Questions about the function and characteristics of RHP pole

[endru]

Hi,

Can anyone explain me why RHP pole can cause unstability?
How does the phase of RHP pole look like? Is it going upward like LHP zero?
Is there any way to compensate it, or make the system stable?

Thank you very much for your answers! =)

 

[qslazio]

rhp lhp zero pole

1. the inverse laplace transform of a RHP pole term will be a increasing exponential (or increasing amplitude sinewave) which is not bounded. This is defined with unstable.

2. yes, it's phase is like LHP zero

3. every practical system should avoid it. for compensation, books about control theory or feedback amplifier design would be useful for you

 

[endru]

rhp poles

Thanks qslazio.

Another question. If I design an op amp, and there exists an RHP pole OUTSIDE the UGF, and I use the op amp in a close loop configuration, will it still stable?

Thanks!

 

[qslazio]

pole rhp

It might be.
But it depends on the number and positions of the poles and zeros you have when loop is opened.
For detail, check the root locus theory. I've forgotten the details.

 

[endru]

op amp pole zero

Ok, thanks! =)

 

[q0w1e2r3]

poles zeros lhp rhp

when you use Laplas transition , you will find waveform will be enlarged . RHZ acts as a left pole

 

[endru]

rhp loop

This is the op amp's bode plot I got.
https://farm4.static.flickr.com/3193/2325347653_769c863947_o.png

Do you think that this op amp is still stable if I use it in a closed loop configuration, e.g. for LDO?

Thanks!

 

[endru]

rhp pole compensation

Ups.. sorry, I mean this is the bode plot of the whole LDO, not just the err amp.
I got this by tapping from the feedback point.
Any opinion?

Thanks!

 

[Sudhir.K.A]

poles rhp lhp

The LDO will be unstable.

Give small voltage step some where in the loop and simulate. The output will ring.

The system will be unstable even if the RHP pole is outside the UGB of the loop.

 

[endru]

rhp pole unstable

I see...

But, could you explain me why the system is still unstable although it locates beyond the UGF?

Thanks in advance!

 

[Sadegh.j]

is a pole at zero considered a pole on rhp

Think of it in terms of Laplace transform, a RHP will result e^(at), if t goes to infinity, will become infinite and thus it is not stable.

 

[Sudhir.K.A]

circuit configuration rhp pole

Hi ,
I agrre with Sadegh.

When there is an RHP pole, this means an exponentially increasing term as your as co efficient if you apply laplace transform.
. Hence the system output is not bounded.

regards