Use of a PNP stage in a chain of NPN common-emitter amp stages

Discussion in 'Homework Help' started by nnxen, Dec 22, 2012.

  1. nnxen

    Thread Starter New Member

    Dec 22, 2012
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    Hi everyone, new member here.

    I'm currently revising an audio electronics module for my course, but I'm struggling to understand how a PNP stage in an amplifier design works and what purpose it provides.

    So far we have been designing a pre-amp consisting of NPN common-emitter stages with a potential divider at the beginning of the chain to provide an initial fixed bias. Obviously the output bias for each stage provides the input bias for a subsequent stage. From what I understand, at a simple level the emitter resistor dictates the current flow through a stage and the collector resistor affects the bias for the following stage. The stage gain is approximately Rc/Re.
    This is all fine and I've made a few simple chains in Multisim to show the theory.

    However, in our final lecture before we broke up for Christmas, we were told that one of the NPN stages could be replaced by a PNP stage, still taking the output from the collector pin and still putting the input from the preceding stage directly to the base. But this time, we have our collector resistor to ground and our emitter resistor to the +12vdc supply rail.

    What purpose would this stage provide? I initially thought it was to reduce the bias voltage so we had more 'headroom' for a higher gain final NPN stage. But I cant understand how this stage would work and how would I go about calculating my Re and Rc resistances for this stage.

    Hope that explains it well enough, if not, I may be able to elaborate further.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Where is your schematic of the chain of NPN common-emitter stages
     
  3. nnxen

    Thread Starter New Member

    Dec 22, 2012
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    Attached :)

    There's no output buffer or output cap yet, but it gives you the general idea.
     
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  4. Audioguru

    New Member

    Dec 20, 2007
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    Most preamps have only two transistors but yours has five!
    I used the very old BC108 transistor 43 years ago in my first job. I still have a few.

    See what happens when you replace one of the NPN transistors with a PNP.
     
  5. nnxen

    Thread Starter New Member

    Dec 22, 2012
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    I thought it was strange he asked us to use the BC108s, I thought they'd become pretty much obsolete since being replaced by newer models. Maybe he's just stuck in his old ways :)

    Part of our revision in to the amps is to create a preamp of a chain of common-emitters with 250x gain, hence the multiple stages. The final design will have 7 transistors once I've added an output buffer and a darlington pair for the input buffer to create an increased input impedance.

    I've actually succeeded in getting 252x gain with just the NPN common-emitter stages, but I'd still like to be able to understand why he's suggested using a PNP stage.

    I've placed the PNP stage at stage 3 as he recommended and used the same Re and Rc values as I'd calculated for the original NPN stage. It just seems to throw my bias settings out and the current draw for the stage is now much greater (I was originally aiming for 2mA per stage.)

    It could be that I've written down his schematic incorrectly, but I was pretty sure it was right. He didn't show us how to calculate our new Re and Rc values for the stage so its probably a case of me not really understanding how this stage functions.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Your new PNP for Q4 is almost saturated and now Q5 is slammed on hard and is completely saturated. The amplifier is dead.
     
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