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

#### Benengineer

Joined Feb 6, 2016
133

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

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#### ericgibbs

Joined Jan 29, 2010
17,983
hi Ben,
Is this a College assignment.?
E

#### Benengineer

Joined Feb 6, 2016
133
hi Ben,
Is this a College assignment.?
E

MOD: hi Ben
Moved to Analog & Mixed Signal Design

Last edited by a moderator:

#### wayneh

Joined Sep 9, 2010
17,405
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.

#### Benengineer

Joined Feb 6, 2016
133
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.
The is circuit I = c dv/dt, Vout = RC dv/dt
The output waveform like this

#### Benengineer

Joined Feb 6, 2016
133

MOD: hi Ben
Moved to Analog & Mixed Signal Design
ok, thank you.

#### Benengineer

Joined Feb 6, 2016
133

#### wayneh

Joined Sep 9, 2010
17,405
Is this output all right?
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.

#### WBahn

Joined Mar 31, 2012
29,150
Can you figure out the equations for the outputs in each case in terms of the circuit parameters?

#### wayneh

Joined Sep 9, 2010
17,405
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.

#### Benengineer

Joined Feb 6, 2016
133
Can you figure out the equations for the outputs in each case in terms of the circuit parameters?
Can you figure out the equations for the outputs in each case in terms of the circuit parameters?

Vi = LdiL/dt + Vo - > Vo = Vi-LdiL/dt iL = ic -> ic = C dVo/dt
Vi - LC (dVo/dt)^2-Vo = 0

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.

#### WBahn

Joined Mar 31, 2012
29,150
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.

#### Benengineer

Joined Feb 6, 2016
133
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.
1 RCL circuit
Vi = LC dVo2/dt2 + R*C dVo/dt + Vo

LC M2+RCM + Vo - Vi = 0

2, LC circuit

Please let me know if I need to do anything

#### WBahn

Joined Mar 31, 2012
29,150
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.

#### wayneh

Joined Sep 9, 2010
17,405
A question like this is looking for big-picture concepts, not detailed minutiae.
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.

#### Benengineer

Joined Feb 6, 2016
133
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.
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?

#### Benengineer

Joined Feb 6, 2016
133
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.
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.

#### WBahn

Joined Mar 31, 2012
29,150
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?
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.

#### BR-549

Joined Sep 22, 2013
4,928
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.

#### ArakelTheDragon

Joined Nov 18, 2016
1,362
You need to know about current being faster than voltage, voltage being faster than current. What is a coil and what is a capacitor.