Biasing NMOS using current mirror

[salil87]

Hi

I need a brief explanation on how a Current Mirror is used to Bias an NMOS using Constant Current Model. My Doubts are :-
* Where the input is given ?
* If the current remains constant how does it work ?
* If the current does not remain constant then why say constant current ?
Bit Confused. Little help needed.

Thanks
Salil

 

[ducvilla]

Firstly, we have a current source to produce the reference current, then we using the current mirror to bias the Iref to the other stage. Why the current from the other stage can keep constant? to reply this question, you should read about: Id of mos, then bias voltage to understand more about current mirror. You can quote the source of "* If the current does not remain constant then why say constant current ?" because i think it's quite strange.
Regard.

 

[salil87]

Thanks for replying ducvilla. Actually I am in the nascent stage of MOSFETs so I am talking about single stage. What I am not getting is if the Id current is biased by a constant current source then how will it change with the input signal so that we get the proper amplified output? Well if it changes then how can we say the mirror is feeding constant current? Sorry if its a stupid question but would be great if all the fog in my mind is cleared.

Thanks
Salil

 

[ducvilla]

Hi salil87!
The transistor in your single stage is biased by what? Usually it is said that by current source, but i think it is biased by the gate voltage, then Vgs -Vt is constant, if you want to change the value of Id, you just change the value of ratio W/L, in this case Id is changed, but the current source current actually is constant. ( It can be changed by another way by changing the value of Iref, because if you change the Iref, the Vg of this transistor also will be changed).
Hope this help!
Regard.