Can you design a two-stage OTA without compensation resist.

Two-stage OTA design!

Hi, all.

I just designed a two-stage OTA.
When I compare with and without compensation resistance, their response were very simliar. Actually, without compensation resistance (even marginally better) response turned out to be better phase margin and gain.
(Actually, both responses are the almost identical, but at very high frequency (not frequency of interest) without compensation resistance has a bit better phase response...I know it doesn't matter since it is way out of my interest.

My question is...is it okay to design a two-stage OTA without a compensation resistance? (Is it possibly safe?).
Do real designers design such a OTA? (believe 100% on simulation tool...or is there other reasons that I have to include a compensation resistance in a two-stage OTA?

Thank you!!!

Re: Two-stage OTA design!

What's the meaning of “compensation resistance”？
You add a resistor or capacitor?
If possible ,you can upload you schematic,and all will help you to analyze.

Re: Two-stage OTA design!

Um.."Without a compensation resistor" meaning a two-stage OTA added only a compensation capacitor(which goes between 1st stage output and 2nd stage output). "With a compensation resistor" meaning a additional resistor in series with a compensation capacitor.

As you know, a compensation resistor (at l called it....I don't know why but I just came up with) is used to reduce the bad response due to a forward zero.

Thanks!

sorry for the comfusion.

Re: Two-stage OTA design!

ee484 said:

Um.."Without a compensation resistor" meaning a two-stage OTA added only a compensation capacitor(which goes between 1st stage output and 2nd stage output). "With a compensation resistor" meaning a additional resistor in series with a compensation capacitor.

As you know, a compensation resistor (at l called it....I don't know why but I just came up with) is used to reduce the bad response due to a forward zero.

Thanks!

sorry for the comfusion.

Click to expand...

It is safe to use the two-stafe op-amps without so called compensation resistor. I am working in the field of Analog VLSI under a very renowned person in this field. He and his PhD students have designed many circuits without compensation resistor. Good Luck! you can continue your simulation work.

Re: Two-stage OTA design!

try to sweep the value of the resistor, actually as the resistor increase the phase margin increase

the value of ur resistor may not be the optimum

Re: Two-stage OTA design!

Thank you for all!

It's good to hear that a two-stage OTA without a compensation resistor is okay. I was always wondering...because almost all books always discuss about this resistor (so I wonder "is it always safe to put a resistor to reduc RHZ effect even if I cannot see in my simulation?")

The whole point is...I designed a two-stage ota based on gm/id method and I have 70.3 phase margin (my target was 72) so I did not want to use a resistor.

Thank you for all!

Re: Two-stage OTA design!

ee484 said:

Thank you for all!

It's good to hear that a two-stage OTA without a compensation resistor is okay. I was always wondering...because almost all books always discuss about this resistor (so I wonder "is it always safe to put a resistor to reduc RHZ effect even if I cannot see in my simulation?")

The whole point is...I designed a two-stage ota based on gm/id method and I have 70.3 phase margin (my target was 72) so I did not want to use a resistor.

Thank you for all!

Click to expand...

Hi ee484.
Just wanted to add a quick comment regarding your initial quation - this question of whether a technique is necessary is very vague in analog IC design because everything is design specific - in this case it depends on the location of the feedforward RHP (right half plane) zero and its proximity to the other poles in the system. In your case, as is the case with many others who have replied that the resistor is not required, it just happens that the RHP zero is far away from the GBW that its effect is not seen and hence the resistor is not required.
Run the same simulation over process corners, supply voltage variations, temerature variations and your phase margin will change.
In other designs, the RHP zero effect may be seen even in nominal temperature/ process corner case, so it make sense to use it in the design. The location of the RHP zero is given by gm2/(2πCc), where Cc is the compensation cap and gm2 is the transconductance of the second stage. The location of this RHP zero and its proximity to the other poles in the system totally depends on the individual design.
So, rather than asking this question, go and figure out the location of RHP zero, other poles and see how it changes over PVT (process, voltage , temperature) variations. That should tell you if you need the resistor or NOT. If the RHP zero is close to unity GBW, it degrades the phase margin - that is the main problem.
RHP zero when moved to become LHP zero removes this problem as it adds phase shift and improves phase margin.

Hope this helps.

Bharath