rcantal
Newbie level 4
Hi Guys,
I have some problems to understand what is wrong in the stb analysis or why in my mind it do not match with the Ac analysis and feedback theory.
To do stability analysis I always have done AC analysis and use the bode theory to understand the stability of op-amps or feedback systems.
When I designed the the first full-differential op-amp I have heard about the cmfb and stb analysis. I did it in this time and I agreed with the tool (Spectre -Cadence).
But now I want to understand and i did a simple OTA with a unitary feedback. To compare the to methods, I ran ac analysis with vdc source at input, I put ac = 1 and measured the open loop gain. it give-me 28 dB. (The input of the OTA was well biased and the transistors are in saturation region).
After I closed the loop and put a vdc source between the loop ( ac = 1, dc = 0), I run a Ac analysis and measured the close loop gain, as the OTA is a buffer configuration the close loop gain is equal to 0 dB.
With the same configuration, I ran the stb analysis than measured the loop gain that was 82 dB. I think it is wrong because the loop gain is the product between open loop and the close loop gain. So it would be 28 dB!!
Anyone can help me to understand this!
I read this paper that look good to understand the advantages between Ac and stb analysis.
http://www.cdnusers.org/community/virtuoso/resources/ams/tp_AllegroMicro_CDN_Final.pdf
Regards,
I have some problems to understand what is wrong in the stb analysis or why in my mind it do not match with the Ac analysis and feedback theory.
To do stability analysis I always have done AC analysis and use the bode theory to understand the stability of op-amps or feedback systems.
When I designed the the first full-differential op-amp I have heard about the cmfb and stb analysis. I did it in this time and I agreed with the tool (Spectre -Cadence).
But now I want to understand and i did a simple OTA with a unitary feedback. To compare the to methods, I ran ac analysis with vdc source at input, I put ac = 1 and measured the open loop gain. it give-me 28 dB. (The input of the OTA was well biased and the transistors are in saturation region).
After I closed the loop and put a vdc source between the loop ( ac = 1, dc = 0), I run a Ac analysis and measured the close loop gain, as the OTA is a buffer configuration the close loop gain is equal to 0 dB.
With the same configuration, I ran the stb analysis than measured the loop gain that was 82 dB. I think it is wrong because the loop gain is the product between open loop and the close loop gain. So it would be 28 dB!!
Anyone can help me to understand this!
I read this paper that look good to understand the advantages between Ac and stb analysis.
http://www.cdnusers.org/community/virtuoso/resources/ams/tp_AllegroMicro_CDN_Final.pdf
Regards,