I have been studying and researching this for a while now and I have come across this a couple or so days ago. I have come across the formula for input capacitance for the common emitter amplifier. Xc< or=0.1×Zin or Rin. The formula is: C=1/ 2π×f(low)×(0.1×Zin).

 

I have been studying and researching this for a while now and I have come across this a couple or so days ago. I have come across the formula for input capacitance for the common emitter amplifier. Xc< or=0.1×Zin or Rin. The formula is: C=1/ 2π×f(low)×(0.1×Zin). Zin(base) is r'e + Re without a bypass capacitor. With a bypass capacitor to bypass the emitter resistor RE is hfe×r'e.

 

And you qestion is?

 

There are so many formulas in the field of electronics for designing and analyzing circuits.
When you are "studying and researching"such objects it is really necessary to understand the physical background of the formulas. Otherwise, you cannot know under which conditions the may apply (or not).
Simply to state "yes - correct" is not enough.