Basic Audio NPN amp, BAD Gain

Discussion in 'The Projects Forum' started by logans-electronics, Sep 5, 2009.

  1. logans-electronics

    Thread Starter Member

    Sep 1, 2009
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    Hi everyone,

    I amp trying to get a basic NPN audio amp to work in Multisim 8 before buying anything.
    I was having trouble with distortion...but after some experimenting I found my input signal too high. As you can see now I have a gain issue (actaully a small loss). I love using MPS A06 and A56 transistors. Any advice on what I should be trying to calculate or determine first before just putting things together?.I would like a gain of 10 or more....max wattage right now is not important.

    ....any way I am trying to build an audio amplifier capable of handling 4 to 8 ohms. Input would be a CDplayer or Ipod. I know once I load this amp down in Multisim it will crash..I am just trying to take one step at a time. Thanks,
    Logan
     
  2. logans-electronics

    Thread Starter Member

    Sep 1, 2009
    36
    0
    Drawing and voltage measurements.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    That is not a practical circuit to drive a 4 to 8 Ohm speaker. You need power gain not voltage gain. Therefore you would be better off with an Emitter Follower, as they can present a low output impedance and power gain. That said, a single transistor running class A would waste appreciable power when quiescent. ;)
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Didn't you calculate anything when you selected the bias resistors?
    Your transistor is biased incorrectly and is saturated with its collector and emitter voltages nearly the same. The voltages will actually be a little less than I show.
    Also it is missing a very important supply bypass capacitor.
    Also its input capacitor had backwards polarity.

    I corrected your circuit and if its load is a high impedance its gain is 4.7k/470= 10.
    It will not drive a speaker.
     
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    first of all the collector resistor should be at least 10 times smaller than the load. So it would not work to design a class A single stage amp for such a load of 4-8 ohms.
    Other components or configurations would be involved.

    But if your just starting out in learning how to bias a transistor for small signal amplification, then here is one
    way to do it, as there are many ways of approaching this, depending on the application.

    1. Choose Vc to be 1/2 Vcc
    2. choose a value for RC ...(R9)
    3. choose RE...(R6) to be around ....RC / V.gain.
    4. calculate IC = 1/2Vcc / RC.
    5. calculate VE = IC x RE.
    6. calculate VB = (VE + Vbe.)
    7.choose RB1....(R4) to be 10 to 20 times greater than RE.
    8. calculate IRB1 = VB / RB1.
    9. calculate RB2...(R10) = (VCC - VB) / IRB1.

    Ex. using your schem.

    1. VC = 15V. (30V. / 2)
    2. RC = 4.7K
    3. RE = 470 ( RC / 10 where V.gain = 10)
    4. IC = 3.19mA (15V. / 4.7K)
    5. VE=1.49V. (3.19mA x 470)
    6. VB=2.19V. (1.49V. + 0.7V...assuming 0.7V. Vbe.)
    7. RB1 = 4.7K (arbitrary choice)
    8. IRB1=465.9uA (2.19V. / 4.7K)
    9 RB2=59.69K ((30V - 2.19V) / 465.9uA)

    then choose the standard values for all resistors.

    RC=4.7k
    RE=470
    RB1=4.7k
    RB2=62K

    then prototype it and take key measurements and adjust values where needed.
     
    Last edited: Sep 6, 2009
  7. Wendy

    Moderator

    Mar 24, 2008
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    If you're circuit is trying to drive an 8Ω speaker, then from an AC signal point of view the 8Ω is in parallel with Rc, not a good situation. When designing a circuit like this you can not ignore the impedance of the sourse or load (mostly the load), since it does interact with the design quite a bit.

    So instead of having a gain of 470/47, it is actually (470Ω║8Ω) / 47Ω.

    You can get around this by using a second emitter follower (AKA common collector) connected to the collector of Q1, and the capactor C2 is connected to it's emitter. Not a great solution, but it will work.

    You can also add a large capacitor to ground fromt the emitter of Q1 to ground. A less satisfactory solution, but it will improve your numbers.
     
  8. logans-electronics

    Thread Starter Member

    Sep 1, 2009
    36
    0
    Thanks everyone for the advice....I will work on this and let you know what happens!!
     
  9. logans-electronics

    Thread Starter Member

    Sep 1, 2009
    36
    0
    This should do it...now after all this work, I realized I could have used a LM833 OP amp:)
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    If your intent is to drive a small speaker, as you said, you would be better off using a LM386. They're available from RS.
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your new transistor has its input capacitor with backwards polarity again.
     
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