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Operational amplifier - strange characteristic?

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egomez

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Hello!
I have to analyse and if necessary optimise this circuit.
norm_oan1.jpg

Technology of the circuit is at0.35n

When I connect it as voltage follower and simulate with spice I get response like this:
norm_odziv.jpg


Why the output don't follow the input until 0.7V. Is this because of the treshold voltage of m1 and m2? Can I fix it in some way or only with changing substrate doping?

Parameters of this transistors are:
m1 w_6 inn w_1 vssa MODNm w=50U l=12U m=6 ad=55P as=55P pd=52.2U ps=52.2U nrs=8M
+ nrd=8M

m2 w_10 inp w_1 vssa MODNm w=50U l=12U m=6 ad=55P as=55P pd=52.2U ps=52.2U nrs=8M
+ nrd=8M

I know that here are people with a lot of experiance but I am not sure that I gave enough information. I will be glad for any comment and suggestion.

Thank you!
Erik
 

Davood Amerion

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Why the output don't follow the input until 0.7V. Is this because of the treshold voltage of m1 and m2?
yes it is.
It is not 'strange characteristic', it is normal.
your circuit isnt designed as rail-to-rail input characteristic.
Many op-amp desiged in this metod. you can search internet for rail-to -rail
(input) op-amp. for example get some datasheet of rail-to-rail op-amp schematic
and evaluate its circuits. :D

Regards
Davood
 

    egomez

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sunking

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yes,the result is right.
do you simulate the CMR of the opa?
it is no much useful to change the doping.
the most is rail to rail, as Davood Amerion said.
 

egomez

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Thank you, Davood Amerion and sunking.

I will look for rail to rail of op-amp's.
 

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