Transistor Amp Simulation Help

Discussion in 'Homework Help' started by Ishtar1, Jul 24, 2013.

  1. Ishtar1

    Thread Starter New Member

    Jul 24, 2013
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    0
    Hi there

    I am currently doing an introductory course that looks at electronics and have been assigned the task of designing three transistor amplifiers. These are a Common Emitter, a Common Collector and a Common Base amplifiers.

    Now I am currently using National Instrumentations multisim to perform simple simulations to look at the responce of an analoge input voltage. During the course of these simulations certain problems have occured that I cant get my head around.

    Now I am not great at maths and have used a simple method in which to design the common emitter circuit.

    The problem that I have been having is that when I introduce the capacitor across the emitter resistor I effectivley create a sqaure wave a picture for which can be found below now.

    Secondly, I have designed a common collector amplifier again using my basic maths. In the case of this it seem that the voltage output I get matches what theory suggest that the voltage drop will be ~0.6-0.7volts. When I look at the current gain however it seems as though there is actully a reduction in current rather than an increase relative to β. Again a picture has been attached below.

    Any ideas?

    Finaly would anyone have a resource that prevides a guide on how to simply design a Common Base transistor the ones that i have found use maths that I am unfamilier with.

    If anyone would be able to recommend a book that explains the above in laymans terms that would be great.

    Many thanks
    Ishtar
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,743
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    What is your small signal gain expected to be for your circuit?

    What happens if you reduce the amplitude of your input signal until the output doesn't clip? You may need to reduce the amplitude to 10mV or less.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    In the common emitter case the addition of the emitter bypass capacitor greatly increases the small signal gain. Reduce the input drive voltage to a few mV and you should see a sine wave at the output. A drive of 1volt peak is too high and this leads to the gross distortion you see.

    In the case of the common collector stage you seem to have the second current probe on the CRO input wire rather than in series with the emitter.
     
  4. LvW

    Active Member

    Jun 13, 2013
    674
    100
    More than that, it would be a good idea to control the operating points for both cases (dc quiescent currents and voltages).
     
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