Why vgs as large value for current mirror matching,small value for voltage matching?

[jarillak]

Hi.....

Can anyone explain these statements in detail?
In MOSFET device matching...

i)Why we keep vgs as large value for current mirror matching?
ii)Why we keep vgs as small value for voltage matching?

If any materials regarding this,pls let me know.

 

[hamletboy]

Hi ..
In current mirror circuit we should follow the vgs>vth . So we were keep a maximum value for vgs. But I do not know about voltage matching .
Can you explain about Voltage matching its more helpful for me

 

[AMSVJ]

Hi Jarilak,

May I Know, You are asking about layout or schematic?

Regards,
Vijay

 

[csolis_GT]

If you have a fixed area, you will have a particular mismatch on threshold voltages: dVth

i)Why we keep vgs as large value for current mirror matching?

In current mirror you want to have a large vgs to have a small gm as possible so the voltage mismatch dVth does not change the current that much (is like having a low gain transistor)

ii)Why we keep vgs as small value for voltage matching?

I am assuming here you are referencing to differential pairs. The dVth mismatch will always be present, no matter where you bias the transistors, however, having the largest gm possible by minimizing vgs, will lower the input referred threshold mismatches of the other transistors in the circuit.