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About 2 stage op amp in 0.35um CMOS process

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tia_design

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2 stage cmos op amp

I'm using 0.35 um CMOS process and designed a typical 2-stage folded-cascode opamp with PMOS diff pair. The second stage is common source one. Vth for NMOS is 0.6V and PMOS is -0.74V. Bias current for input pare is 3uA for M1 and M0 each with size150/1, while output stage bias current is about 11uA and PMOS size is 38.4/0.7 and NMOS size is 19.2/0.7. The first stage gain is 55dB, and second stage gain is 33dB. My concern is that:

1. Why second stage gain is so big. Normally it is about 20-30dB, right?
2. the length of output MOSFET M16 and M17 is 0.7uM, short channel still show up, right? If I choose length as 1uM for M16 and M17, the second stage gain will be 38dB and total op amp gain will be >90dB for those mosfet model.

Thanks for any comments about my question and my design
 

rajanarender_suram

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opamp second stage sizing

tia_design said:
1. Why second stage gain is so big. Normally it is about 20-30dB, right?

The Second stage gain depends on the output driving capability, if this capability is low ur second stage gain will be higher and vice versa

tia_design said:
2. the length of output MOSFET M16 and M17 is 0.7uM, short channel still show up, right? If I choose length as 1uM for M16 and M17, the second stage gain will be 38dB and total op amp gain will be >90dB for those mosfet model.

Thanks for any comments about my question and my design

yes the short channel effects show up.. but u get higher gain right..what is ur problem in this???
 

yxo

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two stage cmos op amp

Moreover, the W/L of the transistors in a second stage is chosen taking into account output load. It is not right to go up length of M16 M17. If you do it you'll decrease load capability and probably will get phase degradation. You may use cascode transostors in you folded cascode or use gain boost technic.
 

yschuang

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two stage op amp

Hello, yxo

I think the length of the second stage depends on what you want. If you use the first stage as an error amplifier to drive a pmos like you design a LDO, it is better to use smaller size. For conventional two stage amplifer design, I think it is better to keep the length of M16,M6,M7 the same to reduce offset, which is said in many textbooks.
 

yxo

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0.35um cmos

yschuang said:
Hello, yxo

I think the length of the second stage depends on what you want. If you use the first stage as an error amplifier to drive a pmos like you design a LDO, it is better to use smaller size. For conventional two stage amplifer design, I think it is better to keep the length of M16,M6,M7 the same to reduce offset, which is said in many textbooks.

:) Offset voltage of an amplifier (Opamp, OTA and so on) DO NOT depend on size of transistor M16. Offset voltage only depend on sqrt (W x L) of the input transistors.
 

rajanarender_suram

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design of a 0.35um cmos op-amp

Yxo, it is right that having different lengths for M16 & M6,M7 will arise in having voltage offset : by having differrent lengths there will be current mismatches between M6(M7) & M16 which show-up as voltage offset when we use Opamp in Feed-back mode(which we generally do).
 

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