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Design >> Analog Design >> Question on current mirror implementation
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Message started by kidman on Jan 31^st, 2009, 3:52pm

Title: Question on current mirror implementation
Post by kidman on Jan 31^st, 2009, 3:52pm

A current mirror that consists of only 2 transistors won't copy the exact reference current due to channel length modulation. The solution to that is cascoding as well know to shield the mirror from VDS variation and copy the exact reference current.

My question is, is the normal current mirror that's not cascoded used in any practical design? I'm asking this because I'm wondering how does a designer get his circuit working after the PVT variations and so on using the basic current mirror that even without PVT variations will not copy the current accurately.

Title: Re: Question on current mirror implementation
Post by loose-electron on Feb 1^st, 2009, 12:28pm

They are used all the time. Many applications for a current source do not require either good absolute value, or good PVT stability. Example - the current source under a differential pair can usually function properly with a lot of variance (2X over PVT without problems)

If you are looking for tight tolerance on any circuit, it is going to require adjustment circuits or the architecture needs to be reviewed to get rid of tolerance needs.

Title: Re: Question on current mirror implementation
Post by ywguo on Feb 1^st, 2009, 7:51pm

Kidman,

The simple current mirror are often used. That depends on the performance requrements.

Yawei

Title: Re: Question on current mirror implementation
Post by raja.cedt on Feb 2^nd, 2009, 3:07am

hi kidman,
Based on the application type mirror being used is decided,for example in differential amplifiers if the i/p variations are not much then simple mirror is the first choise,but if u want really high CMRR then u have to go for cascode.again cascode consume a lot of headroom for voltege,in that case high swing mirror is the best choice.This information u can get from razaavi.

Thank you.

Title: Re: Question on current mirror implementation
Post by kidman on Feb 5^th, 2009, 5:08am

thanks for your i/p guys