3 Stage audio amp. Help!

Discussion in 'Homework Help' started by Poppinfresh, Apr 15, 2011.

  1. Poppinfresh

    Thread Starter New Member

    Apr 15, 2011
    3
    0
    Hello

    Ive been given the following assignment problem

    PROBLEM
    A microphone has a peak open circuit voltage of 50mV with an output resistance of
    15kΩ. Design an amplifier to allow this microphone to drive an 8Ω speaker in such a
    way that the magnitude of the gain – bandwidth product is maximised. The gain
    bandwidth product is determined as
    The following constraints are placed on your design.
    1. The circuit is to be biased from a +10 / -10V DC power supply.
    2. The circuit should produce no clipping or distortion in the output voltage
    waveform when the input voltage is at its maximum value (50mVpk).
    3. You can only use three transistors (and should use all three transistors). You have
    available the following types of transistors: NPN transistor 2N3904 and a PNP
    transistor 2N3906.

    The first problem i had was what exactly should the gain be, i basically decided to choose an output of 5v peak based on a reasonble audio signal, giving a needed gain of 100. Given the largge input and small output impedence requirements, i went with a CC-CE-CC amp setup. This is where i run into problems, ive drawn the circut, managed to calculate some values (which i found very confusing and there is very little in the way of this when it comes to most of the recources i find, any links/info would be mucho appreciated) and simulated it in LTspice (the amp does not need to be built or have any power amp stages, just simulated) and nothing works, ive been at it for god knows how long and im just not getting anywhere on why its not working

    Circuit i made:

    http://img689.imageshack.us/i/invertingsinglece2.png/

    Also i am completley lost as to how to actually choose good values for the capacitors, the ones there are basically just ones i pulled out of the air with no real backround

    Anyhelp would be very much appreciated, this assignments worth quite a bit of my course and im getting very depressed at not having anything decent :(.

    Thankyou.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    ImageShack is asleep. It is too bad that you didn't attach your schematic to your reply here. ImageShack finally woke up and I see your schematic that is covered with chicken-pox dots.

    Your 2nd transistor is saturated so it is not an amplifier.
    The class-A 3rd transistor will melt since it dissipates about 1.25W all the time and its max allowed dissipation is only half that.

    You should make a class-AB complementary output to drive an 8 ohm speaker because your emitter-follower with a very high resistance emitter resistor will produce a very low level into an 8 ohm speaker.

    Note that the max allowed collector or emitter current for a little 2N3904 and 2N3906 transistor is 200ma which is a peak voltage of only 1.6V, not 5V peak.

    Here is a two stage amplifier with a class-AB complementay output. Change its power supply to a dual polarity supply.
    Add another transistor common-emitter amplifier stage to its input.
     
  3. Poppinfresh

    Thread Starter New Member

    Apr 15, 2011
    3
    0
    Ah thanks for that, nothing has been said really about that max op capabilities of these sort of amplifiers, what has been shown in lectures is basically picking arbitrary nice sounding values of current and working of that, which i thought sounded kinda rediculous, thankyou for your circuit idea unfortunatley i can only use 3 transistors in its design (again i think this is rather silly), ill give it a look over anyway.

    edit: Also, why is the second transistor saturated?
     
    Last edited: Apr 15, 2011
  4. Poppinfresh

    Thread Starter New Member

    Apr 15, 2011
    3
    0
    Attached circut on non-crappy image hosting website :p
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your second transistor is saturated because its emitter resistor value is much too high and its base voltage is also much too high.

    In the circuit I posted the gain can be increased if the emitter resistor has a reduced value (56 to 100 ohms) and the input resistor removed. Then the base bias resistor value can be set for symmetrical clipping. Bootstrapping can be added for higher gain, higher output level and reduced distortion.
     
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