NMOS Folded Cascode Differential Differernce Amplifier: Improve Unity Gain Frequency

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deveshkm

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Hi,

I am designing NMOS input folded cascode DDA.
Specification:
Gain : 60dB
UGB: 1MHz
Capacitive Load: 10pF
Input and Output Common Mode Level = 900mV

Current through folding transistors (PMOS) M50 and M59 : 30uA
Tail Current through M80-81 and M82-83 : 20uA

********************
Obtained Results:
Gain : 55dB
UGB : 1.59kHz



In order to bias it, I had chosen following W/L ratios:
PMOS Folding Transistors (M50 and M59) = 14.5u/1u

PMOS (M47 and M48) = 9u/1u


I observed that parasitic capacitances of these 4 transistors are of the order of nanoFarad ( 1E-09 F)

How can I improve the UGB?
 

deveshkm said:
I observed that parasitic capacitances of these 4 transistors are of the order of nanoFarad ( 1E-09 F)
How can I improve the UGB?
Click to expand...

This is impossible.Double-check your circuit..
 

I am biasing it again. While choosing the W/L ratios, I am also observing the parameter ,Cdd for various transistors, esp M50 and M59.
 

Only a discrete power MOSFET ought to exhibit nF-range
capacitances. Start with checking the scaling of your L
and W (i.e. is the value supposed to be microns or meters -
is a 9um/1um device supposed to have W=9, L=1 or are
these property values supposed to be W=9u or W=9E-6,
L=1u or L=1E-6?

Seen PDKs go either way, especially older flows where
nobody thought about going under a micron originally.

You could probably dope this out by placing a layout
PCell with the same properties and look at the size.
 

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