# freq and stability analysis

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#### bharathr87

##### Member level 1
why do we add an inductor and capacitor (LC ckt) in the feed back loop for freq and stability analysis for ex. as shown in this fig?

#### abhisheksbox

##### Member level 1
Where is the figure?

#### bharathr87

##### Member level 1
wait for a few seconds it will appear!!!

##### Full Member level 2
Hello Bharath..

Wht u have done is correct only..it's breaking the loop virtually and it's one of the way for stability analysis...

The easy way is connect the Vin- and Out put through iprobe (which can be found in analogLib)

and add stb analysis ( what ever frequency range u want.)

Vin+ is connected to a Ac Source with Ac magnitude =1 and DC voltage as VDD/2 i mean the common mode voltage.

u will get the loop gain,PM,GM,UGB.

With Best Regards

#### bharathr87

##### Member level 1
Thanks shady25.....i know that it is correct and it works...but can u help me figure out how the LC ckt is useful in doin the stability analysis....

##### Full Member level 2
Bharath..

The actual way of doing with L and C is as follows.

Here the capacitor allows the ac signal and to flow through the OPAMP and evaluates the (gain & Phase response) stability response...the inductor blocks the AC signal coming from the Source. At the other end of the inductor u find the ac signal. from the Ac analysis find the Gain response from output to input and u will get the similar response as that of the stb analysis.

to get the Stability response keep high values of L and C...

Hope this could help u...

with Best Regards

### bharathr87

Points: 2

#### supreet_95

##### Member level 3
Hi,
We only want the DC portion of the output to go to negative terminal, so that the op-amp is properly biased. So we use C and L to send only dc portion in feedback. And then we can apply the ac voltge seperatly on inegative terminal, to see the gain on the output.

Supreet

### bharathr87

Points: 2

#### biff44

##### Advanced Member level 5
Here is a short paper on a way to do it. Scroll to the end to see it.

Rich

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