$($b$)$ The BJT as a switch and current amplifier. Figure below shows a typical circuit where an NPN transistor is used as a switch and as an amplifier of current. The rectangular waveform at the input switches between $0$ V and $+5$ V $.$ The LED $($Light Emitting Diode$)$ has a forward voltage drop of $1\mathrm{.}5$ V when conducting. Resistor values are $\backslash (\backslash$mathrm$\{$RB$\}=33\backslash$mathrm$\{$k$\}\backslash$Omega $\backslash )$ and $\backslash (\backslash$mathrm$\{$RC$\}=200\backslash$Omega $\backslash ).$ The NPN transistor i$.$e$.$ used in this circuit is a $2$N$3904.$ Let's examine how the BJT responds to the $0$ V input and to the $+5$ V input.