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Transistor load and saturation problem

  1. May 6, 2013 #1
    1. The problem statement, all variables and given/known data
    consider the following circuit:
    attachment.php?attachmentid=58540&stc=1&d=1367880937.jpg

    a) Determine the value of the resistor, R so that a current of 0.5 A flows through the load.
    b) What is the maximum value of th load resistor?
    c) If the load resistor is 10Ω, what value of VB will put the transistor into saturation?

    2. Relevant equations

    IC ≈ IE = VE/RE

    IC ≈ (VB-0.6V)/RE

    ICRL+VCE+VE=Vcc

    3. The attempt at a solution
    a) 0.5 ≈ (3.2 - 0.6)/RE
    RE = 5.2Ω

    b) (I am guessing that the VCE is 0.6 V here but I am not sure)
    0.5RL+0.6+0.5 = 15
    RL = 27.8Ω

    c) From what I can tell, the transistor is an NPN and in that case, in order to saturate it, the base voltage must be greater than the collector or emitters'.

    VC=ICRC
    VC=(0.5)(27.8)=13.9V

    VE=IERE
    VE=(0.5)(5.2)=2.6V

    So in order to be greater than both the VB must be greater than 13.9V

    Parts b and c I was not so sure about, thanks for any input.
    (Sorry that the image is not nicely sized, I'm not sure how to fix it.)
     

    Attached Files:

  2. jcsd
  3. May 6, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    a) is right.
    b): where does the 0.5 come from? Fact: Ve + Vce + iRL = 15. You have Ve already (= iR). You can increase the load R until saturation, Vc = Vb. You know Vb so go for it.

    c) Yes, saturation is when Vb = Vc. So let the load resistor = 10 ohms; then how high must Vb be to get Vc = Vb?
     
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