Today as i was installing decoupling caps on a circuit board i thought
to
myself: "Why dont the IC manufactures just incorperate a decoupling cap
right in the ic itself?" Any one have any ideas?
*********************

Andy writes:

  The largest size we can get on a die is only a few pf. And
it takes up as much space as many thousands of transistors.
Furthermore, it is not a very good cap, since there is
significant resistance in the internal track which, along
with the dielectic, limits the "Q". Additionally , any caps
that are used for coupling (RF ckts) internally also have
a stray coupling factor to substrate of about a fifth of their
value (it varies some). Lastly, the substrate for a silicon
IC is not conductive so you have to use interconnect as the
ground leads as well as the hot lead inside the device and it
can lead to undesireable intercoupling.
   Whew!!
  Actually, frequency compensated op amps do use internal
capacitors, but they are very small..... and can only be
used in special areas.

  In summary -- the technology for making acceptable capacitors
is not the same technology for making acceptable transistors...

         Andy in Dallas  (too wordy for a Sunday morning...)

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