Cap/Current Source in Series - Determine Size of Current Source

Discussion in 'Homework Help' started by chrispo86, Jan 28, 2009.

  1. chrispo86

    Thread Starter New Member

    Jan 28, 2009
    Okay, this is my first time posting anything so hopefully I get this right:

    I've been given the following problem to do. It's a very basic circuit, just an unknown current source in series with a 25 fF capacitor going to ground. The question wants to know how large the current source must be so that the voltage across the cap reaches 0.6V in 30ps. Other than saying it wants the answer in mA, that's all you're given. And frankly, I haven't the slightest idea where to begin...

    The focus of the lecture was about how all circuits that contain a single capacitor or inductor have an exponential response vs time. The prof said that the key to remember while doing these problems is the equation t = τ ln(B/A), where B is the value which the exponential is approaching, A is the difference between B and the value of the curve at time t, and τ is the time constant equal to RC. However, I don't know what the voltage across the capacitor is growing to, nor do I have a value for R, so to me this equation doesn't seem to be much help.

    If someone could just give me a jumping off point, I'm sure I'd be able to run with it. Am I even on the right track with that formula? Any help would be greatly appreciated. Thanks in advance.

    And for good measure, the problem is attached...
  2. Tesla00010010


    Jun 20, 2008
    In a capacitor: Q=CV

    the formula can be used expressing change in voltage or charge,the same curve is developed,so you can express simply change in voltage or change in charge by using CV

    If the capacitor is discharging,the graph is the same for change in current also,where you can use ohm s' law

    then,if the voltage is 0.6V then you can obtain the charge by multiplying voltage by capacitance.Then divide the charge in the time given and u get the current needed to charge the capacitor at 0.6V in that time,assuming of course the capacitor was at 0 volts when the circuit was closed.
  3. hgmjr


    Jan 28, 2005
    You should be able to use the equation given in this section in the AAC ebook on capacitors and constant current sources.

    You know delta v (dv) and you know delta t (dt) and you know capacitance C so you should be able to plug the values into the equation and arrive at the value for current i.

    Be sure that you know what the units for femto-Farads are.

  4. chrispo86

    Thread Starter New Member

    Jan 28, 2009

    Ahh... I actually looked at that before and it went right over my head apparently. Now that you point it out it makes perfect sense. I was able to do the math and figure it out, and it checks with my simulation on the computer.

    Thanks for the help!!