Problem with AC simulation of LDO

[myblues]

Hi, I'm studying ldo design. I've met a problem of ac simulation. In figure below, one of input of error amp is connected to the feedback net.
When I do .ac simulation, I put a spiral between input of EA and feedback net. So the DC voltage of input achived and ac signal is blocked. But how can I add ac signal? Ples. see below:
v1 in 0 ac 1
Is this ac sentence right? It has no dc voltage! Other words the dc voltage is zero.
If I give a dc voltage like this:
v1 in 0 dc 1.2 ac 1
then the dc voltage from freedback net is not taken effect.
To solve this problem I put a capacitor between the ac signal and the input. But in this way I cannot see the ac characteristic in low frequency.
So I have no idea about it.
Thanks!
(By the way, are there any problem in my picture?)

 

[myblues]

Re: How to simulate .ac of LDO?

I saw a ldo circuit in a paper 'Performance Evaluation of CMOS Low Drop-Out Voltage Regulators' showed below:

This circuit puzzled me. Should the feedback be connected to M2 gate? Why not M1 gate?

The paper is below.

 

[baniliu]

AC simulation of LDO

Dear myblues,


it will be no problem with inductor L1 connected the feedback loop in order to give DC operation, a much large resistor(100Mohm) may also be an alternative instead of inductor.

So if you give a dc voltage to IN, suppose the VREF=1.2v, a systematetic offset will easily destroy the DC balance.

And my suggestion is that you can try give a ac signal in VREF side.


R1 IN NEt4 100M
C1 IN 0 1000
v1 VREF 0 dc 1.2 ac 1



Baniliu

 

[haoshs]

AC simulation of LDO

you can break the net from inductor L1 to in and insert a ac voltage source, like this:
vac in net6 ac=1
L1 net6 net4 5