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Mayank

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Re: need help choosing op-amp config
Reply #15 - Nov 12^th, 2009, 7:58am

Sorry forgot the other part of your post.....

As for the Models,
Your point is totally correct....Models for new processes are nascent and show reasonable difference in CAD vs. SILICON comparisons.....Models mature as the Process matures...Can't do anything about it....Whenever i switch foundries, i play defensive.....Ensure reasonable margins everywhere (i try & account for 5/6 sigma variation)

BIPOLAR is cool, though I have a certain inherent apprehension of it (:-P maybe coz i most of my exp is on MOS).....
BUT You can't deny the fact ----> When God did his masters in Analog/RF, he designed BJT & Si-Gae as a part for his Thesis.... Tongue

CMOS is the plebian version of BJT,designed for digital, but sadly what most of analog guys also have to work upon....coz it's cost-effective....That's what we all want in the end...

--Mayank.

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sos Community Member Offline Posts: 32	Re: need help choosing op-amp config Reply #16 - Nov 12^th, 2009, 9:06am Thanks for the explanation Mayank, you make the gm/id approach sound pretty reasonable. It sounds like perhaps more work to set up, with corresponding more insight into circuit operation when done. Steve
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carporsche

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Re: need help choosing op-amp config
Reply #17 - Nov 16^th, 2009, 11:08pm

@Mayank

I have implemented the gm/id method for the folded cascode topology.
Got all the data in MATLAB and plotted the following curves.

id/W vs gm/id
id vs (2*id/gm)
gm/gds vs gm/id
ft vs gm/id

I seem to get the design working for values of unity gain frequencies around 100MHz.
But for higher values of like the one am designing for (500MHz) the gm/id values are way too high and not obtainable from id/W vs gm/id graphs.

I dont know whether am missing any design approach here. Am sometimes confused as to how to use the graphs generated methodically!

Can someone help please!

thanks

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Mayank

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Re: need help choosing op-amp config
Reply #18 - Nov 17^th, 2009, 1:55am

Hello Caporsche,
i have designed 2-stage OTA's with UGB in the range of 400 MHz or so......I didnt understand a few points in your post :--

1. What is the significance of id vs. (2*id/gm) Plot ??

2. I guess you did not take into account the cgd & cdb values....
In your plots, also use cdb/cgg & cgd/cgg plots....Then using the value of cgg found from fT/gm plot, you can find cdb & cgd....
Incorporate Cgd & cdb caps also into your calculations when you are working in 500 MHz range or above.....
Use MATLAB to handle the complex expressions formed. You can also optimize the ckt using these plot values in any mathematical tool.

3. If you are sweeping vgs & vds completely uptil vdd, the values of gm/id in these plots are the maximum that you can attain in any circuit....I guess you probably meant Higher Gm values for higher UGB requirements... ...Once Gm/Id is sufficiently high, Increasing current is the only option we hv got

4. In some technologies, intrinsic gain of the transistor gm/gds is quite small....Increasing current increases Transconductance, but decreases gain at the same time...You might have to increase the length to keep gm/gds high....

5. GBW can be achieved easily by increasing the current...But If you are still not able to achieve the Gain requirements even after increasing lengths to acceptable values, Revisit your topology

Hope that answers you....

--Mayank

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carporsche

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Re: need help choosing op-amp config
Reply #19 - Feb 17^th, 2010, 1:32pm

Hi Mayank

Have been looking into this gm/id method for quite sometime now. I still need some clarifications from you.
You did mention tht u have designed rz-cc 2 stage op-amp and thats exactly what am designing. Later on i want to design a Fc OTA too.
Some points
1. Assume currents in various branches
2. I break my OTA into basic analog structures,
Diff pair = supplying gm / gain
Load struct = operating in strong inversion (here i use the vurve gm/id vs vdsat and find the corresponding gm/id ratio and proceed further)
bias structures = depending on the current to be supplied etc etc
3. Suppose i need a buffer after my OTA. how do i design tht?
4, Also could you give me an insight into choosing values for rz(transistor implementation in triode) and CC values using gm/id graphs.

Do you follow the textbk equations and design ?
Would be really helpful if you could provide me some hints which you have picked up while designing high bandwidth amplifiers.

One more point , i always proceed with designing for bandwidth.
Still havent got a hang while designing for gain. Could you please point out the things while designing for gain too.

Have put in too many questions, kindly excuse that :). But i do feel the gm/id method is really helpful.

thanks again for your time.

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