Size of inverters

Question

Explain the phenomenon of gradually increasing the size of the inverters in the buffer design while trying to run high capacitive load? Why not provide the output of circuit into one large inverter?

Answer 1

We could not use the big inverter in order to run the large output capacitance as, who will run the big inverter? The signal which has to drive output cap will now see the larger gate capacitance of BIG inverter. So it will results in slow raise or fall times .A unit inverter may run approximately an inverter that is 4 times larger in size. Let say we require driving the cap of 64 unit inverter, and then we try to maintain the sizing such as say 1, 4, 16, 64 so that every inverter sees the same ratio of output to the input cap. This is the chief reason behind going for the progressive sizing.