1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

OP-Amp connected with Transistor whose Base and Collector are short

  1. Jun 4, 2013 #1
    1. The problem statement, all variables and given/known data

    Problem number 6a in the picture.
    lic2.jpg

    2. Relevant equations
    Transistor:
    Ic = βIb

    Ic = αIe

    Ic = Is*e^(Vbe/nVt)

    OP-Amp:

    Vo = Acl*Vi

    Acl = 1 + (Rf/Ri)

    3. The attempt at a solution

    I tried to solve but just couldn't understand the behavior the transistor.
     
  2. jcsd
  3. Jun 4, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    First, find Vo1.

    Then find Vo.

    Note that Q1 and Q2 behave like p-n junction diodes.
     
  4. Jun 4, 2013 #3
    I tried that.
    Now Q1 must be forward biased so that the constant current Iref flows through Q1 and the voltage drop across Q1 is 0.7 Volts that is

    Vbe1 =.7 Volts

    Vo1 = Vi*(R4/R4+R3) - .7

    Now what will be the behavior of transistor Q2? The Voltage across it depends on Vi

    Vbe2 = .7 - Vi*(R4/R4+R3)

    So how can i take diode 2 as forward or reverse biased without knowing Vi?

    If the diode 2 is forward biased then where does the current go at NODE Vo1. Does it go into the OP-Amp1?

    Taking the Diode 2 reverse biased the circuit is open and Vo2 becomes 0 Volts

    PS: I don't have a camera right now or i could have shown you the full attempt.
     
  5. Jun 4, 2013 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You can't!
    What current? Vo1 sinks currents from both diodes. Obviously, all net diode current has to go into op amp 1's output.

    Q1 current is always = I_ref.
    That is correct.

    ********************

    Hint: You got Vo1 right.

    This circuit is a Rube Goldberg comparator. The output Vo tells you whether the input is above or below a certain threshold voltage. What is the correspondence between input voltage and output voltage? Vo is one voltage when Vin is < threshold and another when Vin > threshold.
     
    Last edited by a moderator: Jun 4, 2013
  6. Jun 5, 2013 #5
    Thanks for help. Now tell me is this the answer

    1) When Vi < .7*(R4 + R3)/R4, Vo = 0 Volts

    2) When Vi > .7*(R4 + R3)/R4, Vo = R3*Is*e^((.7 - Vi*(R4/R4+R3))/nVt)
     
  7. Jun 5, 2013 #6

    rude man

    User Avatar
    Homework Helper
    Gold Member

    'Fraid not.

    First, what is Vo1 as a function of Vi?

    Second question, what happens if Vo1 is < -0.7V or > -0.7V?

    BTW your expression for Vo must be compared to the saturation output voltage. Your op amp can't go more positive than a certain voltage, say +12V for a 741 op amp running on +/- 15V supplies. This applies to terms like your Vo = R3*Is*e^((.7 - Vi*(R4/R4+R3))/nVt) which might be correct if your op amp had no saturation limit.
     
  8. Jun 5, 2013 #7
    Vo1 = Vi*(R4/R4+R3) - .7

    1) Vo1> -.7 that is Vi > 0, Vo = 0 Volts

    2) Vo1 < -.7, Vi < 0

    In this case the diode would be forward biased and second op-amp would behave as a negative comparator and then op amp will saturate at positive saturation voltage that is 12 volts.
    Is this the right answer?
     
  9. Jun 5, 2013 #8

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Absolutely correct. Good work!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: OP-Amp connected with Transistor whose Base and Collector are short
  1. Ideal op amp problem (Replies: 8)

  2. Op-Amp Circuit ANalysis (Replies: 16)

  3. Op amp (Replies: 1)

  4. Op-Amp question (Replies: 13)

Loading...