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How to size the transistors in an opamp

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Dec 21, 2005
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For the folded cascode opamp in the pic, How do you size the transistor M1 to M4.
It was used in high speed circuits. The minimal L is 0.35u, then can I size the L of M1 0.35u? I see some circuits use bigger size such as 0.5u for M1, 0.35u for M2. I want to know how you size the transistors.


To avoid the effect of channel length modulation on the mosfet performance mostly L ia not taken minimum.
Again instead of taking large W (increases Capacitance) it is always better to select possible low value of L.

1) The noise and offset contributions of the rest of the opamp is attenuated by the gm(diff pair)/gm(M1) ratio. So, you wnat the diff pair gm large and M1 small.

2) You need good matching between M1 and its pair for good copy of the currents. So, no minimum L. If you look into the technology docs, sometimes matching is not even modeled for min L. They have too large mismatch. M4 is also copying the current from a reference (not shown), so you also need good matching there.

3) M2 & M3, you don't care. They are just cascodes, so, you can put them with min L.

Good luck


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Larger L of M1-M4 can increase output resistor and larger M1-M4 W/L increase the output swing.Hower ,at the cost of noise attenuation.

W*L (M1) is important in flicker noise contribution.

Added to okguy
also M4 device will contribute lots of noise!

Please refer to the attached file ~

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