Need advice for 3 stage audio amplifier!

Discussion in 'The Projects Forum' started by mitens, Nov 4, 2012.

  1. mitens

    Thread Starter New Member

    Nov 4, 2012
    I have a project for school to create a 3 stage audio amplifier and I have some questions hopefully some one can answer.

    This is the basic requirements of the project:
    - Maximum working frequency of 20KHz
    - Input amplitude of 50mVpk
    - Load impedance of 8 Ohms
    - Supply voltages of +/- 3V
    - Number of stages limited to 3
    - Maximum gain possible with
    - Minimum distortion obtained

    also we can only use transistors, 2N2222, 2N3904 to be specific.

    To start off my slew of questions... how can you have a low pass bjt 3 stage amplifier when generally coupling capacitors are used to block the lower frequencies, mainly to stop the dc voltages. How can there be a maximum of 20kHz with low distortion and when its a low pass design :/

    I was planing on using a diff pair amp with active load, but that uses npn transistors from all the schematics ive seen, is there a way to actively load a diff pair amp using only pnp's of either of the 2 i stated above?

    also, if the first stage is a diff pair amp with active load, there would be a gain of roughly 1/2(gm)(Rout) so would this be a voltage output or would this stage have a current output...

    and for the second stage, i was going to use a common-emitter with a emitter resistance for my gain stage. If the max vcc/vee is +-3 then would my maximum gain would have to be relatively low? or would i just need to use lower resistances in order to make sure that the bjt is properly biased.

    the third stage i was thinking of doing a simple emitter follower just as a buffer to the load, the speaker.

    This is all just thoughts at the momment, i havent yet done any calculations because..well im kinda at a loss where to start.
    Any answers or advice is appreciated, also if you think this design is a terrible idea please let me know so i don't waste too much time pursuing it lol.

  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    2N2222, 2N3904 are NPNs.
  3. mitens

    Thread Starter New Member

    Nov 4, 2012
    Opps typo'd, yeah i meant to say is there a way to actively load a diff pair amp using only npn's. I only see models with pnp's**

    edit* there are 2 pnp's and 2npn's in the models i see, 2 npn's in the diff pair and 2 pnp's for the active load