TIP31 Biasing Problems?

Discussion in 'General Electronics Chat' started by crazyengineer, Jan 11, 2012.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Okay, so once again, I created another orcad simulation so I can try biasing a TIP31 transistor. I wanted to bias so that the ac waveform entering the bas will be centered around 8 volts

    [​IMG]

    However, when I made a plot of the ac voltage entering the 8 ohm resistor, the shape of the 2.83 AC signal entering the base was recreated, but it was reduced somewhere around the millivolt range.The red line indicates the DC voltage at the collector, and the green line indicates the AC voltage at the 8 ohm resistor.
    [​IMG]

    Here's a larger picture of the ac voltage
    [​IMG]

    Did I screw up with my biasing or is this one of the problems of a class A amplifier: the fact that the efficiency is much less than other amplifiers?
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If you look closely, you will see that the peaks of the loudspeaker waveform do not coincide with those at the collector. There is an obvious reason for this.

    What is the reactance of a 10μF capacitor at 100Hz? Is it low compared to the speaker's 8Ω impedance? If not, what might be done about this?

    Edit: There are other issues here, but let's deal with one thing at a time.
     
  3. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Let's see

    The reactance of the capacitor (assuming xc=1/(2*pi*f*C) is 159.55 when a 100 hertz signal is applied. Since the capacitor is connected in series with the 8 ohm resistor, I need a smaller capacitor in order to get more ac power into the resistor. Am I correct?
     
    Last edited: Jan 11, 2012
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
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    Not quite: Xc is inversely proportional to C.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
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    I said this, among other things, in reply to an earlier thread:

    But 10μF is too small.
     
  6. w2aew

    Member

    Jan 3, 2012
    219
    64
    Part of the problem is that you are using a Common Emitter amplifier. A CE amplifier's output impedance is essentially equal to the collector resistance. Thus, when you load it with a low impedance like 8 ohms, you completely attenuate it's output.

    To drive a low impedance like a speaker, you have to use an amplifier with a low output impedance. There are a number of ways to do this:

    1) Use a audio transformer in the collector as your load, with a turns ratio that creates a reasonable collector impedance on the primary side and a low impedance for the secondary for the speaker.

    2) Use the CE amp as you have shown, but follow it with a second stage that can drive a low impedance load - something like a common collector (emitter follower) circuit.

    3) etc...
     
  7. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Okay! I see my mistakes. Although I'll take w2aew suggestion about using a common collector configuration in future designs, I still want to play around with this circuit some more.

    Here's the updated schematic with a 470u capacitor instead of the 10u
    [​IMG]

    VC and 8 ohm resistor voltage plots
    [​IMG]

    8 ohm voltage plot closer up
    [​IMG]
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    ...almost line up with those of the collector load.
     
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