bipolar analog IC design ?

[Svilen]

Hi,


Up to now I have
designed analog circuits only in CMOS. Now I have
to
do some stuff in BiCMOS and I'm just trying to make
the link between bipolar and CMOS approach to
design.
My question is basically how to size the bipolar
transistors? Generally speaking one sizes a MOS
transistor based on the current that flows through,
choosing the appropriate W/L. If gain and high
output
resistance is needed than choose bigger L; if more
speed is required then L is shorter. But how to
size
the BJTs - it is the area of the emitter that
matters
there. I have looked in the books but didn't find
any
guidance about sizing the transistors. Everything
there is based on the exponential relationship
between
Vbe and Ic and the area is represented by Is but it
is
a technological parameter and I'm not sure it can
be
used for sizing. For example if I want to have a
current of certain value going through a bjt, what
should be the size of this bjt or what is the
criteria for choosing the size?

Sorry if I annoyed you with such a stupid remarks,
but
I guess I need just a little bit of help.

 

[Jim Thompson]

Hi,


Up to now I have
designed analog circuits only in CMOS. Now I have
to
do some stuff in BiCMOS and I'm just trying to make
the link between bipolar and CMOS approach to
design.
My question is basically how to size the bipolar
transistors? Generally speaking one sizes a MOS
transistor based on the current that flows through,
choosing the appropriate W/L. If gain and high
output
resistance is needed than choose bigger L; if more
speed is required then L is shorter. But how to
size
the BJTs - it is the area of the emitter that
matters
there. I have looked in the books but didn't find
any
guidance about sizing the transistors. Everything
there is based on the exponential relationship
between
Vbe and Ic and the area is represented by Is but it

"IS" is *not area*... it is "transport saturation current", a device
parameter that you cannot change. Use parameter "Area" to re-size
devices, *if* that's allowed by your process. Some processes have a
"library" of devices that you must pick from, or use multiple
*instances* of devices.

is
a technological parameter and I'm not sure it can
be
used for sizing. For example if I want to have a
current of certain value going through a bjt, what
should be the size of this bjt or what is the
criteria for choosing the size?

Sorry if I annoyed you with such a stupid remarks,
but
I guess I need just a little bit of help.

I *generally* size devices so that they are operating at the peak of
their beta curve.

Do this simulation of a bipolar transistor:

Hold the collector voltage at a constant appropriate for the device.

Do a .DC analysis varying a current source into the base.

Plot IC/IB (Y-axis) of the device with X-axis chosen as IC.

You will observe that beta is low at low and high currents, but peaks
in the middle... use this "sweet spot".

[And learn how to set up your newsgroup reader to improve the
word-wrap of your posts :-]

...Jim Thompson

 

[Svilen]

Thanks to both of you for your replies. I think I've got the idea and
now I have a guiding point to start from.

Svilen