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Understanding amplifiers

Discussion in 'General Electronics Discussion' started by walkingbeard, Oct 24, 2010.

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  1. walkingbeard

    walkingbeard

    6
    0
    Mar 24, 2010
    Hi folks,

    I've got this group project at university to design and build an amp to run an 8 Ohm, 0.5 Watt speaker from an mp3 player. My own experience from lectures would tell me to build a simple class AB amp with two or three BJTs.

    Our lab programme suggests though, that we use an op-amp-based pre-amp, tone controls and then an output amp based on the BJTs.

    I don't understand why you would use a pre-amp unless you were planning to mix the output with signals from other sources.
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    You'll need to increase the amplitude from the mp3 player. That's what the first transistor in a three-transistor AB output amp does, it's a pre-amp.
    This first transistor can be replaced with an op-amp, which has the potential of resulting in better distortion data.
     
  3. walkingbeard

    walkingbeard

    6
    0
    Mar 24, 2010
    Thanks! That makes a lot more sense. :)

    However, I still don't understand why you would have the second amplifier. Do you have any idea on that, or is it just some precedent?
     
  4. davenn

    davenn Moderator

    13,812
    1,945
    Sep 5, 2009
    You misunderstand what a pre-amp is for ... its just that a preamp. The level out of it is still going to be low but enough to drive the final amplifier stage to the required output level

    Resqueline's comment on using a Op-Amp for the pre-amp is good :)

    A mixer/mixing is a different section for a multi-input system .... A mixer will have pre-amps for each input that can be adjusted to suit the different input levels from different devices

    A preamp in your single input situation just gives the input a small boost to make up for the losses incurred by the tone control stage.

    Dave
     
    Last edited: Oct 24, 2010
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Yep, the preamp can hardly drive a 32 ohm headset. The output (buffer) transistors amplifies the current (not the voltage) to enable driving a low impedance (8 ohms).
     
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