# Current Mirror OTA gain calculation

1. ## Current Mirror OTA gain calculation

Dear all,

I designed a current mirror. Some papers such as http://www.imec.be/esscirc/essderc-e...ons/063_50.pdf said the DC gain is

A0 = gm1 x B x ro3.

I computed it with mine, but, in the simulation, the B factor doesn't seems to have any impact on the DC gain, only on the GBW. Not sure if there is a reason, my inputs transistors are working in subthreshold region.

Any Idea, suggestions?

Thank you...

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2. ## Re: Current Mirror OTA gain calculation

Originally Posted by Fabien
A0 = gm1 x B x ro3.

... the B factor doesn't seems to have any impact on the DC gain, only on the GBW.

Any Idea, suggestions?
The DC gain ~ B * ro3

Now read the 1st paragraph of point 2.3. of your a.m. paper!

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3. ## Re: Current Mirror OTA gain calculation

OK, thank you for you answer. So the B factor doesn't have any impact as the ro will change with B...
But, if the gain is boosted B times and the output resistor is reduced B times, it means the DC gain is gm1 x ro3, doen't it?
If yes, the gm1 is 12.8µS (qID/nkT) and ro3 = 5.5MOhms... it gives only 70 so 37dB...

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4. ## Re: Current Mirror OTA gain calculation

Originally Posted by Fabien
But, if the gain is boosted B times and the output resistor is reduced B times, it means the DC gain is gm1 x ro3, doen't it?
Fabien: No! DC gain is gm1 x B x ro3 , as you stated yourself above!

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5. ## Re: Current Mirror OTA gain calculation

OK, so I probably made a mistake when I calculate ro3...
For gm1, in subthreshold I've only this equation: gm1 = qId/nkT (from Martin's book)

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6. ## Re: Current Mirror OTA gain calculation

Originally Posted by Fabien
OK, so I probably made a mistake when I calculate ro3...
For gm1, in subthreshold I've only this equation: gm1 = qId/nkT (from Martin's book)
This is quite ok, I think. The gain enhancement doesn't depend on B , but is based on the (1−k)*ID1 current shunting, see and study equns. (5) & (6) of item 2.3. of your a.m. paper thoroughly!

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