I designed a low pass filter using Differential difference amplifier for cutoff frequency 150Hz.When i simulate in ngspice i got correct cutoff frequency,but when i extract the magic layout and simulate using spice i'mgetting very low cutoff frquency around 40Hz.What may be the reason?............
if extracted spice model works otherwise , but only cutoff frequency is reduced then you may have to relayout the leyers with reduced capacitance effect.
For such a low frequency, layout capacitance is likely not a factor. I suspect there is in error in how it extracts the layout. Compare the two spice files to see what the difference might be. Perhaps there's an error in your layout.
how can we reduce the capacitance effect while drawing the layout.I tried by redraw but no effect,amplification of differential difference amplifier is very low.What may b the reason.................
if diff amplifier amplification is low , then parameters do not match to your designed value. one is , layout according to your requirement and dipending on thechnology files params will vary.
(assuming that your circuit designed is according to your spec)