Measuring open loop gain of LDO in LTspice

[DjAci]

I am using LTspice to model an LDO with error amplifier and an NMOS pass device and my goal is to model the open loop gain.

However i have trouble cutting the loop for AC and leaving the DC bias intact. I am using a 2 resistor voltage divider as part of the feedback loop. I can increase the value of one resistor in AC simulation to effectively cut the loop, but then i also created a massive load impedance for my AC simulation that makes problems with the output capacitor.

Does anyone know how to elegantly separate AC & DC feedback without using high value inductors and capacitors?

Edo

 

[--jason--]

you need to design an appropriate filter in order to cut off the ac and dc signal. at the first, select the suitable cut-off frequency and design on the filter afterward.

 

[ashish_chauhan]

did you try loopgain element as shown in loopgain2 example in LTspice examples...

I think thats the best possible way...

 

[LvW]

This problem - simulation of loop gain without destroying the dc operating point - has been discussed very often in detail - in this as well as in other forums. Spend some minutes and search around.

For example: