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[SOLVED] Basic question about the layout (common mode centroid configurations) - current mirror?

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palmeiras

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Basic question about layout (commom mode centroid configuration) - current mirror?

Hi guys,

I'm designing the layout of a current mirror (Q1 and Q2), where Q1 has multiplicity = 1, and Q2 has multiplicity = 8. I put them in a common mode centroid configuration. The current mirror is part of a Bandgap voltage reference circuit.
What is the problem of doing the connections between the gate of transistors done by poly, as shown in the attached figure (poly is written in red color)?

Is there any problem of doing these connections by poly instead of M1?

Thanks,
 

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Re: Basic question about layout (commom mode centroid configuration) - current mirror

What you have will be fine. You need to ask yourself if dummy devices are need. If so, then you will need a 5x5 array. A smaller alternative is to lay them out vertically in a row, connecting the gate in metal. You would only need dummies on the ends. I have also done them horizontally in a row connecting the gates in poly.

Rob
 
Re: Basic question about layout (commom mode centroid configuration) - current mirror

Thanks, Rob.

thus, Is the only drawback of my layout the bigger area?

What about layout matching? Will it be ok?
 

Re: Basic question about layout (commom mode centroid configuration) - current mirror

Matching should be fine with what you have drawn. However, you are the one that has to determine the level of matching you need.
 

Re: Basic question about layout (commom mode centroid configuration) - current mirror

My question is: Does this configuration have worse matching than that you suggested? If yes, why?

I need to achieve the best possible matching.
 

Re: Basic question about layout (commom mode centroid configuration) - current mirror

palmeiras said:
Does this configuration have worse matching than that you suggested? If yes, why?
Click to expand...

No, for a current mirror it's really sufficiently well. Note that in most cases there will be current deviations anyway due to different Vds voltages of master & slave transistor - if not equalized by an additional control loop.
 
Re: Basic question about layout (commom mode centroid configuration) - current mirror

Try to avoid poly connection if you can
 

Re: Basic question about layout (commom mode centroid configuration) - current mirror

RS83 said:
Try to avoid poly connection if you can
Click to expand...

why? in digital circuits, ok (there is a speed issue). How about this case? I´m worry about matching.
 

Re: Basic question about layout (commom mode centroid configuration) - current mirror

palmeiras said:
How about this case? I´m worry about matching.
Click to expand...

I think for your current mirror it's fine. Poly width = gate length is quite practical and achieves a compact design. Anyway the mirror probably isn't thought for high frequency, perhaps even for DC, I guess.
 
Re: Basic question about layout (commom mode centroid configuration) - current mirror

palmeiras said:
why? in digital circuits, ok (there is a speed issue). How about this case? I´m worry about matching.
Click to expand...

I am pretty sure to have read it in "the art of analog layout" but I don t want to give you a bad answer so I m looking for it. If you can take a look at it (free pdf on internet) you have others things to check
 

Re: Basic question about layout (commom mode centroid configuration) - current mirror

In general there is no issue with gates connection via poly.
However, polysilicon has ≈ 20Ω/sq sheet resistance and could contribute to the noise (usually in extreme cases).

The real issue is bounded with technology. If used process has relatively high gate leakage, poly connection resistance provides voltage drop and lower effective gate voltage for devices placed farer than biasing diode. In addition, some of the processes provides DFM rules forbidding gate connection only by poly.
 
Re: Basic question about layout (commom mode centroid configuration) - current mirror

Thanks guys for all replies.

I guess it was possible to achieve a more compact layout doing in this way, and then, a better layout matching
I think I'm going to layout my next DC circuits in this way!!
 

Re: Basic question about layout (commom mode centroid configuration) - current mirror

The best way for doing this is to place vertically which is to place and route and you can add dummy both the sides.
 

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