adding ressistor on the output to cure problematic slope in bode plot

Y

yefj

Advanced Member level 5
Joined
Sep 12, 2019
Messages
1,515
Helped
1
Reputation
2
Reaction score
5
Trophy points
38
Activity points
9,188
Hello , on the third minute of the manual shown in the link at minute 3 he says that when we add a resistor our added pole is 1/(Rop+Radd)C
Then he says that when the impedance of the capacitor in the same order ar the added resistor then we have a ZERO effect and the slope gets less steep.
How does a pole can turn to zero?
What is the math behind it?
Thanks.


 

Sort by date Sort by votes

When Impedance of C matches series Rs then you are near critical damping.
My RLC impedance 4D graph helps look up these values.

e.g. here L=C=1e-9 and Zo=sqrt(L/C)= 1 so adding Rs=1 make it "near" critically damped with a very low peak in frequency response and shifted slightly lower. (Q = 1⁄2) is said to be critically damped.




There are tables with direct correlation to 2nd order Q and overshoot.

Q factor - Wikiwand

In physics and engineering, the quality factor or Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defi...
www.wikiwand.com

 

Attachments

  • 1706896332197.png
    273 KB · Views: 79
Last edited:
Reactions: yefj

    Y

    yefj

    Points: 2
    Helpful Answer Positive Rating
Upvote 0 Downvote
If we ignore the internal z's of the OpAmp output, we have a series
R and C to ground, so

Z = R + 1/sC = (RCs+ 1) / sC = [ RC (s + 1/RC) ]/sC, hence you can see the zero
in numerator.


Regards, Dana.
 
Reactions: yefj

    Y

    yefj

    Points: 2
    Helpful Answer Positive Rating
Upvote 0 Downvote
danadakk said:
If we ignore the internal z's of the OpAmp output, we have a series
R and C to ground, so

Z = R + 1/sC = (RCs+ 1) / sC = [ RC (s + 1/RC) ]/sC, hence you can see the zero
in numerator.


Regards, Dana.
Click to expand...
but at unity gain frequency then internal Z of op Amp may be relevant if R approaches Zo.

e.g. Op Amp Zo may vary from 20 to 250 ohms in BJT type

So for critical damping step response, include internal Zo and choose Rtotal = 1.85 to 2 * X(f) at breakpoint, or fo resonance

It depends if you want slightly faster step response with overshoot, or no overshoot.
But recall Vin+ always has greater BW.
 
Last edited:

Upvote 0 Downvote
You must log in or register to reply here.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…