Finding the 3dB point of a transfer function.

Factual Questions
1

I am trying to figure out how to find the -3dB point for the transfer function 8(1+2s) and for some reason I am having a brain fart. I did this a million times the first 2 years of school but haven’t done it in a long time so I have forgotten.

I know it is when the power is 1\2 so I am thinking that I find when the Tf= 1/2 ? Then 10log of that? I think I am wrong here. Can someone starighten me out?

2

0.5=8/(1+2s)
s=?
duh?

3

Make the bode plot in Matlab:

Draw a horizontal line from the y axis parallel to the x axis starting from 0.7 (-3dB) till you reach your amplitude curve, from there drop vertically down and read off the value from the x-axis. Thats you bandwidth.

Alternatively, replace s by j w (w = omega), then solve the equation :

.7 = Magnitude(8/1+2jw)

Hope that helps.

4

Yes, andy_fl is right, but it requires knowledge of whether the questions was phrased in voltages or powers.

I should have said:

either

0.5 = 8/(1+ 2s)

or

0.707 = 8/1 + 2s)

s=?
duh?