Feedback cap

Discussion in 'General Electronics Chat' started by DexterMccoy, Mar 11, 2014.

Not open for further replies.
1. DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Inverting or Non Inverting Op amps amplifier using Rin Resistor , Rf feedback resistor

When placing a capacitor in the feedback of a inverting or Non Inverting Op amp amplifier

At high frequencies the feedback capacitor's impedance is low, ESR is low, so it passes the high frequency but the Op amps gain is LOWER because the capacitor impedance and Rf feedback resistor is in parallel so the Gain is lower at higher frequencies?

The Caps ESR value is in parallel with the Rf feedback resistor which makes the op amps gain lower?

The Caps Impedance at high frequencies is in parallel with the Rf feedback resistor which makes the Op amps gain lower?

Nov 12, 2008
3,657
2,817

3. DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Why does the Gain get lower?

The Caps impedance is AC resistance , which is a low resistance value in parallel with the Rf feedback resistance

I would think the Gain would be Higher

I mean the Caps Equivalent series resistance is "in parallel" WITH the RF feedback resistor

Why does this cause the Gain to get lower? i would think the op amps gain would get Higher

4. w2aew Active Member

Jan 3, 2012
219
64
In the inverting amp case, the closed loop gain is equal to -Rf/Rin. Thus, if Rf is lowered, the gain is lowered. Same thing for non-inverting case.

5. DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Right, but the Capacitors impedance is low and the ESR value is low

So the capacitors impedance value is low which is in parallel with Rf = low gain?

How is it low gain? I could under stand if the capacitors impedance value was a High value because it would be in parallel with RF which would lower the value of RF , which causes the op amp to have high gain

6. LvW Well-Known Member

Jun 13, 2013
779
105
The task of such a capacitor is to improve stability.
This is achieved by increasing the feedback factor (hence, lowering the closed-loop gain) in the critical region. That is the frequency region where the loop gain is approaching unity.
Very often, stability is degraded due to the capacitance at the opamps input terminals. The feedback capacitor can - up to a certain degree - compensate this influence. This effect is comparable to the working principle of an oscilloscope probe (frequency-compensated voltage divider).

7. DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
How does it improve the closed loop gains stability?

The Gain gets lower at high frequencys
The Gain gets High at low frequencys

How is that a closed loop gain stability?

8. LvW Well-Known Member

Jun 13, 2013
779
105
In my former reply I have tried to roughly explaine the effect of stabilization.
However, for a good understanding it is necessary to become familiar with the stability criterion.
Do you know what "loop gain" is?
Do you know the stability requirements ?
Do you know which role the lop gain plays for stability?
Do you know how a frequency-compensated voltage divider works?

9. DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
The capacitor stabilizes the op amps gain by filtering out the ripple and voltage fluctuations?

The Feedback capacitor and the feedback resistor = impedance feedback

The impedance feedback = the feedback capacitors impedance/ESR that is in parallel with the feedback resistor

At high frequencies on the input , the feedback capacitors impedance/ESR is low in resistance which will have low gain output

High frequency input = Op amp low gain output
Low frequency input = Op amp High gain output

10. ronv AAC Fanatic!

Nov 12, 2008
3,657
2,817
High frequency input = Op amp low gain output
Low frequency input = Op amp High gain output

Right you are....

11. DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Yes is that? The capacitor changes the gain, how and why?

12. LvW Well-Known Member

Jun 13, 2013
779
105
No - sorry, that is not correct.
As mentioned, you can understand the stabilization effect only if you are able to answer the questions as written in my post#8.

13. DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Why isn't this correct?

The capacitor stabilizes the op amps output voltage fluctuations and AC ripple, noises

What is your theory on how the capacitor stabilizes the op amp?

14. LvW Well-Known Member

Jun 13, 2013
779
105
May I ask you: What is your theory on "output voltage fluctuations and AC ripple"? In which book did you read something about these terms? Why do you expect such output disturbances?

I only can repeat: It is the task of the feedback capacitor to increase the phase margin, thereby improving stability of the whole circuit.
"My theory" is connected with those terms as contained in my questions (see post#8). Primarily: Loop gain, stability criterion (Nyquist).

15. Billy Mayo New Member

Mar 24, 2013
70
0
So the feedback cap increases the phase shift of the op amps output to input phase which stabilizes the gain?

Feb 17, 2009
4,697
1,321
17. ronv AAC Fanatic!

Nov 12, 2008
3,657
2,817
Ahh, I get it. Two answers to 1 question. I think the OPs question is for a filter, not stability, but we can find out.
Here are 3 pictures of the same circuit with 3 different frequencies as the input - 100Hz., 1000 Hz., and 10000 Hz. In all cases the input is 2 volts peak to peak.
At the low frequency the impedance of the cap is high compared to the feedback resistor so not much change. At the high frequency it is low compared to the feedback resistor so the gain is lower. In the middle-well it's in the middle. A little lower gain.

File size:
161.7 KB
Views:
18
File size:
160 KB
Views:
19
File size:
161.9 KB
Views:
20
18. MrChips Moderator

Oct 2, 2009
18,215
5,722
Did anyone consider looking up active low pass filter?

19. Veracohr Distinguished Member

Jan 3, 2011
670
101
In a nutshell:

At some frequency, an opamp's response will have a 180° phase shift. If the gain at that frequency is greater than 1, it can result in net positive feedback and thus an unstable system. The capacitor changes the opamp's own frequency rolloff characteristic so that the gain is less than 1 at the frequency where the phase shift is 180°, stabilizing the system.

Now you should read up on the things LvW mentioned to really understand it.

20. LvW Well-Known Member

Jun 13, 2013
779
105
It`s really interesting to see how many different approaches are possible to explain the effect of this additional capacitor (frequency domain, time domain, loop gain, opamp´s gain roll-off, compensated voltage divider,..).
And all these explantions come to the same conclusion:
Improvement of stability.