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understanding discrete circuits

Discussion in 'General Electronics Discussion' started by Rajinder, Nov 19, 2017.

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  1. Rajinder

    Rajinder

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    Jan 30, 2016
    Hi all,
    I have a Btec OND in electronic engineering.
    Which I qualified for a few years ago.
    My question is that I find it difficult to analyze circuits that I have seen to figure out what is going on. Something with say 3-4 transistors NPN and PNP starts to baffle me. I look at a circuit and can't understand what is going on, this is more of a problem with analogue electronics circuits.
    Has anyone else had the same issues and could someone help, any advice would be appreciated. I have tried to reach learn a few of the basics again but just can't seem to fathom out what happens in some of these designs.
    Any help would be appreciated.
    Best regards,
    Raj
     
  2. davenn

    davenn Moderator

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    honestly .... how can you have that and not understand circuit basics ?
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Maybe he did his degree during the valve era?

    @Rajinder, can you show us an example of what confuses you?

    What level of understanding are you after? Do you just want a qualitative understanding, or do you want to be able to derive a full quantative understanding?
     
    Arouse1973 and hevans1944 like this.
  4. davenn

    davenn Moderator

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    maybe ;) :rolleyes:
     
  5. Rajinder

    Rajinder

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    Jan 30, 2016
    Hi, yes I will show some circuits.
    I will post a couple later, the issue being I can't see what the interconnecting components and circuits in general is trying to do.
    Thanks for your reply

    My OND was done around 10 years ago.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Use it or lose it. I believe most high school graduates here in the United States, immediately after graduation, remember little to nothing of what they studied immediately before graduation... and much less than that ten years after graduation. And I am being generous, assuming they actually studied... and presumably learned something.

    So many modern youth consider education to be right up there with serving a prison sentence. It never occurs to them that an education might be useful in their daily lives.

    Go to the library and check out some basic electronics texts to refresh your memory. Try taking a few free on-line courses in electronics. Post to this forum those circuits you are having trouble understanding. It would help if you try to explain what you DO understand, and where you are becoming confused. People here will try to help you if we see you are putting forth some effort.
     
    Anton V likes this.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Ok, so OND is similar to A levels. I was thinking it was a higher level (I thought undergraduate degree).
     
  8. Cannonball

    Cannonball

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    May 6, 2017
    Nuff said. Bring on the circuits and we will see if we can help.
     
  9. Rajinder

    Rajinder

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    Jan 30, 2016
    Thank you all.
    I will post some circuits.
    Best regards,
    Raj
     
  10. Rajinder

    Rajinder

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    Jan 30, 2016
    Hi all,
    Here is a typical example of a circuit that I can't understand and explain fully. I know it is to drive a loudspeaker, but why all the transistor interconnections is what that confuses me first. transistor example.jpg .
    Any help would be appreciated.
    Best regards,
    Raj
     
  11. BobK

    BobK

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    This is a push pull driver, which you can find lengthy descriptions of many places.

    Not sure what you want us to do here. I could give you a description of it including what every part does, but I don't think that is what you are looking for, and you would find a better one in many textbooks or online tutorials than I could give in a forum post.

    So, what is your question?

    Bob
     
  12. Rajinder

    Rajinder

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    Jan 30, 2016
    The question is I wouldn't know how to describe the circuit function. For example what is the purpose of C2 and R2, is it a low pass filter? Then why is this connected to C1? What happens when Q3 switches on?
    I put this circuit as an example to highlight my lack of understanding.
    So could someone describe this circuit function for me?
    Thanks
     
  13. BobK

    BobK

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    C2 is there to pass the AC signal from the input to the base of Q3, while blocking the DC level of the input. R2 is negative feedback from the output to the input to set the DC bias level of Q3, consequently of Q1 and Q2. The result should be to set the emitters of Q2 and Q3 to anout half the supply voltage when there is mo onput signal.

    Bob
     
    Anton V likes this.
  14. Cannonball

    Cannonball

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    May 6, 2017
    Hello.

    This should be a simple circuit but when I try to trace a signal through it I see things that I never seen before. For instance see how Q3 gets it's bias. From the 9 volt power supply through a 25 Ω speaker coil, a 1kΩ resistor,Q1 base emitter, r2 to the base of Q3. Q3 is the audio driver and it's bias is modulated by the output of the speaker. I've never seen what is supposed to be a class A amp biased this way.

    I've seen this type audio output circuit but the transistors were biased differently. I hope this helps.
     
    Last edited by a moderator: Nov 22, 2017
  15. Audioguru

    Audioguru

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    Q3 does not "get switched on", instead it is always conducting, a little more or a little less with the audio AC. Its base is not modulated by the amplifier output at the speaker, instead its base is modulated by the input signal and is biased by R2 and has negative AC and DC feedback from the amplifier output by R2. Negative DC feedback keeps the average output DC voltage at about half the supply voltage and AC negative feedback reduces distortion, increases high frequency response and reduces gain.

    The speaker connects to the positive supply instead of to ground and R1 connects to the speaker side of C1 for a cheap way to do bootstrapping (look in Google). Bootstrapping produces more gain in Q3, a higher maximum output power and less distortion. A normal way to do bootstrapping that you have probably seen has the speaker connected with one wire at ground and uses an additional resistor and capacitor.

    This is a class-AB amplifier. Q3 is low power class A but the higher power output transistors are biased with a little quiescent current limited by the diodes so that they are class-A for no output and a very low level output, but are class-B for high level outputs for efficiency. An amplifier with a class-A output is a heater, not an amplifier. It produces a massive amount of heat even when it has no output signal.
     
    Cannonball and Harald Kapp like this.
  16. Rajinder

    Rajinder

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    Jan 30, 2016
    Thanks for the help guys. Perhaps this was a bad example.
    I will post another circuit later that a friend of mine did.
    Thanks again.
     
  17. Cannonball

    Cannonball

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    May 6, 2017
    You are welcome. I am sorry if I wasn't much help.
     
  18. Anton V

    Anton V

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    Jul 11, 2015
    I learned. Thanks
     
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