# Question on transfer function equation

Discussion in 'Electronic Basics' started by MRW, Dec 4, 2006.

1. ### MRWGuest

Hi again!

http://focus.ti.com/lit/ml/sloa088/sloa088.pdf

In page 16-12 (Figure 16-12) they are showing a first order
non-inverting low pass filter. I tried calculating the transfer
function and came up with the following:

Vo / Vin = (1 + R2/R3) / ( s*C1*R1 + 1 )

Their transfer function is:

Vo / Vin = (1 + R2/R3) / ( wc*s*C1*R1 + 1 ), wc = omega sub c (I think
this represents the cut off frequency).

My question is: how did they get wc?

Thanks!

2. ### Andrew HolmeGuest

Your transfer function is correct. They are wrong.

3. ### EeyoreGuest

It's a mistake. s = jwc

Graham

4. ### The PhantomGuest

It's not wc (omega times c); it's omega sub c. Go back and read page
16-2, where they say:

"For a normalized presentation of the transfer function, s is referred to
the filter's corner frequency, or - 3dB frequency, w sub c, and has these
relationships:"

5. ### MRWGuest

Yep, I actually wrote down wc = omega sub c. Thanks for pointing out
that page. I'm going to re-read it again and see if I can understand
it. I'm not too comfy about s = sRC. To me, it's like saying 1 = 1*2*3.

6. ### The PhantomGuest

I would definitely agree. I would have made up a new symbol for the
normalized s, something like s' or s sub n or something. It's very bad
practice to use the same symbol for two different things!

I think I would just rewrite all the pertinent equations with a different
symbol for normalized s before I tried to follow the analysis.