Electronic – Current Notation in BJT


Circuit demostrating notation.

We are currently studying BJTs in electronics I and I am a bit confused on the notation of the current, specifically the collector current here. Throughout this chapter they will swap \$I\$ and \$i\$ to seemingly refer to the exact same thing. The book itself doesn't explain it either. There seems to be no consistency. Why wouldn't it just be \$I_{C1}\$ and \$I_{C2}\$? I understand that usually lower case refers to instantaneous voltage or current, but when solving this example, for instance, \$I_c\$ and \$i_c\$ are simply different examples used to solve the \$i_c = I_{s}e^{V_{BE}/V_T}\$ equation to give the \$V_{BE}\$ needed for the design choice.
I cannot find any other info on this so I figured this was a good chance to ask here.

Best Answer

I is for large signal (eg. bias) values, and i is used for small-signal (eg. time-varying as @DKNguyen says).

Usually you will be dealing with linear equations for i(t) and with nonlinear equations for I.