# Transfer function needed -- help please

1. Mar 18, 2016

### Suiluas

Hi guys, I'm new to this engineering problem solving and I just wanted to ask for your help getting a transfer function of the input voltage Uin(t) and the output voltage Uout(t).

1. Uin(t) = U_R(t) + u_out(t)
2. i(t) = i_L(t) + i_C(t)
3. u_r(t) = R_i(t)
4. u_out(t) = L d_iL / dt
5. i_C = C d_uout / dt

3. The attempt:

Dv_c(t) / dt = 1/C i_c(t) = [ 1/C (i(t) - i_out(t) ] = - 1/RC_vc(t) + 1/C i (t)
Di(t) / dt = 1/L v_L(t) = 1/L...

Last edited by a moderator: Mar 18, 2016
2. Mar 18, 2016

### Staff: Mentor

Welcome to the PF.

Is there a circuit diagram that you could UPLOAD for us to look at. For me, it's hard to visualize the problem. Thanks

3. Mar 18, 2016

### Suiluas

Sure, attached is the circuit. :) http://postimg.org/image/7a4mj5c6b/

<< Link removed by Mentor >>

4. Mar 18, 2016

### Staff: Mentor

5. Mar 18, 2016

### Suiluas

Sorry for that, here is the picture of the system.

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6. Mar 18, 2016

### Staff: Mentor

No worries. Thanks for the diagram.

I'm not able to follow what you are doing with your equations, but it seems like you are working with each component individually (I could be wrong). Instead, I would write the one KCL equation for the node between the input resistor and the RLC load. Write that differential equation and solve for the currents and voltages. Can you give that approach a try?

7. Mar 18, 2016

### Suiluas

I will try, but it's not as easy for me. anyways, thanks for advice.

8. Mar 19, 2016

### LvW

Suiluas - are you aware that a system`s "transfer function" requires to find voltage-current relations in the frequency domain?
You have started in the time domain - this is not necessary and requires application of the Laplace transformation. Instead, you can start directly with impedances in the frequency domain
Examples: inductice impedance: ZL=jωL, capcitive impedance: ZC=1/jωC.

Last edited: Mar 19, 2016