[SOLVED] Self compensated OTA

[Junus2012]

Dear All

for the high output impedance OTA, the circuit lead to be self compensated from the load itself. the high output impedance make the distance between the dominant and non dominant poles far enough to have good phase margin.
the dominant pole is at f-3 dB = 1/2.pi.Ro, because Ro is high thus this frequency is the first pole frequency

Here is my question.

why increasing the capacitive load make the circuit more stable ??.

According to me, pushing the non dominant pole away from the GBW is making the OTA more stable because it will push away the accumulation of the phase when it becomes 180. but reducing the pole frequency has nothing to shift this value


Many thanks to you

 

[rakshitdatta]

Junus2012,

I believe the compensation whatever you are talking usually corresponds to a single stage OTA. In that case, adding any extra capacitive load at the output will push the pole associated with it to lower frequencies. If this happens to be your dominant pole, then with increasing capacitive load, the dominant pole moves to lower and lower frequencies and there by giving you a larger phase margin or "making the circuit more stable". Also, 1/2.pi.Ro is not dimensionally consistent with frequency. Can you please go through it again.

 

[Junus2012]

Dear rakshidatta

Thank you for your reply

Actually my question how this lead to increase the phase margin when the dominant pole is at lower frequency ?

Junus2012,

I believe the compensation whatever you are talking usually corresponds to a single stage OTA. In that case, adding any extra capacitive load at the output will push the pole associated with it to lower frequencies. If this happens to be your dominant pole, then with increasing capacitive load, the dominant pole moves to lower and lower frequencies and there by giving you a larger phase margin or "making the circuit more stable". Also, 1/2.pi.Ro is not dimensionally consistent with frequency. Can you please go through it again.

 

[rakshitdatta]

If the dominant pole moves to lower frequencies, the unity gain frequency also moves to lower frequencies proportionally. Effectively, the non dominant poles are more farther from the unity gain frequency. Hence phase degradation due to these non dominant poles will be less. Hence the better phase margin.

 

[Junus2012]

Dear rakhitdatta

Thank you for your nice interpretation

but what moving the dominant pole to the lower frequency and keeping the GBW the same ??????????????

in the folded cascoded OTA, the dominant pole is f-3dB=1/2.pi.Ro.CL, the GBW is gm1/CL. So i can decrease the dominant pole by increasing Ro and hence the GBW will not change

If the dominant pole moves to lower frequencies, the unity gain frequency also moves to lower frequencies proportionally. Effectively, the non dominant poles are more farther from the unity gain frequency. Hence phase degradation due to these non dominant poles will be less. Hence the better phase margin.

 

[rakshitdatta]

Yeah you are correct. Increasing the Ro will move the pole to lower frequencies without changing the unity gain frequency. This is because the dc gain(low frequency gain) will increase proportionally. But the discussion is about increasing load capacitor. If the load capacitor is increased, the pole associated moves to a lower frequency. However, the dc gain(low frequency gain) will not change. Hence, along with the pole, the unity gain frequency will also move to lower frequency.