Can two first order transfer functions be added together in matlab to make a second order transfer function

As the title says is this possible I have

g=(31.2)/(155*s+1)

and

g1=(4)/(172*s+1)

if i then do

g2=(g+g1) does this give the correct second order tf

Answer 1

[Daniel]

Thanks so that also means putting two first order transfer functions in series on simulink does not make it a second order system?

Given the two transfer functions i have for two responses to level and pressure when a given step change was implemented in a valve how would i make this second order transfer function

[Paul]

If you have two systems in series you should multiply, not add. So, g2= g*g1=

             124.8
  ---------------------
  26660 s^2 + 327 s + 1

Answer 2

[Paul]

Yes, unlike Mischa said you do get a second order tf because g and g1 do not have a common denominator. The result is:

g2 =

      5986 s + 35.2
  ---------------------
  26660 s^2 + 327 s + 1

[Mischa Kim]

Paul is correct. Of course, the order of the denominator determines the order of the system. Unless you have other effects (e.g. pole-zero cancellation).

[Azzi Abdelmalek]

Mischa, when I read Paul's answer, something is wierd. Did I by accident, delete your answer?

[Walter Roberson]

Is the numerator the one with 5986 s in it, or is the 124.8 that has no s in the numerator ?

[Paul]

Depends on what you do. Add them and you get this one, multiply and you get the 124.8 one. In the question Daniel said he wanted to add them, but in a reply he said he wanted two transfer functions in series so you have to multiply.

Answer 3

[Daniel]

spot on that thanks lads

[Azzi Abdelmalek]

Daniel, If you want to add a comment, just click on [comment on this answer]