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Transistor amplifier

Discussion in 'General Electronics Discussion' started by FutureFuzz 26, Nov 15, 2012.

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  1. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    ok guys, im new here
    i wanted to start by saying that i am a regular on CarAudioForum, which is what leads me here

    i have been into car audio my entire life, and now i want to take my skills a bit more. i have been experimenting with a few transistors to figure out how an amplfier works.

    yesterday i went and bought some transistors and drew up a design of how i thought i could make it amplify a AC signal, went over to the welding shop and found a piece of scrap aluminum to use as a heatsink. then i took my design to a buddy that is an electronics engineer, he simply looked at it and said that it wont work. but i was certain that it would

    i took home all my new parts and set it all on the kitchen table (alongside my Fluke DMM)

    this may be difficult to follow so please pay attention, i shared a common ground between the input and battery on the emmiter pin, the input positive on the Base pin, and the output negative from the collector pin, the output positive came directly from the battery positive. at the input i was getting 121MV at 60hz, on the output i was getting 143MV at 60hz, thats an amplification correct? that proves that it works?

    [​IMG]

    i know this as all basic to you guys, but im only 17 and am trying to learn the inner workings of an amplifier

    P.S. please pardon my poor circuit drawing skills
     
    Last edited: Nov 15, 2012
  2. BobK

    BobK

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    I am surprised you get any output if the input is as described. It takes about 600mV on the base of the transistor to turn it on. In any case, even if it does amplify, it will do so with so much distortion as to be useless.

    A resistor from the battery + to the base, chosen to pull the collector to 1/2 the battery voltage, and a capacitor to couple the input to the base might make a working class A amplifier, which is not a practical amplifier in terms of efficiency. And it will be operating with the speaker pulled to one side of the center all the time, limiting the output to about 1/4of the power it could normally handle.

    Bob
     
  3. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    yes, i understand that it is relitivly useless, i am using this as a learning experience.

    please explain to me how the resistor and cap would be put into the circuit, and how does it do what it does. im sorry i am full of questions, but im intrested to learn
     
  4. BobK

    BobK

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    Jan 5, 2010
    The resistor would go from the battery + to the base of the transistor. You want it to make the collector 1/2 the battery voltage when there is not input signal. Start with 1K, if the collector voltage goes too low, raise it, if not low enough lower the value of the resistor. This technique is known as biasing, which means putting the transitor into the region where it is partially conducting. (by the way, I assume the battery voltage is low, like 6V or less, otherwise you might blow out the speaker doing this.)

    After the correct resitor is connected, you will find that the voltage on the base is somewhere around 0.6 to 0.7 V. Now, if you connect an input directly to that, you will be fighting this voltage. So you put a capacitor between the input + and the base, which allows these voltages to differ, while allowing changes in the voltage (i.e. the AC signal) to get through to the to base. These changes should be amplifed at the collector. The capacitor should be about 1uF.

    As I said before, this is not an ideal amplifier, but it is about the simplest amplfifier you could make to work. From your previous comments, I assume the transistor is a power transistor. Is that correct? What transistor exacly is it?

    Bob
     
  5. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    It's a big wide transistor off of an older Kenwood amplifier, I suppose it's a MOSFET

    And I was using a 9v battery as my power supply
     
  6. BobK

    BobK

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    Your terminology led me to believe you had an NPN bipolar junction transistor. MOSFETs don't have an emitter, collector or base. If you don't know what kind of transistor it is, how can you know which lead is which, there is not standard for pinout. I suggest that you start by getting a known component.

    bob
     
  7. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    I saw it on a website, to wire it that way

    How about we start over and you tell me what I need. But yesterday i got a MOSFET power transistor, that's what I need for this project correct?

    As you can see my knowledge is basic at best, I'm looking to learn
     
  8. BobK

    BobK

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    Which MOSFET?

    Bob
     
  9. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    Will a bi-polar NPN work, or must it be a FET?
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    you could use a MOSFET or a bipolar transistor.
    you need to understand that you need many more components around the device(s) before you have anything even closely approaching a useable amplifier. Which is why you EE buddy said that your current setup wont work.

    Do some googling on audio amplifier circuits ( schematics) and start getting an idea of whats involved in building an amp


    Dave
     
    Last edited: Nov 16, 2012
  11. stanfidavid

    stanfidavid

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    Nov 16, 2012
    It takes about 600mV on the base of the transistor to turn it on.[​IMG]
     
  12. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    Ok guys, I know I seem stupid. But I'm ignorant. But I did build a very small low quality amplifier. Well it's actually a pre amp, but drives a pair of headphones pretty decently (but has a lot of distortion) but hell, alteast its a working start

    I'm sure you think it's useless to build something soo weak and so low sound quality, but I did achieve my goal, read a schematic and build a circuit. And I can use this as my beginning of experimenting (as im not going to disassemble my many hundred dollar amplifiers around the house) I'll post pics of the schematic I used and the resulting circuit
    [​IMG]

    [​IMG]

    Feel free to tell me what I did wrong, and/or what I did right. And do you have any obvious clues as to why it gets soo distorted when the volume is over 60%? From my experience in car audio it does not sound like clipping, just simply distortion
     
  13. wingnut

    wingnut

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    Aug 9, 2012
    Well done at making this circuit, which you got from ...
    http://hackaweek.com/hacks/?p=327

    Maybe a good next step would be to buy a LM386 audio amplifier to add to your pre-amp. These are less than $1.

    For better quality you need more complexity.
     
  14. BobK

    BobK

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    Jan 5, 2010
    As a microphone preamp, that circuit is probably okay. What are you using for input? Any amplifier will distort when the input times the gain exceeds the voltage that it can output.

    If you want to make a simple (inefiicient) amplifier that will drive a speaker, do the following:

    1. Replace the 2N3904 with a TIP31 and heat sink it, it is going to get hot.
    2. Replace the collector resistor with a 15 Ohm (not K) resistor.
    3. Replace the output capacitor with 220uF
    4. Replace the resistor between collector and base with something in the range of 100 to 1K that brings the collector voltage to 1/2 the battery voltage with no input signal.
    5. Get rid of the resistor from the V+ to the input and drive the input from an MP3 player or other high-level source.

    It will exahaust your battery rather quickly though.

    Bob
     
  15. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    I was using my IPhone as an input, I ofcoarse wasn't looking to build a 2k Rms @ 1 ohm amplifier, I was just looking to put a simple one togeather to get an understanding of how it works

    I have another quick question? What it the advantage of using a MOSFET over a bjt, I am under the impression that mosfets handle more power, but are not linear as far as output
     
  16. BobK

    BobK

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    Jan 5, 2010
    Neither the BJT nor the MOSFET is very linear. A good amplifier will rely on negative feedback to make it linear, which can be done with either.

    Bob
     
  17. FutureFuzz 26

    FutureFuzz 26

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    Nov 15, 2012
    so why do amplifier manufacturers advertise "Mosfet Design" when a bjt will do the job?

    i was under the impression a BJT is more linear that a Mosfet, but Mosfet will handle more power and has faster switching speeds
     
  18. BobK

    BobK

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    Jan 5, 2010
    Why do bathroom cleaners advertise scubbing bubbles?

    Bob
     
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