Hi I've been trying to solve this problem on finding the voltage out by using the Laplace transform. I know it's pretty basic but I can't get the solution after trying many times. I'm starting now to study this, and I'm a bit confused because it's a bit different to the way I'm used to solve this kind of circuits.

This is the problem:

Problem Laplace Transform

Solution: $$v_{o}(t) = (20e^{-2000t} - 10e^{-1000t})u(t)$$

I have tried to associate Ls and R in parallel and then associating (R||Ls)+(1/Cs) but I don't get to the solution I should obtain. Can someone help me and tell me what should I do to solve this kind of circuit? (I don't want you to solve it, just need you to give me a hint on what's the operation I should do to obtain Vo(s) ) Thank you very much.

EDIT: So this is what I have to develop?

$$V_{o}(s) = \frac{10}{s} \cdot \frac{\frac{RLs}{R+Ls}}{\frac{1}{Cs}+\frac{RLs}{R+Ls}}$$


1 Answer 1


You are right about combining Ls & R in parallel. Next do a voltage divider with that combination and 1/Cs to divide 10/s to Vo.

  • \$\begingroup\$ I don't get the same solution as given either. \$\endgroup\$
    – Austin
    Mar 16, 2015 at 19:32
  • \$\begingroup\$ Whoops, I do get the right answer, just read my calculator wrong. It's a little off but it rounds to what you gave. \$\endgroup\$
    – Austin
    Mar 16, 2015 at 19:48
  • \$\begingroup\$ Thanks @austin. Can you look at my edit to see if I understood right what you told me? Thanks sir. \$\endgroup\$ Mar 16, 2015 at 19:54
  • \$\begingroup\$ @user3780731 Yes, your edit is correct. I got most of the way doing the algebra on paper but gave up and used a TI-89. \$\endgroup\$
    – Austin
    Mar 16, 2015 at 20:01

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