Analog circuit, such as RC, LC and RLC for step response

Discussion in 'Analog & Mixed-Signal Design' started by Benengineer, May 15, 2018.

  1. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
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    upload_2018-5-15_1-3-2.png

    Please analyze those three output and what really happen to capacitor and inductor. Thanks.
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi Ben,
    Is this a College assignment.?
    E
     
  3. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    No, interview questions for undergraduate

    MOD: hi Ben
    Moved to Analog & Mixed Signal Design
     
    Last edited by a moderator: May 15, 2018
  4. wayneh

    Expert

    Sep 9, 2010
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    Well you've named them properly. What's keeping you from solving them? Start with the definitions of how (ideal) resistors, inductors and capacitors behave. For instance ohms law, V = I•R defines the behavior of a resistor. The other two are a little trickier but it's all math.
     
  5. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    The is circuit I = c dv/dt, Vout = RC dv/dt
    The output waveform like this
    upload_2018-5-15_11-47-7.png
     
  6. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    ok, thank you.
     
  7. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    Is this output all right?
     
  8. wayneh

    Expert

    Sep 9, 2010
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    The general shape is ok but it shouldn’t trail off at the end. And although the axes of the input wave are not labeled, I’d probably label mine. I’d also note major features such as the halfway point and such.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    Can you figure out the equations for the outputs in each case in terms of the circuit parameters?
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Consider also that there are different cases. If RC is really small compared to the x scale, the output will look like the input. If RC is huge, the output will be nearly flat.

    You really ought to show a family of curves, or create an x axis scale as I mentioned earlier.
     
  11. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    upload_2018-5-16_22-31-14.png
    Vi = LdiL/dt + Vo - > Vo = Vi-LdiL/dt iL = ic -> ic = C dVo/dt
    Vi - LC (dVo/dt)^2-Vo = 0

    upload_2018-5-16_23-4-10.png
    Vi =LdiL/dt + R*C dVo/dt + Vo -> iL = ic -> ic = C dVo/dt

    Vi = LC (dVo/dt)2 + R*C dVo/dt + Vo

    The above are the equations.
     
  12. WBahn

    Moderator

    Mar 31, 2012
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    Check the units on your final equation. They don't work out. It's not the only place you made that mistake, either.

    Keep in mind that (dVo/dt)² is NOT the same as dV²/dt². The former is the square of the first derivative and the latter is the second derivative.

    Once you have fixed that, then solve for the output voltage in terms of time and the circuit parameters. Then plot them on a reasonable set of axes.
     
  13. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    1 RCL circuit
    Vi = LC dVo2/dt2 + R*C dVo/dt + Vo

    LC M2+RCM + Vo - Vi = 0

    upload_2018-5-18_0-25-44.png
    upload_2018-5-18_0-26-48.png

    2, LC circuit
    upload_2018-5-18_0-27-38.png
    Please let me know if I need to do anything
     
  14. WBahn

    Moderator

    Mar 31, 2012
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    Put yourself in the interviewer's place. If you gave someone what you posted in the first post, what would you like to see them able to do within the limited time and resources of an interview.

    I would expect someone to be able to answer all three reasonably well in less than two minutes.

    A question like this is looking for big-picture concepts, not detailed minutiae.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Exactly. If you've worked enough of these problems, you can draw the plots and note the major features almost from rote.

    It's been a long time since I solved these classics but the math above doesn't look even vaguely familiar to me.
     
  16. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    Your guys makes me confused. What shoud I answer it? My awnsers are right? What concept shoud I answer? This is the part your guys can help me with. According to your statements, how do i answer them in 2 minutes? What is key concept?
     
  17. Benengineer

    Thread Starter Member

    Feb 6, 2016
    128
    2
    I know how to do those math, but what does an interviewer really want me to answer them? Please help me. Thank you. They don't need me to solve them. They want me to describe the output for those questions.
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    Take a shot at it and then let us provide feedback on YOUR answer.

    Take each one and give yourself a two minute time limit (per circuit) and describe everything you can think of about that circuit in two minutes. Pretend it is a written interview.
     
  19. BR-549

    Distinguished Member

    Sep 22, 2013
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    I don't think the interviewer wants math equations. He wants to see 3 output lines of that cap charging. This will show him if you understand circuit configurations. The character of those 3 lines will tell him.
     
  20. ArakelTheDragon

    Active Member

    Nov 18, 2016
    510
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    You need to know about current being faster than voltage, voltage being faster than current. What is a coil and what is a capacitor.
     
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