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This is little arbitary. To decrease the spread of VBDG the voltage
VT*ln(N)
should be as high as possible. That is because of the voltage offset of an individual NPN or PNP is e.g. 500uV, the offset voltage of the loop expression defining the k*T current source is
Vo,loop=Vo*(1+sqrt(1/N))
So the spread of the k*T current source is
Vo*(1+sqrt(1/N))/(VT*ln(N))=12.5e-3 for N=8 and Vo=500uV
for N=48 the avove spread is 5.7e-3
So you can benefit a little bit if you increase N. Also the shot noise and the resistive noise from RB and RE improves with N but the expression and derivation is too long here.
So it seems to me that the choice N=8 is done in most cases w/o investigation area versus tolerance.
I am afraid that if n=24 then every line 5 bjt in it
so the first and the last one will be faraway from the center
can you take it sure that they will match to the mirror one?
Placing of the single diode is not as important than to place dummy diodes on the periphery of the array. These dummys present the same lateral diffusion profile for every diode. The dummy could be smaller or only an fragment. But it should be an allowed element. You can take a look in your designkit if your foundry have ever heard of that.
M:N=1:8 ratio fits best with the common centroid layout scheme and thus reduces different process varitions (like Vbe, Is, beta etc.). The config will be like following...
N N N
N M N
N N N
The centre of M and spreaded N is on same point.
Also other combinations can be used, but this one better.
For better Maching of the BJT,it's safe to keep 2,16,because if the two BJT's kept in centre and surrounded by 16 other one,from center of axis the process variation(delta) gets nullified.
It is not compalsary to use the ratio B:A as 1:8,U can go for 1:15 or 1:24 or 1:35....
but the smallest possible best matching combination in the layout point of view is the 1:8 The advantages in using this is the less area,best matching.the arrangement is :
A A A
A B A
A A A
Even u can go for the 1:24 or 1:35 there also u will get the common centroid but the area will be huge compared to the rest of the circuit.
u might get the DOUBT that B:A = 1:3 also the smallest possible common centroid arrangement,but it is not,since the arrangement u will get is:
A A
A B
In the above arrangement to provide the same environment for the all BJTS u need to place again 5 more dummies, which comes to the same arrangement as stated B:A=1:8.
So most of the BGR designs will contain the 1:8 ratioed BJTS
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