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James Bond Community Member Offline Posts: 82	large input swing for fully differential opamp Oct 15^th, 2007, 11:05am Hi, all I need to design a fully differential amplifier to handle the large input swing up to 1Vpp differential signal, my supply is 1.2V and ground. Can anybody give me some suggestion about this ? such as how to design the input stage. Thank you very much. james
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carlgrace Senior Member Offline Posts: 231 Berkeley, CA	Re: large input swing for fully differential opamp Reply #1 - Oct 17^th, 2007, 10:31am You probably want to use complementary input structure, that is a PMOS diff pair in parallel with an NMOS diff pair. There are a lot of circuits in the JSSC that use this approach. Carl
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ACWWong

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Posts: 539
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Re: large input swing for fully differential opamp
Reply #2 - Oct 17^th, 2007, 11:12am

A simple differential pair or either N or P type could suffice given a suitably well defined and limited input common-mode level. Then it might not be necessary to implement a complementary input.

But in the case N and P are required, this paper represents a fair survey of toplogies for rail-to-rail input stages.

http://amesp02.tamu.edu/~sanchez/lvtutorial-2000.pdf
section 4.2/table 3

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vivkr

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Re: large input swing for fully differential opamp
Reply #3 - Oct 18^th, 2007, 2:39am

HI James,

What exactly are you trying to do with your opamp? Also, when you say 1Vpp, does it mean that (Vip-Vim) varies from -0.5V to +0.5V,
or do you mean that it varies from -1.0V to +1.0V? As per your statement, I would imagine it to be the former, in which case each signal swings
about +/- 0.25 V around a fixed input common-mode. This can be managed without complementary structures and it is probably unwise to use
complementary structures unless there is no way out.

Secondly, is this signal swing only at the input of the overall feedback amplifier or really at the inputs of the opamp? If the former, then you have some
sort of resistive or other element interposing between the inputs of the overall amp, and the inputs of the opamp.

Regards
Vivek

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James Bond

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Re: large input swing for fully differential opamp
Reply #4 - Oct 19^th, 2007, 1:14am

Hi,

Thanks for all the suggestions. I have built the opamp with only pmos input pair. the input signal is -0.5V to 0.5V input signal , supply is 1.2V. The thd is less than 0.1%. At some time the input pair goes to the weak inversion, but it still can handle the input signal quite well, I also got some suggestions about adding a complementary pair. it's a good manual. Thank you so much.

Another question:

Does anybody how to build a model for the fully differential op with ideal component or veriloga code ? I want to evaluate the performace for it, e.g bandwidth, gain affect. Please instruct me again.

BR

Best Regards
james

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