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Design >> Analog Design >> Left half plane zero & Right half plane zero
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Message started by henrytqy on Nov 8th, 2011, 11:08pm

Title: Left half plane zero & Right half plane zero
Post by henrytqy on Nov 8th, 2011, 11:08pm
The following picture has two circuits that contain the different way of compensation for polos & zeros. As I know that the 1-st(left)one has a zero on the LHP that approximate equals gm/Cc, the 2-nd(right)one has a zero on the RHP that approximate equals gm*rds*gm/Cc. I may ask that which compensition is better?
thands~[img][/img]

Title: Re: Left half plane zero & Right half plane zero
Post by raja.cedt on Nov 9th, 2011, 1:00am
hello,
could you please verify the zero location for the first(left) ckt, i guess it is also same as 2nd one, because from schematic you can see small signal equivalent is same for both .

Thanks,
raj.

Title: Re: Left half plane zero & Right half plane zero
Post by Alexandar on Nov 9th, 2011, 2:08am
In general a LHP zero (if properly placed), has a positive influence on stability, as it 'pulls' your root locus to the left. And vice versa for the RHP zero of course.

Title: Re: Left half plane zero & Right half plane zero
Post by henrytqy on Nov 9th, 2011, 5:21am

Lex wrote on Nov 9th, 2011, 2:08am:
In general a LHP zero (if properly placed), has a positive influence on stability, as it 'pulls' your root locus to the left. And vice versa for the RHP zero of course.

Can I take you mean that the 1-st one is better?I don't know if the LHP zero on the 1-st is too small?
Are there any other factors should be taken into considerition,like PSRR?
Thanks~

Title: Re: Left half plane zero & Right half plane zero
Post by henrytqy on Nov 9th, 2011, 5:30am

raja.cedt wrote on Nov 9th, 2011, 1:00am:
hello,
could you please verify the zero location for the first(left) ckt, i guess it is also same as 2nd one, because from schematic you can see small signal equivalent is same for both .

Thanks,
raj.

I don't think the small signal equivalent is the same for both.I draw it as following:
I have calculate the transferfunction Av=Vout/Vin with the help of Maple.It shows that it really has LHP zero as I mentioned before.

Title: Re: Left half plane zero & Right half plane zero
Post by raja.cedt on Nov 9th, 2011, 5:30am
hello,
no man both ckts gives RH zero's

Thanks,
raj.

Title: Re: Left half plane zero & Right half plane zero
Post by henrytqy on Nov 9th, 2011, 5:31am

raja.cedt wrote on Nov 9th, 2011, 1:00am:
hello,
could you please verify the zero location for the first(left) ckt, i guess it is also same as 2nd one, because from schematic you can see small signal equivalent is same for both .

Thanks,
raj.

[img][/img]
This is the small signal of the 1-st ckt.

Title: Re: Left half plane zero & Right half plane zero
Post by henrytqy on Nov 9th, 2011, 5:36am

raja.cedt wrote on Nov 9th, 2011, 5:30am:
hello,
no man both ckts gives RH zero's

Thanks,
raj.

I'm sorry that I don't undersdand this"no man both ckts gives RH zero's".
I am new here. does ckts short for circuit?
Thanks~

Title: Re: Left half plane zero & Right half plane zero
Post by raja.cedt on Nov 9th, 2011, 5:56am
hello,
i am sorry for my previous post. So if you have razaavi could you please verify page no..371. He has derived TF for the left ckt, it is gm*rds*gm/Cc. You get gm/cc only if there only cap feedback aorund the transister. By refering razavi page no 176, it has simple method to estimate zero location, i would suggest you to estimaate with this method without deriving full equations.

And comming  to smal signal models, whats the diference between both of these ckts, in one case you have PMOS commongate amplifier and in another case you have NMOS commongate amplifier, so small signal model don't care about NMOS and PMOS. One more thing both circuits have zero in RHP, because forward path an feedback path are having oppsite polarity gains.

Once again sorry for the previous post.

Thanks,
Raj.

Title: Re: Left half plane zero & Right half plane zero
Post by thechopper on Nov 9th, 2011, 4:47pm
I agree with Raja. Both are RHP.
The second circuit might be better since both the Miller and cascode stages are NMOS. Thus parameters will track and stability over temp will also be better.


Best
Tosei

Title: Re: Left half plane zero & Right half plane zero
Post by henrytqy on Nov 9th, 2011, 4:52pm

raja.cedt wrote on Nov 9th, 2011, 5:56am:
hello,
i am sorry for my previous post. So if you have razaavi could you please verify page no..371. He has derived TF for the left ckt, it is gm*rds*gm/Cc. You get gm/cc only if there only cap feedback aorund the transister. By refering razavi page no 176, it has simple method to estimate zero location, i would suggest you to estimaate with this method without deriving full equations.

And comming  to smal signal models, whats the diference between both of these ckts, in one case you have PMOS commongate amplifier and in another case you have NMOS commongate amplifier, so small signal model don't care about NMOS and PMOS. One more thing both circuits have zero in RHP, because forward path an feedback path are having oppsite polarity gains.

Once again sorry for the previous post.

Thanks,
Raj.

I have read the pages of Razaavi you mentioned, but Razaavi says"this topology contains a zero in the left half plane" just up his equation 10.42.
I tried the method on the page 176 for the left ckt.In this way I got a RHP zero. It is ovious that the two results are in contradiction.

By the way, I think the forward path of the left ckt is somehow weird,since the signal will go from the drain to the souce of the common gate PMOS?

Thanks~

Title: Re: Left half plane zero & Right half plane zero
Post by raja.cedt on Nov 10th, 2011, 10:54am
hello henrytqy

there is nothing weird in the left, infact both ckt's form same forward path, but again one side nmos and another side pmos.

Thanks,
raj.

Title: Re: Left half plane zero & Right half plane zero
Post by sean.geng on Nov 13th, 2011, 11:58pm
In fact, this two sch are the same in small signal analysis, because both of them are providing a feedback signal path with  gm if PMOS is almost the same with NMOS.

and the difference between them is:
at high frequency, the second sch may cause a straight path from input to output, which lower the gain at high frequency.

Title: Re: Left half plane zero & Right half plane zero
Post by raja.cedt on Nov 14th, 2011, 12:17pm
hello,
at what frequency you are assuming, it is far from the UGB...

Thanks,
Raj.

Title: Re: Left half plane zero & Right half plane zero
Post by sean.geng on Nov 15th, 2011, 2:04am
Hi,
if a step-up signal is assigned to input, the voltage of output will increase almost as input, this could pose a stress at the source and gate of MOSFET
raja.cedt wrote on Nov 14th, 2011, 12:17pm:
hello,
at what frequency you are assuming, it is far from the UGB...

Thanks,
Raj.

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