question about high-swing bias

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twlin1

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It is a slide of lecture note.

There is one question that why the lengths of current source and cascode need equal.

Could someone answer the question? Thanks!
 

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The only reason I can think of :

the output impedance of the current mirror is rdscascode//rdsmirror.

These impedances are largely dependent of the length. (the higher the length, the bigger the rds.)

So the most optimal choice is to have the lengths equal to have the impedances +- equal.

Greetz, J
 

Hi,
The note Lcurrent-source=Lcascode maybe insisting on keeping the L(M4)=L(M2). Typically, the cascode transistors are sized bigger i.e W/L(cascode)> W/L(current-source). General practice is to keep L(current-source) >L(cascode).

regards
 

There is another question about device sizing of the other high-swing bias circuit.
What does the objective "VDS1=R*Vd1,sat" mean ?
And why Vd6,sat equates Vd1,sat ?


Thanks in advance!
 

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