What are the differences between \$I_B\$, \$I_b\$, \$i_B\$ and \$i_b\$ in a transistor?
\$I_B\$ means the DC current quantities
\$i_b\$ means the incremental current quantities such as AC signals
\$I_b\$ is used to denote the amplitude of a sinewave
\$i_B\$ is used to represent total instantaneous quantities, for example \$i_B = I_B + i_b\$

Refer the attached image


Reference: Microelectronic Circuits ~ Sedra/Smith