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analogrf

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OpAmp GainBW estimation
Dec 18^th, 2009, 8:16am

Hi everyone,

I am designing a low voltage opamp. The target specs only state the f3db, but not ft (GB product), of my opamp. I am in need of GB before i calculate my aspect ratio of input transistor. (hence i dont know gm1 either).

Now assuming a two pole transfer function, after f3db = 500Mhz, can i esitmate at 12db/octave rolloff to guess my correct unity gain frequency. So incase my DC gain was 60 db, my gainBW should be around 500 x 5 = 2500Mhz at around the 0db gain ?.

Could any of you verify the above method for estimation of my gainBW?

Regards

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aamar

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Re: OpAmp GainBW estimation
Reply #1 - Dec 18^th, 2009, 8:32am

Hallo,

What you said is Ok, but it is easier to multiply your Gain and 3dB BW, this means in your case 500M*1000. On the other hand, the unity gain point or ft depends also on the position of your non-dominant pole. Ideally if the non-dominant pole is at very high frequency then your ft converges to be equal to the GBWP.

Best regards,

aamar

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analogrf Junior Member Offline Posts: 16	Re: OpAmp GainBW estimation Reply #2 - Dec 18^th, 2009, 8:52am OK, thanks a lot for clearing up my confusion! For the higher frequency pole i tend to push them beyond my GBWP, to get a decent PM.
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buddypoor

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Re: OpAmp GainBW estimation
Reply #3 - Dec 18^th, 2009, 10:51am

aamar wrote on Dec 18^th, 2009, 8:32am:

Hallo,

What you said is Ok, .....

aamar

Sorry, but I am afraid, it is not OK.
1.) I suppose you don�t want a slope of -12dB/Okt. . Instead it should be only -6dB/Okt (equivalent to -20 dB/Dec).
2.) Therefore, 60 dB is equivalent to 3 decades with -20 dB each.
That means: Unity gain frequency is three decades above 500MHz.

That's theory. Whether you can realize 500 GHz is another question.

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LvW (buddypoor: In memory of the great late Buddy Rich)

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Mayank

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Re: OpAmp GainBW estimation
Reply #4 - Dec 19^th, 2009, 12:30am

Hi,

@ buddypoor :
Quote:

1.) I suppose you don�t want a slope of -12dB/Okt. . Instead it should be only -6dB/Okt (equivalent to -20 dB/Dec).

I agree. Maybe analogrf was trying to convey this.

Quote:

2.) Therefore, 60 dB is equivalent to 3 decades with -20 dB each.
That means: Unity gain frequency is three decades above 500MHz

I slightly Disagree on this point. This point seems to depend on where you start counting your decades from. 500MHz is the -3dB freq. where gain has already fallen down to 57dB.

Acc to how i would calculate the UGB, GBW product is constant. Gain at -3dB freq = A_DC / sqrt(2) . So UGB = 500MHz * 1000/sqrt(2).
Now this 353.xyz GHz UGB is not realizable.

Coming back to AnalogRF ,
Pls check your specs once again. 3dB Freq is mostly quoted for Filters. You Hardly quote -3dB freq. for an opamp / error amplifier as a spec. 3dB freq is mostly quoted for A CLOSED LOOP which is UGB in OPEN-LOOP. I am highly suspicious that 500MHz you are tryin to design is a CLOSED LOOP 3dB BW of some Loop, which will require you to design an opamp with 500MHz UGB.

Disclaimer :-- The above assumption is based on my CMOS exp. If you are talking about Si-Ge, that's a different matter altogether.

--
Mayank.

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buddypoor

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Re: OpAmp GainBW estimation
Reply #5 - Dec 19^th, 2009, 2:02am

Mayank wrote on Dec 19^th, 2009, 12:30am:

3dB freq is mostly quoted for A CLOSED LOOP which is UGB in OPEN-LOOP. I am highly suspicious that 500MHz you are tryin to design is a CLOSED LOOP 3dB BW of some Loop, which will require you to design an opamp with 500MHz UGB.

I slightly disagree with the above statements.
Open loop UGB is only (approximately) identical to the closed loop 3-dB-BW in case of 100% feedback (closed loop gain 0 dB)!!

Added somewhat later:
Hi MAYANK,

I also slightly disagree with another point of your posting: The principle of calculation of the UGB.
Assuming a first order gain roll-off the transit frequency will be exactly three decades above the 3dB frequency!
In your calculation you have forgotten that the slope of the gain function in the vicinity of the 3dB point will be lower than -20 dB/dec.

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LvW (buddypoor: In memory of the great late Buddy Rich)

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analogrf

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OpAmp pole estimation
Reply #6 - Dec 19^th, 2009, 4:52am

Thanks mayan and buddypoor for your inputs

I realize that f3db spec. is perhaps at close loop, but i need to reconfirm it.
If above turns out to be ture, then since Wu = A x Wp1, my first pole drops at 500K.
Now should this pole that actually comes from my first parasitic cap Cx combined with C-comp, 'be set' or mapped at 500K by the altering the aspect ratios ??

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