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Homework Help: Need Help With a Few Basic Electrical Questions

  1. Aug 4, 2013 #1
    Hi All

    It's been years since I've had to do any of these sort of problems and I've forgotten how to do these types of problems so I'm hoping one of you smart people can help. As much detail as possable please

    First Question

    I fairly certain I need to use ohm's law on this one but can't seem to figure it out. I need to know the voltage at point A?

    Second Question

    Could someone please explain to me what the characteristics and configuration of the following transistors?

    Third Question

    What will you observe on the screen of the oscilloscope at points A,B,C (I need the wave forms and voltages please)
  2. jcsd
  3. Aug 4, 2013 #2


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    Welcome to PF, gbeach01. It is not the policy at PF to simply supply answers to questions until the poster has researched the problem and has attempted to solve itr. Read up on ohm's law and parallel and series resistances for the first one. You'll need to study transistors for the second, and semiconductor device operation as well as reactive circuits for the third. You are welcome to come back with questions about specific points where you get stuck.
  4. Aug 4, 2013 #3
    For the first question, what is the relationship between the two vertical resistors that are connected to ground?

    The second question involves a so called "Darlington Pair," which you can look up.
  5. Aug 4, 2013 #4
    Thanks for the reply I think problem one is a trick question I think the two parallel resistors have nothing to do with the problem cause correct me if I'm wrong electricity takes the path of least resistance. So to find the voltage at a I would use the equation V = I x R where I = 1ma and R = 10 so the answer would still be 10V at A correct or am I way off. Oh and I don't even know where to start with question 3 anyone have any hints.
  6. Aug 4, 2013 #5
    Problem 1: Think of the point A as not even being there. It's part of the node where the current splits between the two 10K resistors, it's just there to show you where the voltage is being taken at.

    Then you can easily find the current through the entire circuit by combining the parallel resistors, then combining that with the series resistor, finding the current, and finding the voltage drop across the first 10K resistor for the voltage at node A. (It's not 10V.)

    Problem 2: I know nothing about BJTs.

    Problem 3: You have two regular diodes and a zener diode. Regular diodes operate by allowing current through if the diode is forward biased (current flowing from base of triangle to tip of triangle), and they have a voltage drop of approximately 0.7V. The last diode is a zener diode. These are specifically made to operate in the reverse bias region, I forget the specifics but a small change in current amounts to a big change in voltage is what these basically do.

    I don't remember the specifics for circuit analysis with zener diodes though...

    The capacitor I think acts as a short if your frequency is not too high or not too low (mid-band).
  7. Aug 5, 2013 #6


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    Hints for problem three...

    a) Temporarily pretend the 1K resistor and the zenner diode aren't there...

    b) The two ordinary diodes on the left will conduct if the left hand side is more than 0.7V higher voltage than the right . So when Vin = +50V point A cannot be lower than ???. Likewise point B and the capacitor voltage cannot be lower than ???.

    c) So knowing that the voltage on the capacitor is the value calculated in b) .... what happens if Vin suddenly falls to -50V ?

    d) Once you have figured out what happens to the voltage on the capacitor add back the resistor and the zenner.
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