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Transistor Audio Amp Design

Discussion in 'Electronic Basics' started by Steve, Dec 22, 2003.

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  1. Steve

    Steve Guest

    Hi All,

    I've designed a basic class-A common-emitter audio amp using a single transistor. This is biased correctly, and develops the voltage over an emitter resistor - the output of which can be seen clearly on a scope.

    Now, whilst it's acting as an amplifier - as soon as I connect a speaker across the output (DC blocked with a capacitor) and ground, evrything fails miserably. I'm not sure if this is mainly due to the impedence of the speaker changing the operation of the circuit - or if it's simply an impedence mis-match (I guess the speaker is essentially 'shorting' the output to ground via a very low resistance).

    I've seen two designs which seem to resolve this problem:-

    Firstly, the use of an audio transformer in place of the collector resistor, with secondary wirings connected to a speaker. I don't like this - as the transformer would have to be pretty large to handle any reasonable currents.

    Secondly, the use of a common-collector stage, which has less than unity voltage gain, but of which can provide the current requirements for a low impedence speaker. Is this right?

    Assuming the second option is the best ... how would this work in reality? For example - If the first stage of the amplifier produces a signal 20 volt peak-to-peak, this would by far exceed the 5v maximum base voltage of the second stage ... even using a PNP and NPN in push-pull configuration would only give a possible input of 10 volt peak-to-peak ...

    Should I somehow forget about the common-collector current stage ... and somehow modify the first stage to delivery the necessary current? I can easily develop a large current with the common-emitter design, but this current flows through the collector load resistor, collector, and emitter. How do I use this current to drive a loudspeaker?

    I hope someone can understand the above - and point out where I'm going wrong. I obviously lack understanding somewhere ... but I'm not sure where.

    At the moment I'm only interested in standard BJT's, not FET's or similar. I want to understand basic transistors before moving on to others ...

    For Ref - the rough circuit (view in fixed width font):-

    + ----------------------------------------- +
    | |
    RES RES
    | |----CAP---O output
    input 0---CAP--------TRAN
    |
    RES
    |
    GND----------------------------------------GND

    Any help would be very much appreciated!

    Thanks,


    Steve
    Bristol, UK
     
  2. Ian Bell

    Ian Bell Guest

    A common emitter amplifier normaly develops its output between its collector
    and ground. However its output impedance is relativeley high (of the the
    order of several thousand ohms) and therefore not suitable for directly
    driving a loudspeaker. You can easily measure this yourself. First
    measure the output voltage with no load. Then try different value load
    resistors until the output is halved. The load resistor is now equal to
    the output resistance. Note: the circuit you show at the bottom is this
    post is *not* what I would call 'properly biased.
    A possibility.
    Sorry, you lost me here. Do you mean the base/emitter voltage?

    .... even using a PNP and NPN in push-pull
    Are you sure? What about the dissapation of the transistor?
    You couple via a capacitor as before. however you must remember that as
    this is a class A circuit the efficiency will never be more than 25% so a
    lot of power will need to be dissipated in the transistor and its emitter
    resistor.
    One problem with a simple common collector (or emitter follower as it more
    commonly known) is that its output impedance is asymatric for large signals
    because current is sourced by the transistor and sunk by the emitter
    resistor. A better design uses a current source in place of the emitter
    resistor.
    You would probably benefit from reading Douglas Self's excellent series of
    articles on power amplifier design that he wrote for Wireless World. He
    has a web site at:

    http://www.dself.dsl.pipex.com/

    HTH

    Ian
     
  3. A common solution to your problem is a complementary emitter follower
    (common collector). The NPN has its collector connected to the
    positive supply, and the PNP has its collector connected to the more
    negative supply rail. Their emitters are connected together and to
    the coupling capacitor to the speaker. Often there are a pair of low
    value resistors (10 ohms or less) between the emitters, with the
    coupling capacitor connected to the common point between the
    resistors.

    The trick is to apply the same AC signal to both bases, while biasing
    them to slightly different voltages to make both transistors conduct
    just a little when there is no signal. The simplest way is to connect
    two diodes in series between the bases, with the PNP base connected to
    the collector of the previous common emitter amplifier, and the
    collector resistor for that stage connected to the NPN base, co the
    collector current of the voltage gain stage passes through the two
    diodes, producing enough voltage drop to put both of the common
    collector transistors into slight conduction. You can increase the
    voltage swing of the output by splitting the collector resistor in
    half (two equal resistors in series, and connecting a coupling
    capacitor from the output back to that node. No need to worry about
    base to emitter voltage, because the emitter voltage follows behind
    the base voltage, never getting much more than a diode drop away from
    it.
     
  4. Although the output impedance of the common emitter amplifier is high,
    this has nothing whatsoever to do with the ability to use such a
    configuration in driving a low impedance speaker. What matters is the
    ratio of maximum voltage and maximum current that the transistor is
    capable of. There are many commercial designs that use a push pull
    collector output. Feedback is usually used to reduce the output
    impedance to a low value.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    http://www.anasoft.co.uk/replicators/index.html
     
  5. Steve

    Steve Guest

    Hi,

    Just a quick note to thank all those that responded. I now have a few more
    ideas to work with ... and am very greatful for all your suggestions :)


    Regards,


    Steve
    Bristol, UK
     
  6. Bill Bowden

    Bill Bowden Guest

    A common collector configuration is sometimes called
    a "emitter follower" where the emitter voltage follows
    the base voltage. So, you don't have to worry about exceeding
    the base to emitter voltage because they are almost the same.

    There is an example push-pull emitter follower circuit
    using 3 transistors here:

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page8.htm#amp.gif

    It only delivers about 50 milliwatts from a 9 volt battery, but
    you can run the supply at 12 volts for more power.

    -Bill
     
  7. ddwyer

    ddwyer Guest

    surely a common collector
     
  8. Ian Bell

    Ian Bell Guest

    I am sure you are aware that common collector and emitter follower are the
    same thing.

    Ian
     
  9. N. Thornton

    N. Thornton Guest


    Hi Steve. Lets get your circuit working. Firstly remove the collector
    resistor and put the speaker where it was. You wont need the
    capacitor. Before someone yells, yse I know it will put sc thru the
    speaker, but for a basic starter it'll work. Later we can replace that
    with L-C-LS.

    Now, add a resistor from base to ground so the transistor's bias point
    is controlled sensibly. Set it up so you have say half a volt sitting
    on the emitter resistor. Thus the base needs to be at 1.1v.

    That'll get you up and running. Some more info on what power out you
    want, supply voltage, tr type etc would help.


    Regards, NT
     
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