Cload influence on the two-stage cascode compensated OTA

[hsiangleung]

Could anyone give some explanations on the effect of load capacitance on the two-stage cascode compensated OTA design. What's the influence of Cload on the choice of compensation capacitor value and GBW , PM etc.? If Cload is too large or too small, what's the difference? thanks a lot!

 

[tompham]

I think in 2 stage opamp design. To compensate you need Cc > 0.22 Cload
1/ BW = gm(input)/Cc ---> if Cload large --> Cc large and BW decrease. If want high BW need to increase gm or increase power
2/ Slew rate = Itail /Cc and Iout/Cc +Cload ---> if Cload large SR decrease. Want icrease need icrease current or icrease power.
3/Settling time of opamp depends on static and dynamic error settling(Depend on SR and BW) and this will effect by high Cload

 

[Dominik Przyborowski]

Everything whats You need is explained in Copeland, Ribner paper from SSC vol. 19, no 6, pp 919-925, 1984, in section 3b on pages 4-5 of this document.

 

[hsiangleung]

your explanation is quite helpful and is an inspiration for me . thanks very much!

I think in 2 stage opamp design. To compensate you need Cc > 0.22 Cload
1/ BW = gm(input)/Cc ---> if Cload large --> Cc large and BW decrease. If want high BW need to increase gm or increase power
2/ Slew rate = Itail /Cc and Iout/Cc +Cload ---> if Cload large SR decrease. Want icrease need icrease current or icrease power.
3/Settling time of opamp depends on static and dynamic error settling(Depend on SR and BW) and this will effect by high Cload

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Hi Dominik Przyborowski, I have read this document you recommended and it is really helpful. thanks very much for your so detailed information.

Everything whats You need is explained in Copeland, Ribner paper from SSC vol. 19, no 6, pp 919-925, 1984, in section 3b on pages 4-5 of this document.