Fully differential op-amp-----Requesting for Urgent Help.

• Feb 12, 2007

• #1

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yonzzan

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I am designing fully op-amp with cadence and I was wondering what topollogy I have to use and how to do hand analysis to determine the aspect ratio and bias voltage. I will really appreciate if you can give me some help.
Thanks
The specification is below:
TSMC 0.18 micron technology
DC gain:60dB
Gain bandwidth product:500MHz
Phase Margin: 60
Output swing: 1v P-P
Output Load: 2pF
Slew rate: 20v/usec
fully differential..

 

• Feb 13, 2007

• #2

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nxing

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I would suggest you start with the classic two stage opamp, but I am little bit concern the Gain bandwidht product without the real try.
Follow Philips E. Allen's CMOS analog Circuit Design, that will help.

Good Luck

 

• Feb 13, 2007

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yonzzan

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is this the classic two stage op amp?

 

• Feb 13, 2007

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pfd001

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M3 ,M4, M5 and M6 are not included in classic two stage opamp

 

• Feb 13, 2007

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Btrend

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Re: Fully differential op-amp-----Requesting for Urgent Help

u can also refer to this
**broken link removed**

 

Last edited by a moderator: Jul 13, 2021

• Feb 13, 2007

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gingerjiang

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Re: Fully differential op-amp-----Requesting for Urgent Help

a classic two-stage operational amplifier is good startpoint
but from the GBW specification, maybe a single-stage amplifier is preferable
if the output swing isn't a limit, folded-cascode amplifier is a good choice
lucky

 

• Feb 13, 2007

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MSSN

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Re: Fully differential op-amp-----Requesting for Urgent Help

Do u want to design an op-amp or OTA ??, i.e. do u want a low output resistance, or it doesn't matter

 

• Feb 14, 2007

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yonzzan

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Re: Fully differential op-amp-----Requesting for Urgent Help

As long as satisfying with the given specification, there is no other conditions to consider.

 

• Feb 14, 2007

• #9

gingerjiang

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Re: Fully differential op-amp-----Requesting for Urgent Help

it has 2pF capacitance load and the specification 20v/us, so an OTA is ok.

 

• Feb 14, 2007

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zenisle

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fold casecode op is a good choose

 

• Feb 14, 2007

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MSSN

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Re: Fully differential op-amp-----Requesting for Urgent Help

As long as satisfying with the given specification, there is no other conditions to consider.

Click to expand...

So i think u can achieve these specs with a single stage(folded or telescopic) much easier than two stage ota and if u r gonna use sc cmfb it will be much easier to use a single stage ota

 

• Feb 15, 2007

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Re: Fully differential op-amp-----Requesting for Urgent Help

you can start with two stage opamp and then if the gain is not sufficient then you can add one more stage,while choosing the devices the input device sizes need to be bigger(w/l) ratio ,then little small,then you need to have bigger device(w/l) so that your rout will be more it will be in Megaohm.
Choose proper biasing current ,first stage 2uA,second stage 6uA...

 

• Feb 15, 2007

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vijay2912

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Re: Fully differential op-amp-----Requesting for Urgent Help

yes two stage opamp is more than enough for the specs

you'i better get the whole design in allen holberg

 

• Feb 15, 2007

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hunter694

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You should try with folded cascode first since:

1/ Your output swing is not too demanding (1Vpp) assuming 3.3 V power supply

2/ With a single stage amplifier you dont have to worry about stability as you have to do with two stage amplifier

3/ Gain-bandwidth is typically low for folded cascode but 500M should be achievable in 0.18u

 

• Feb 16, 2007

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yonzzan

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Can anyone explain how the second stage of this topollogy works? The first stage looks like a telescopic op amp but I am not sure about that as well. I have never seen this type of topollogy. I would also appreciate if you can let me know any text book covers this topology.

With much appreciation

Added after 50 seconds:

also, I was wondering how size of transistor effects speed of op-amp such as GBW.

 

• Feb 16, 2007

• #16

gingerjiang

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Re: Fully differential op-amp-----Requesting for Urgent Help

as you said, the first stage is a telescopic amplifier. i think the tail current of second stage is unnecassary. i don't know the use of the capacitor connecting the two outputs. this opamp gain is too much.

 

• Feb 28, 2007

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jiaming_lee

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Re: Fully differential op-amp-----Requesting for Urgent Help

gingerjiang said:

as you said, the first stage is a telescopic amplifier. i think the tail current of second stage is unnecassary. i don't know the use of the capacitor connecting the two outputs. this opamp gain is too much.

Click to expand...

The output capacitor is not the design itself, just the loading. connecting like this equals double of capacitance connected to ground at two outputs respectively.

 

• Feb 28, 2007

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aibrahim

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Re: Fully differential op-amp-----Requesting for Urgent Help

I think Razavi book will be useful check chapter 9 it's about operational amplifier design.

 

• Feb 28, 2007

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Alles Gute

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Re: Fully differential op-amp-----Requesting for Urgent Help

yonzzan said:

Can anyone explain how the second stage of this topollogy works? The first stage looks like a telescopic op amp but I am not sure about that as well. I have never seen this type of topollogy. I would also appreciate if you can let me know any text book covers this topology.

With much appreciation

Added after 50 seconds:

also, I was wondering how size of transistor effects speed of op-amp such as GBW.

Click to expand...

This OPAMP has two stages. The first stage is a classical Telescopic cascode and the second stage is a classical Differential pair. Then there is a classical miller compensation capacitance together with a nulling resistor for frequency compensation. The GBW is determined by gm/CL.

There is nothing special in it. I think you'd better firstly read some classical analog IC design text books before posting questions here.

Some nice literatures as a starting point:
Philip E.Allen, Douglas R.Holberg, “CMOS Analog Circuit Design”
Behzad Razavi “Design of Analog CMOS Integrated Circuits”

and the paper you can find in
PAUL R. GRAY, “Mos Operational Amplifier Design: A Tutorial Overview”IEEE J. Solid State Circuit, Vol. SC-17, No. 6, December 1982

 

Last edited by a moderator: Jul 13, 2021

• Mar 1, 2007

• #20

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wael_wael

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Re: Fully differential op-amp-----Requesting for Urgent Help

ok, i will till that i f you are going to design sample and hold 60db gain is not enough, so increase the gain, other thing that is so difficult to use telescopic OP AMP with 1.8v, i suggest that fully folded cascade with p channel as input, u can reduce the VDD until 1.5v or less depend on ur phase margin.
i have tried what i am saying no, so don't try just go ahead as i told you if u want some paper i can give u also
best regards