a better understanding & also want to make my own valve guitar amp in the future as I think I will appreciate it much more than just buying one. (Lower cost & more appreciation) =)

    I suppose also I want to make sure I know what I'm doing with solid state parts before messing around with valves as they are expensive to break! hehe. (Also going down this path due to the inherent danger with high voltages while working with valves).

    Has anybody else found that making electronic circuits as a beginner is very time consuming? I spent 3 hours in the shed with a breadboard to try and get a B class working last night with no success.
     
  9. BobK

    BobK

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    Jan 5, 2010
    My circuit is actually class AB. The transistors are both on in idle mode passing about 2mA of current. If you did not do this, the sine wave would have an ugly jag in it around the zero point. And the two diodes make up for the base-emitter voltage drop in the two output transistors so that they switch on and off seamlessly.

    Bob
     
    hevans1944 likes this.
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