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Explain to me this OTA current mirror

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surianova

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hi all!

Just want to check with OTA in the attachement

transisitor M5 and M7 need to be ratio since it is a current mirror?

What is the effect if w/L size of M5 and M7 is totally different?

Thanks for everyone comment!
 

compensation current mirror ota

Its only problem in layout. you cant match the transistors with different W/L s in the layout properly. thats why you use fingers with same W/L's
 

Re: OTA current mirror

rajanarender_suram said:
Its only problem in layout. you cant match the transistors with different W/L s in the layout properly. thats why you use fingers with same W/L's
Click to expand...


ok, tx.. What will happend to the operation of the opamp if the layout of M5 and M7 not match? Is it very critcal issue?
 

OTA current mirror

Generally, M5 and M7 can have different sizes if this OTA isn't used in precise applications.
Because matching between M5,M7 and M3,M4,M6 will give you an advance. In this case you will aling potential at nodes 1 and 2 (when Vin.diff=0), so u decrease the systematic offset of OTA.
 

OTA current mirror

(M7/2M4)=M5/M6

Added after 1 minutes:

surianova said:
rajanarender_suram said:
Its only problem in layout. you cant match the transistors with different W/L s in the layout properly. thats why you use fingers with same W/L's
Click to expand...


ok, tx.. What will happend to the operation of the opamp if the layout of M5 and M7 not match? Is it very critcal issue?
Click to expand...

there would be a system offest

Added after 1 minutes:

make (m7/2m4)=m5/m6
 

OTA current mirror

M6/M4 = 2* (M5/M7)
This condition should be satified and this relation can be used in determining the w/l ratio.
This conditions is to cancel the systematic offset voltage.
You can refer Ken Martin 5th chapter on opamp compensation
 

OTA current mirror

M6/M4 = 2* (M5/M7) , please refer to Martin's book
 

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