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# a left zero before the second pole, is the system stable?

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#### shanmei

is a system with pole at zero stable

I design a op amp, the bode polt in the figure, a left zero before the second pole, is the system stable?
For the reason that at the unit frequency point, the phase margin is bigger than 45(which is bigger than 90), the the system is stable ? right?
anyone ? help! thank you

##### Member level 1
left zero can make gain +20db/dec and phase +90deg, so it is no problem, but in your figure, it seems PM is too small, I think you'd better design the circuit with z1 and p2 both away from your gbw better

### shanmei

Points: 2

#### caosl

Re: a left zero before the second pole, is the system stable

hi,shanmei
In your figure , the stability is not a problem, and make a notice that the PM is at the lowest point of phase at the condition of (MAG>0), not at UGB because of existence of zero. As castrader said, design the circuit with z1 and p2 both away from UGF will be better, this will not bring out the p-z doublet to propagate the response time, but in some situation, this is difficult to implement, for example in LDO.When the stability is the first requist, the pz cancellation is a good choise.

#### paley

##### Member level 4
I think it is no problem

#### hhplay

##### Newbie level 4
Re: a left zero before the second pole, is the system stable

Left zero helps your circuit to be stable.

Be careful of your phase margin. It is not big enough!

#### arghpok

##### Member level 3
Re: a left zero before the second pole, is the system stable

You absolutely want that zero over there, even when I don't quite understand the behavior of your hand-made phase-shift plot.

If you are using traditional LDO topology (I think you are), the frequency of the zero would be 1/2piResrCout where pi is pi, Resr is the ESR of the output capacitor Cout.
Also you should have another pole beyond the P2 frequency and it's load dependant as well as the zero. Also, the zero is (as you can see) Resr dependant; you cannot decrease it (ESR) indiscriminately.

#### lijianheng

##### Full Member level 2
Re: a left zero before the second pole, is the system stable

It is stable, but transient responce maybe a little different.

#### Hunter_BJUT

##### Member level 2
stabe, but may suffer from settling problem.

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