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How to determine transistor size of folded cascode ota?

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rose_song

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How to determine transistor size of folded cascode ota(gain=80db,fu=30Mhz)?
Thanks!
Rose
 

For a cascode, the gain is approximately
= gm*ro^2 = gm * (1/(Lemda*Id))^ So if you keep the bias Id unchanged while increase the width you can boost your gain. However, large transistor increase the node capacitance hence limit the bandwidth. Run the simulation to dertermin the trade off.

Be aware not to simply increase the bias Id as gm=sqrt(2*Id*Beta), increase Id will LOWER your gain!
 

hi,
If the ID and gm are fixed, how to determine transistor size and bias voltage of cascade output stage.
thanks!
Rose
 

If the bias current is constant, then the swing range will be ratio to w/l, the max output voltage= vdd - 2*Vds, the min output voltage=2*Vds
 

1. if gm and Id are known then u can compute W/L for ur process using following eqn
gm=(2BId)^0.5
where B=Beta=Un Cox W/L

2. If u know swings pecs in ur design u can decide supplies, although it depends on ur technology. u cant choose arbitrary value
 

"Design of analog cmos integrated circuits" by Razavi has a good example to determine transistor size of folded cascode ota, perhaps it can give u some guide.
 

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