how will you place 10 pnp transistors?

[holddreams]

In Razavi's book, Figure 11.20 -----Generation of a temperature-independent voltage.
The configuration has Q1,Q2,Q3 pnp transistors.

As described in bandgap circuit design,the two pnp transistors ratio usually equal 8:1,so when doing layout,we can place the pnp transistors like the following,

A A A
A B A
A A A

But in Figure 11.20, if Q1:Q2:Q3=1:8:1,that is total 10 pnp transistors,how will you place these 10 pnp transistors?

Thanks.

 

[Syukri]

I can quite understand what do u want clearly...

How to draw it or how to place the transistor in symmetry...

How a bout
A A
AABAACAA
A A

This is only a bad idea eh... he he

 

[holddreams]

How about this?

AABAA
AACAA

 

[sunking]

it is area ratio.
so change it to 2:16:2
AAAABCAAAA
AAAACBAAAA

 

[holddreams]

it is area ratio.
so change it to 2:16:2
AAAABCAAAA
AAAACBAAAA

But if change the area ratio to 2:16:2,the layout area will increase a lot.

 

[sunking]

Dont worry about it.
Use the min size pnp.
it only a little increase for the whole chip which this circuit is needed.
you can calculate it.

 

[holddreams]

for a bandgap circuit, the pnp will cost the most area.
so if double, the area will increase a lot.

 

[YESH_23]

ya u r correct.the way u told i.e

aabaa
aacaa

is the correct way which suits u

 

[vijay.kumarreddy]

The best way is to take 2 dummy pnps and u can plase in 3*4 or 4*3 array.There is one more best way is take 6 dymmys and place 4*4 u can get beat matching and can minimize the stress and temperature effects to aa greater exteny.

 

[mahendra]

The main points when BJTs matched:

1.Aspect ratio
2.Gradient cancellation


Gradient cancellation my be done with normal centroid based arrangement.But still
u have options.

1.AABAA
AACAA


The above configuration saves the area BUT by increasing mismatch.

ASPECT RATIO: 5:2 =2.5:1

Ideal aspect ratio is 1:1.DON'T PROMOTE MORE THAN 1:2.

2. Du A A B
A A A A
C A A Du Du=Dummy

The above configuration increases area by reducing the Mismatch greatly!!!
Aspect ratio:4:3=1.33:1

With smaller contributiontion to the mismatch from aspect ratio part.

3. The same pattern can be implemented to reach the Ideal aspect ratio

1:1.BUT by increasing area parasitics&noise.Having 4:4 for better match is not optimal solution.

Regards
Mahendra

 

[dervisakyuz]

AAAABCAAAA
AAAACBAAAA