Audio Amplifier Problems?

Discussion in 'General Electronics Chat' started by crazyengineer, Jan 9, 2012.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Okay, so I started learning about class a amplifier will add a current to the input. So I thought a class A amplifier whould be prefect to deliver the necessary power to drive a speaker.

    The speaker is modeled as an 8 ohm resistor and needs at least .5watts of power to work. So I figure I need at least 500ma to be delivered to the 8 ohm resistor. The electret microphone is modeled as a 1mv sine wave. I'm familiar with the fact that there's a lot of distortion in class A amplifiers.

    Here's the schematic in OrCad.
    [​IMG]

    Here's the current passing through the 8 ohm resistor plot
    [​IMG]

    Here's the voltage of the 8 ohm resistor plot
    [​IMG]


    What I want to know (other than potential problems with the circuit) is whether or not it will be enough to deliver power to the speaker.
     
  2. PaulEE

    Member

    Dec 23, 2011
    423
    32
    Correctly built, Class A amplifiers have the least amount of distortion of any of the amplifier classes. They are also the least efficient. Yanking 500 mA out of a single 2n2222 implies that you're dissipating a watt or more in the transistor itself, which exceeds its maximum power rating.

    I would first buffer the signal with a common emitter amplifier that provides power gain, and then follow that with a common collector stage, with a bigger power transistor. An example of "bigger" is a tip31 or something.
     
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  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Your amplifier uses a resistive load of much less resistance than the speaker. The efficiency will therefore be dismally low.

    DC current passes in the speaker. This will deflect the cone, and a small speaker voice coil will overheat [about 2W dissipation!].

    The little 2N2222 transistor is not big enough for this job, particularly if you use an inefficient class A design.

    The transistor is not biassed properly or stably. Even with a properly sized transistor, power amplifiers need to be biassed with some feedback (emitter resistance, usually). Otherwise, as the transistor heats the collector current may increase unacceptably.

    This can wind up in a dance of death called "thermal runaway". That's actually less likely, or at least less lethal when a resistor load is used, but resistive loads are normally considered too in efficient in power amplifiers.
     
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  4. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    So the biggest problem is that I'm delivering too much power to the speaker. If I bias the transistor to be between .9 and 1 volt and have current going through the speaker to be 500ma, will that prevent the speaker from overheating? Or will more problems occur?

    Also, I'm thinking about using this transistor instead of the one in orcad (the problem I keep running into orcad is that it does not have the parts that I need).

    I'll rework the circuit to include an emitter resistance.
     
  5. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Thanks for the suggestion! I just want to learn how to build power amplifier/ audio amplifiers for the time being. That's why I have been posting these random questions
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The BC547 has an even lower current rating than the 2N2222 - see datasheets.

    http://www.sparkfun.com/datasheets/Components/BC546.pdf http://www.stanford.edu/class/ee133/datasheets/2n2222.pdf

    The DC is not useful power, it just pushes the coil away from its best position and makes it get hotter. Very low power circuits may get away with this, but 0.5A continuous in a little speaker is not good.

    It is normal to avoid passing DC through the speaker. One very common way to do this is by putting a capacitor in series with it. Many hundreds to thousands of microfarads are required, depending on how low the speaker impedance is, and how much bass response is required. If you have only a small loudspeaker that only responds down to a few hundred Hz a few hundred μF may do.

    You might just about manage 0.5W output with transistors like 2N2222 / 2N2907 in a normal class AB push-pull, but if you insist on class A, you need something bigger.
     
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  7. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    I'm sorry, but there's a lot of things I need to understand a little bit better.

    I understand that both transistors are too flimsy to use. I'll look else where.

    I guess the biggest thing I'm getting confused about is power for a speaker. If passing dc is very bad for a speaker, and you must have a blocking capacitor in series with the speaker (I'm assuming to only pass the ac value of the signal), then why is delivering power to the speaker such an important parameter?
     
  8. PaulEE

    Member

    Dec 23, 2011
    423
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    Think about the AC and DC equivalent circuits. The biasing of the transistor allows the transistor to "idle" such that, between the transistor and collector resistor, there is about half of the supply voltage. This allows any perturbation at the input (base) to induce a current, which in turn is amplified via beta. The resulting current in the collector/emitter fluctuates, causing the voltage at the output to fluctuate around the idle point (half supply).

    You are interested in the AC from this signal, which is superimposed onto .5*Vsupply at output. Instead of putting the speaker in series with the DC section, capacitively couple it to the output and replace the speaker with a resistor. When the resistor gets smoking hot, you'll understand why class a is inefficient.

    Enjoy :)
     
  9. AetherWerx

    New Member

    Dec 30, 2011
    16
    2
    Why not use an amplifier that is capable of providing you a BTL output? Try the NCS2211 from ON Semiconductor.
    There is no need to provide a DC blocking capacitor in BTL since both sides of the speaker are biased to the same DC level. Besides, you get 4x the power from a BTL architecture than you do from a SE design.
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Let's be quite explicit about this: loudspeakers make sounds in response to audio-frequency Alternating Current (AC)* signals. It is these signals that deliver the required power. Audio frequencies are those which human beings can hear. The frequency range is usually quoted as being from 20Hz to 20kHz, although many people cannot hear all of the upper range, and the lowest frequencies require large and expensive loudspeakers to reproduce fully.

    Direct Current (DC) does not make sound in loudspeakers, and can be harmful, so it is best excluded from them. (Very small circuits passing really low currents may be OK.)

    *Not to be confused with AC power, typically at 50Hz or 60Hz, as supplied by the electric mains. This is of course in the audio range, but is best completely excluded from audio circuits. At is usual full voltage it is dangerous, but if even a little leaks in, it can result in humming noises that many people dislike.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    I'd suggest making one out of an IC, there is One DIP Class D Amp That delivers 10W/Channel into 8Ω.

    For a linear version:
    Analog TDA2009A 10Wx2

    Both require only a single supply, making life much easier.

    If you want to stay with discrete components, I'd suggest building a Class AB amplifier, with dual rails (+20/-20) for a full education on what can go wrong in amplifiers. Once you have it correct, there will be practically no quiescent current through the speakers, lots of dynamic range, and plenty of power.
     
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