multistage audio amplifier

Discussion in 'Homework Help' started by marting, Dec 3, 2010.

  1. marting

    Thread Starter New Member

    Dec 3, 2010
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    i have to work on the multistage audio amplifier but i feel uncmfortable in designing the circuit.


    The design requirements are given below:

    • Av ≥ 2000 when driving a 100 ohm load, with a source resistance of 1 Kohm at 1 KHz.
    • Rin ≥ 15 K with RL = 100 ohms at 1 KHz
    • Lower cutoff frequency ≤ 10 Hz with RL = 100 ohms
    • Upper cutoff frequency ≥ 25 KHz with RL = 100 ohms
    • Volume/gain control to vary the gain of the amplifier from ≈ 0 to 2000 with RL = 100 ohms. You will require a potentiometer/variable resistor to vary the gain. While varying the gain of the amplifier, the lower and upper cutoff frequencies at any intermediate gain should meet the above requirements.
    • Must be able to accept input either from a microphone or from music player through a male jack. Must be able to drive a speaker.
    • Nice loud output from the speaker with no audible distortion.
    • Supply voltage = ± 15 V (dual supply).
    • All the input sources should be AC coupled.
    • Output DC voltage ≤ 50 mV.
    • The output swing with a 100 ohm load ≥ 20 V pk-pk
    The final output stage (the one that drives the speaker) must be a Class AB power amplifier.




    please anyone help me out in deigning the circuit.
    thanks.
     
  2. tyblu

    Member

    Nov 29, 2010
    199
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    What do you have so far? Are you using transistors, op amps, or both?
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    There are a few ways to do it. Visit http://sound.westhost.com for some examples.

    Make sure you have a stable dual rail power supply (omitted in the amp drawings at link above), the power supply is important for good sound.
     
  4. marting

    Thread Starter New Member

    Dec 3, 2010
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    i have a choice of either using the op-amps or BJT. but i wud prefer BJT(biploar junction transistors).
     
  5. tyblu

    Member

    Nov 29, 2010
    199
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    So, what do you have so far? It starts with characterizing each stage. Do you should know the characteristics of all single stage BJT amplifiers and the AB stage? This is a good thing to research, straight away. The first thing to do is to come up with a 2-port network that satisfies your requirements; the second is to split that network up into practical BJT stages.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    This Class B is a good example of multistage with BJT design.

    It should give you ideas for starting if you are stuck on a layout. They have a lot of amp schematics.

    Do you need to build it, or does it just need to work in simulation?
     
  7. marting

    Thread Starter New Member

    Dec 3, 2010
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    i have a good idea of the BJt but vaguely have any idea of the AB stage.. can u help be the circuit schematics of ur own. iwud b grateful!!
     
  8. tyblu

    Member

    Nov 29, 2010
    199
    16
    Can you see the different stages in the schematic thatoneguy provided? Input impedance is set to be different for AC and DC by C2; a CE amp starts, followed by a CE; then there is the AB output, implemented what looks like complimentary darlington pairs (but really both use an NPN to drive the load as apparently PNPs were cruddy way back when this circuit was designed); the output impedance is also set to be different for AC and DC by C6.

    The linked page goes through some design considerations, but they're pretty advanced if you don't know the pros/cons of each type of simple stage. The output AB stage will be generating more than 8W. They are also called a "push-pull" design.
    http://en.wikipedia.org/wiki/Class_B_amplifier#Class_B_and_AB
    So, research these stages: common emitter, common collector, common base, long-tail pair, push-pull output, preamplifier and power amplifier (figure out why you're using AB). Then you'll be able to analyze example circuits and judge what needs to change to suit your design.
     
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