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Help with transistor amp circuit

Discussion in 'General Electronics Discussion' started by McDroogie, Jan 10, 2013.

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  1. McDroogie

    McDroogie

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    Dec 25, 2012
    I am trying to build this transistor circuit. Can anyone tell me if they see any mistakes in my wiring?
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, firstly, you have wired it up so that the connections to the transistor would be E-B-C, looking at the flat face of the package. If this is indeed a BC547, the datasheet indicates that the leads go C-B-E.

    The easiest fix is to simply pull it out and turn it around.

    The second issue is that the input lead is connected directly to the base. The resistor R2 the base, and the input lead are all shorted together.

    The easiest fix for this is to connect the resistor from the base to another row of pins, and then connect the input to that.

    What you appear to have done is tried to have built the circuit literally from the (visual appearance) of the schematic rather than following the logical function of it. This is a trick you need to be able to do. The actual circuit layout may look quite unlike the schematic.
     
  3. McDroogie

    McDroogie

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    Dec 25, 2012
    I did as you suggested and it works now. But when I remove the transistor, it still runs as normal. How can I see if the transistor is actually affecting working with the circuit?
     

    Attached Files:

    Last edited: Jan 11, 2013
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If you remove the transistor and it still works then you have a problem because without the transistor in the circuit there is no path from the input to the output so the output will remain as a fixed and unchanging 5V.

    But there's way more wrong.

    Now you show more of your circuit and what you're trying to do with it (and that's helpful)

    Somehow you have decided to connect your input straight across your battery. That is nothing like what is shown on the schematic. Why have you connected the base resistor to your power supply? This should be connected to your input.

    Also now I can see that you're using this to amplify audio. Whilst it's kinda going to work, there are major problems as this circuit is not designed to amplify audio. At low voltages like this the speaker won't actually be damaged, but the DC you're putting through it is not what it is designed for.

    However, let's get it working (as I said, you're unlikely to damage anything).

    Connect R2 between the base and your input signal, and DO NOT connect it to anything else.

    Do you know which way the conductors run on these prototyping boards? You may be confused by it. This site explains it well. Your use of the power lines is fine (it differs from his recommendation)

    edit: the break in the power bars is not always present. It pays to check. Making an assumption either way can spoil your day.
     
    Last edited: Jan 11, 2013
  5. McDroogie

    McDroogie

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    Dec 25, 2012
    OK I connected the input to the resistor. I get very poor sound and when I remove the transistor the music stops so it's working. And I'm using a 9v battery. Could this be the problem or could it be the resistor values? Or the lack of a capacitor? I do get the basics of the protoboard, however sometimes I under look sections and have them connected vertically rather than horizontally.
     

    Attached Files:

  6. davenn

    davenn Moderator

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    Sep 5, 2009
    what is the signal source ? phone? .... mp3 player? ... something else

    the 2 mentioned things have much more output than what this single transistor amplifier can produce so you would need the output level of the source turned down really low

    As Steve said, "it will work ... sorta "
    this is just a really basic circuit, not really designed for an audio amplifier
    to improve it a little bit you could use capacitors in what they call a decoupling mode,
    this keeps the DC supply to the transistor from going places it shouldnt

    so get a couple of 1uF or 4.7uF caps ( value for experimenting not too critical), put one
    in series with the signal input resistor to the base of the transistor and put the other one in the output lead to the speaker

    make a note of the difference it makes

    Dave
     
  7. McDroogie

    McDroogie

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    Dec 25, 2012
    It's my phone's mp3. I tried to connect the caps but had no success. I am just trying to look for simple circuits to construct that will help me build on the basic fundamental of electrical circuits. As sSteve mentioned I try to simulate the circuit exactly as it is drawn.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Dave, the amplifier is not biased at all, so the sound will always be poor due to massive distortion.

    McDroogie, I would recommend that in addition to what Dave has suggested, you place a 100k resistor between the collector and the base of that transistor. This won't be perfect, but it should be better.

    Also, you may need to turn the volume on your player right down so that you keep the transistor operating in a relatively linear region.
     
  9. davenn

    davenn Moderator

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    haha yeah I knew there was something else I was supposed to mention ;)

    I was distracted by the BOSS coming into the workshop every 5 mins

    Dave
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Something like this is what we are suggesting:

    [​IMG]

    This isn't a really good design, it's just an incremental improvement. It's also not really appropriate for driving a loudspeaker (except at very very low volume).

    C1 should be (say) 1 uf, C2 can benefit from being larger. What value have you used for R2?
     

    Attached Files:

    Last edited: Jan 11, 2013
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