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Find Vs and the voltage gain

  1. Mar 4, 2016 #1
    1. The problem statement, all variables and given/known data
    assume β=200,Vt=26mV,Vbe(on)=0.7V
    Find the voltage gain Vo/Vs

    How to find the Rth , Vs ,then find the voltage gain?
    I try to draw the circuit diagram below
    2. Relevant equations


    3. The attempt at a solution
    Vth=[R2/(R1+R2)]*15
    Vth=1.63V

    Which method should I use to find Rth?
    Rth=R2//R1
    =5.53k ohm
    or
    I see this formula Rth=0.1Re(1+β) on my book
    Rth=0.1Re(1+β)
    =10.3k ohm

    I assume Rth=R2//R1
    Vth=Rth*Ib+Ie*Re+Vbe
    Vth=Rth*Ib+(1+β)*Ib*Re+0.7
    Ib=8.57μA

    Rπ=Vt/Ib
    Rπ=26m/8.57μ
    Rπ=3.03k ohm

    Assume my small signal model diagram is correct
    Vb=ib*Rπ
    Vo=-βib*Rc

    But how to find the Vs
    then I can find the voltage gain(Vo/Vs)?
    =[Vo/Vb]*[Vb/Vs]
     

    Attached Files:

  2. jcsd
  3. Mar 5, 2016 #2

    gneill

    User Avatar

    Staff: Mentor

    Vs is arbitrary, representing some small signal. The gain will be a ratio, so the particular value of Vs is not important. What may be important, though, is the frequency of Vs.
    upload_2016-3-5_11-17-59.png

    Notice where Vo is on your circuit diagram: It's not the voltage at the collector! What effect do you think the 20 μF emitter bypass capacitor is going to have? You may need to take a closer look at how the capacitors in the circuit are going to affect things, and perhaps examine the emitter circuit in more detail for your small signal equivalent.

    Your circuit diagram shows the source resistance labelled with a capacitance value. Presumably that was meant to be associated with CC1. Is there a resistance value for the source resistor?
     
  4. Mar 5, 2016 #3
    Thank you
    the resistot with wrong label is the internal resistor of the signal generator
    I think it can be neglected during the calculation since the value is not given
     
  5. Mar 6, 2016 #4
     
  6. Mar 6, 2016 #5

    gneill

    User Avatar

    Staff: Mentor

    Is the given circuit diagram not correct? It shows Vo connected to the emitter.
     
  7. Mar 6, 2016 #6
    Yes , I make some mistakes when drawing the circuit
    Vo should be at the collector
     
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