[Moved]: Does small W/L ratio affect transistor matching ?

[zhangljz]

Hello,

I am designing a circuit that many current mirrors are needed but only several current references, which means one current reference will drive more than 100 mirrored transistors, but still I want to a proper matching. I am wondering if small W/L ratio of mirror transistors will affect the matching, such as W/L = 1 or 1.5.

Thank you.

 

[BigBoss]

Re: Does small W/L ratio affect transistor matching ?

Yes, it affect absolutely the matching.Larger devices are less sensitive to matching.Smallers are more..

 

[Dominik Przyborowski]

Re: Does small W/L ratio affect transistor matching ?

It depends to inversion region of your transistors. In deep strong inversion (inversion coefficient >1000) your matching is determined by current gain factor mismatch, while for depply weak inversion (IC < 0.1) mismatch is determined by threshold voltage coefficient normalized to thermal voltage.

 

[csolis_GT]

Re: Does small W/L ratio affect transistor matching ?

It will depend on the absolute values of the W and L. For instance W/L = 100/100 has a 100 times better matching than W/L = 1/1 even though they have the same ratio. If W and L is considerably high, having a smaller W/L helps to increase the overdrive voltage and improve the dID/ID error because it decreases the gm of the current mirror transistors.

 

[crutschow]

Re: Does small W/L ratio affect transistor matching ?

There is an absolute tolerance on the etching accuracy of the W and L so the larger the W and L (not just W/L), the less this tolerance will affect the matching.

 

[zhangljz]

Re: Does small W/L ratio affect transistor matching ?

Thank you all. I see the points. Maybe I can do monte carlo simulation to see the difference.

 

[nitishn5]

Look at the paper by Pelgrom "Matching Properties of MOS Transistors" It will give some insight on Area-Matching trade-offs.