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TucsonMike
TucsonMike
2/4/2019 9:33:00 AM
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Using the right frequency in the Cf equaition
Hey Loren, nice post -yea the first pass at getting Cf=0.175pF seemed a low. 

I am guessing the reason the original calculation for Cf was too low was the unity gain closed loop bandwidth of 1.8GHz was used in Eq. 2 where that should have been the GBP, more like 900MHz - Eq. 1 is implicitly solving to place the feedback pole at Fo. that will give a Q=1 and likely the final Cf shown was slightly increased to empirically reduce the peaking (1.25dB)  from that condition. 

I touched on these issues a bit in a new blog showing up on PLanet Analog, Feb. 3, 2019. 

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