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The most simple amplifier

Discussion in 'Electronic Basics' started by [email protected], Jan 3, 2006.

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  1. Guest

    Hi Guys

    I'm trying to experiment with the most simple amplifier. (A transistor,
    couple of resistors and maybe one or two caps).

    For now I was just trying to see if the mic is working but I didn't
    have any success. I used my computer's headset mic but I don't know if
    it is dynamic or electret. So far I was unable to get ANY signal from
    the mic (I know it is working on the PC). It will be nice if someone
    can point to me to Maplin.co.uk parts if I need any.

    Can someone please give me some guidance as to what I need? I prefer a
    dynamic mic because it seems simpler. Also is there some web source
    that describes everything from mic to speaker (preferable showing
    waveforms)

    So far the best resource I have is:
    http://www.tpub.com/content/neets/14179/css/14179_77.htm
    but unfortunatly it doesn't show the whole process from MIC to speaker.

    Does a speaker need positive and negative voltage or if your soundwave
    goes from 0 to 10V e.g. will still be able to be played over the
    speaker?

    Thanks in advance for any tips / links
    CE Auke
     
  2. Noway2

    Noway2 Guest

    First, make sure that your ampliifer (transistor) is properly biased
    and your circuit is working correctly. Second, double check the
    electrical specfications of the components and make sure that you have
    connected the mic in a manner that is compatible with your circuit.
    Keep in mind the "loading" concept and make sure that you don't have a
    divider circuit that prevents a signal from appearing at the "input" to
    your ampliifer.

    A speaker will respond to the AC component of the signal, which is why
    a DC blocking capacitor is typically used in the circuit. As far as 0
    10V, working with your speaker, this will depend on the characteristics
    of the speaker (i.e. impedance, power disipation capability, etc).

    If you are still having problems, a simpler way to start may be either
    an opamp or an audio amp (basically an opamp geared towards the audio
    band).
     
  3. Tim Williams

    Tim Williams Guest

    You probably have an electret, which needs a bias resistor of say 4.7k from
    +5 or +10V. Then take the signal off with a coupling capacitor and have
    fun.

    Tim
     
  4. Guest

    Thanks for the reply!

    What is the difference between the different amps? Seems like there are
    at least 3 kinds of amps.

    1. Whichever one you were discussing.
    2. As an alternative: an opamp; or
    3. audio amp.

    I don't know thw specifications of the mic in my computer headset. Is
    there ANY way I can test the signal with my voltmeter/ampmeter? since
    the voice on the mic will be a wave, I suppose the voltage generated
    will change between -x and x. So how do I determine the range of
    voltage for the mic? I know what all the other components are e.g. the
    multiplication of the transistor etc, so if I know what I get in, I can
    calculate what I get out.
     
  5. Also, be aware that your single-transistor amplifier probably does not have
    much gain - depending on the circuit you're using, maybe you have a gain of
    10. The signal from the microphone (even when it has appropriate bias
    voltage, using Tim's suggestion) is only going to be a few millivolts. So
    the voltage to the speaker will be only a few tens of millivolts. That is
    not very much.

    I would start by using a signal source that is stronger, and easier to work
    with - for instance, the output of a sound card or an iPod, which will be a
    few hundred millivolts, and which have the advantage that when you're
    listening you're not also talking.

    And also, you need to think about impedance matching. If you're driving a
    speaker, it probably has 8 ohms or so of impedance. You need to think about
    that in the context of your circuit, because it is effectively in parallel
    with whatever is driving it. Suppose you're using a circuit like this:


    Vcc -----o-----.
    | |
    | .-.
    | | |Rc
    | | |
    .-. '-'
    | | | ||
    | | o---||-.
    '-' | || |
    | | | __ /|
    || | |/ '--| | |
    IN -||--o---| .--|__| |
    || | |> | \|
    .-. | |
    | | |
    | | | |
    '-' .-. |
    | | | |
    | | |Re |
    | '-' |
    | | |
    GND -----o-----o------'

    Then the speaker impedance is effectively in parallel with Rc (for AC
    signals; you have to remember that the AC difference between Vcc and GND is
    zero). If you've been assuming gain = Rc/Re, then that will throw off your
    calculations by quite a lot. A simpler way to think about that is that in
    the above circuit, the only thing the transistor can do is let more or less
    current through itself. Even if it's letting zero current through, so that
    all the current flows through the speaker, the amount of current is limited
    by Rc; if Rc is, let's say, 10k ohms and Vcc is 9V, then 0.9mA is the most
    that can flow. That 0.9mA, through the speaker's 8 ohms, equals 0.72mV -
    probably even less than the mic is putting out, and that's the MOST the
    speaker voltage can ever be.

    So, to get from a mic element with impedance around 1k ohms, to a speaker
    with impedance 8 ohms, you need impedance "gain" as well. One way to get
    this is by using a transformer, say a 1k:8 ohms transformer.

    All that said... if I were you, depending on what your goal is, you might
    consider using an LM386 as your amplifier. If you do, make sure to read
    about "bypass capacitors".
     
  6. Guest

    Whao Walter, thanks for the reply (and the schematic)

    I understand your recommendation of using a line-out rather than a mic
    as input. However, if I use the soundcard's out, that will already be a
    amplified signal, so I won't know if the transister did any
    amplification? Same for the iPod. Or am I missing something?

    How will my experiment be affected if I use a class A amp? (Ie. I want
    to get rid of Re and remove the other resistor and the wire that is
    next to Re) The reason I'm asking is that I can understand the class A
    better and it will make experimentation better for me.
     
  7. On Tue, 3 Jan 2006 09:32:43 -0800, "Walter Harley"

    [...]
    A tad impractical. He'd learn more and it would be easier just to add
    an emitter- follower stage to the output from his amp. The first stage
    could then concentrate solely on voltage gain; the second on
    current/power.
    He'll learn Jack about amplifiers by copping-out with a 'black box
    solution' like that.
     
  8. So, turn the volume on the iPod down to the point where if you plugged the
    speaker directly into it, it wouldn't be very loud.

    The schematic I drew is class A. Any transistor amplifier where the
    transistor is biased into conduction for the entire signal wavecycle, is
    class A; that includes virtually any audio-frequency single-transistor amp.
    Classes AB and B refer to circuits where there is typically more than one
    transistor, and where each transistor is conducting for only part of the
    wavecycle.

    If what you mean is that you want to tie the emitter directly to ground,
    that will increase the gain slightly, at the expense of making it impossible
    to control how much gain you have and more difficult to bias properly. But
    it will not solve the basic problem you have, which is that you can't easily
    drive an 8 ohm speaker with a common-emitter transistor circuit (that's one
    where the load is connected to the collector).

    You'll have an easier time driving 8 ohms with a common-collector amp
    (that's one where the load is connected to the emitter), but that won't give
    you any voltage gain. So, what most practical circuits do is they have a
    common-emitter stage to give voltage gain, followed by a common-collector
    stage (aka an "emitter follower") to give current gain. As I mentioned
    before, you need both, to get between a microphone (low voltage, low
    current) and a speaker (higher voltage, higher current). Amplifier chips
    such as the LM386 contain several gain stages of various appropriate sorts,
    conveniently packaged up into a blob of plastic.

    Horowitz and Hill's "Art of Electronics", 2ed., contains an excellent
    discussion of the issues you're trying to understand.
     
  9. Jasen Betts

    Jasen Betts Guest

    usually they are electret mics - this is a good thing, but
    you'll need to send some DC its way for it to produce any output
    they need more amplification
    They will work with a one-sided signal, they work better with an signal that
    goes positive and negative.

    Bye.
    Jasen
     
  10. Bob Masta

    Bob Masta Guest

    On 3 Jan 2006 03:12:15 -0800, wrote:

    Note that for simple testing purposes a headphone
    element is effectively the same thing as a dynamic
    mic element, though certainly not optimized for that
    use. So if you have an old pair of headphones,
    you might want to give them a try for this use.

    Best regards,



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  11. You can think of a "0 to 10V" signal as being the same as a -5V to +5V AC
    signal, plus a constant 5V DC signal. If you feed a speaker a signal that
    contains a DC component, the speaker can't turn that into acoustic energy
    (in other words, its frequency response does not include DC); so instead, it
    turns it into heat, in the voice coil.

    If your power levels are high enough, or your speaker is small enough, the
    result is that the voice coil warps or melts, destroying the speaker. On
    the other hand, as Jasen suggests, if the speaker is rated for enough power,
    it just gets a little warm, no problem.

    A fellow sound guy once told me the story of a power amplifier failing with
    a short to the DC power supply, at a concert he was working in the '70s.
    This was a big amp and a big speaker, so when the speaker failed, there was
    apparently enough AC component left to eject the flaming voice-coil ring out
    of the speaker cabinet and into the audience. Pretty cool effect. Your amp
    is unlikely to do that, though :)

    To get rid of the AC component, all you need to do is put a capacitor in
    series with the speaker, as I showed in my schematic. The capacitor needs
    to have a big enough value that it lets enough low frequencies through. C =
    1/2*pi*R*f at the rolloff point, so for 8 ohms and 80Hz (which is lower than
    any little speaker is going to reproduce, there's no point in going to
    20Hz), you get C = 1/2*pi*8*80 = 248uF. You can use a 220uF capacitor, it's
    a more common value. Make sure that you aim the '+' pin of the capacitor
    the right way, towards the amp rather than towards the speaker. The
    capacitor needs to be rated for the DC voltage it's going to withstand: in
    your 0-10V example it sees 5V, but I would play it safe and get one rated
    for at least 10V.
     
  12. Guest

    Hi Jasen

    Thanks for your brilliant description of 0 to 10V question I had!
    One more thing about the capacitors. Why do you have one on the MIC
    side. You know you'll get both positive and negative volts. I don't
    think in my humble electronic mind that the cap hurts but I dont'
    really see a use for it there.

    And then a final question that I will probably get a slap for is... Is
    there a difference between connecting the speaker/cap wire to the
    collector or the emittor. And I think the answer has to do with: maybe
    if you put it on the emitter, you'll only get the posive part of the
    waves. I'm not sure though.

    Seems like this whole experiment would have been much easier if I had a
    1000€ osciloscope.
     
  13. I think you're actually responding to my post? I'm Walter; Jasen is someone
    else.

    Think of a capacitor as being a way to block DC from flowing, while
    permitting AC to flow.

    The DC voltage on the transistor's base is set by the resistive biasing
    network. It wants to be about 1/2 Vcc, roughly speaking. The DC voltage on
    the microphone wants to be zero (unless it's an electret mic, in which case
    it wants to be whatever the preferred bias voltage of the mic is; 1/2 Vcc
    might be fine). Since those are different DC voltages, you need some way to
    separate them, while allowing the AC signal to pass. A capacitor does that.

    If you're using an electret mic and 1/2 Vcc is a good bias voltage for it,
    then you don't need the capacitor. But you do need to think about how much
    bias current the electret pulls (1mA would be a decent guess) and think
    about what that will do to your bias voltage. For instance, if you used 10k
    resistors for your bias network, and you had a 9V supply, you might think
    that would bias the transistor and the electret mic both at 4.5V; but you'd
    be wrong, because you can't get 1mA of current through a 10k resistor with a
    9V supply, according to Ohm's Law. It would work to use 1k resistors, but
    then you have to realize that the mic is seeing a load of less than 500 ohms
    (that is, 2 1k resistors in parallel) and it may not be able to drive that.
    Sometimes it's easier to use a capacitor anyway, and bias the transistor
    separately from the microphone.

    An example of this is the circuit at
    http://www.epanorama.net/circuits/micamp.html. Note that circuit provides
    voltage gain but not enough current gain to drive a speaker; but it is an
    example of biasing a common-emitter transistor stage driven by an electret
    mic.

    There is a lot of difference, as I was trying to explain. If you connect to
    the collector, you can get voltage gain, but not a lot of current gain. If
    you connect to the emitter, you can get current gain, but no voltage gain.
    Which one you need depends on what you're trying to do. (To drive a speaker
    from a microphone you probably need both; so you probably need more than one
    stage of gain, that is, you need more than one transistor.)

    It's not a scope you need, though it's a handy thing to have. It's a decent
    textbook, and a pencil and some paper.

    Analog electronics gets frustrating quickly unless you understand at least a
    little bit of theory: if you just hook some stuff together without knowing
    why, the odds of it working are low. If you don't have some theory, the
    best you can do is build circuits that other people designed.

    But the amount of theory you need is really not that big. It involves no
    math beyond basic algebra, and it can be described in a couple of chapters.
    Good books that will give you the knowledge you need to answer these
    questions include Horowitz and Hill's "Art of Electronics" 2ed., and the
    Handbook of the Amateur Radio Relay League (comes out annually; any edition
    after about 1980 is fine). In each case, although they are thick books, you
    only need the first 2 or 3 chapters to start getting somewhere.
     
  14. Jasen Betts

    Jasen Betts Guest

    if you need it becase the transsitor wants only positive voltage to operate
    correctly. it allows the transitor's base to be positive while the
    microphone is producing an AC signal.
    in the simple one transistor amplifier circuit most of the output signal
    is on the transistor's collector.

    there are other designs that use the emitter as the output terminal....
    yeah...

    as for wanting to use a dynamic microphone with a one transistor amplifier
    I get enough gain from a one transistor amp (configurted for maximum gain)
    to connect a 600 ohm dynamic mic to a soundcard input. it'd take a few more
    transistors to be able to drive a loudspeaker.

    Bye.
    Jasen
     
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