BJT Amp Design Project

Discussion in 'The Projects Forum' started by Lonewolf1991, Dec 13, 2011.

  1. Lonewolf1991

    Thread Starter New Member

    Dec 13, 2011
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    For my school project, I am supposed to make a BJT Voltage Amp (using 2N2222 Pspice model) that has small signal voltage gain of 25. It is given in the specs that Beta is 150, the maximum power dissipation by the transistor is 50mW, resistor-only biasing, only one DC supply can be used, all Caps used are at 1mF and a load of 10k.

    Now the thing is once I would be able to get some numbers to work with, I should be able to do it. The problem is, I don't know where to start...
    To me it looks like it should be a common-emitter amp, since we want a high gain, but there is very little information given. Is it possible to do it with just the given info, or do I need to make some assumptions, like assume Vcc and Icq?
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Another poster has a very similar problem, except for a bizarre DC load resistance value of 16.81kΩ. Apparently no resistors are to be greater than 500kΩ, but otherwise there is no information not given in your problem. http://forum.allaboutcircuits.com/showthread.php?t=63343

    I agree that the problem as stated gives only sparse information, so you will indeed have to make some assumptions The figures you are given do constrain things though. To dissipate less than 50mW, Icq * Vcc < 50mW. If you are using the normal four-resistor bias circuit for a common-emitter amplifier, remember to allow for the base chain current. That is going to be a modest percentage of the collector current.

    The gain required by this problem being quite small, you may find it convenient to have a non-bypassed element to your emitter resistance, so that the circuit ends up with five resistors rather than four.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    This is for reference only.

    Unsure if your supply voltage is allowed to be so high, or if this meets the other requirements, but you can redraw it in spice and tweak for the results you need. I calc this as under 50mW, but I'm not sure what the required gain is, plus, I'm not very good at this stuff :p. Distortion is 0.4% THD

    Change Rc and Rl to match your load, just for kicks.

    [​IMG]
     
  4. Lonewolf1991

    Thread Starter New Member

    Dec 13, 2011
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    Hmmm I tried tackling it by assuming Vcc to be 12V and tried designing it in a way that the transistor was using 25mW of power, so that if I need to adjust anything later, I would have some room to work with. After some algebra...I got lost so I'm gonna push it to do later...too much guess work as it is
     
  5. Lonewolf1991

    Thread Starter New Member

    Dec 13, 2011
    3
    0
    Okay, so I have a circuit that looks like the one above, but with no values in. What I did was I assumed Vcc to be 12V and then wanted to assume the current as well, but a problem came up.
    [​IMG]
    I believe this is the equation that will give me the voltage gain, PLEASE correct me if I am wrong. Then thing is, when I assumed Icq to be 1mA, and assuming that I wanted to dissipate only 25mW, Vcq was at 25V...more than double than Vcc...is that even possible? Should I lower my assumed current?
     
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