Input High Pulls Output Low - Electronic Circuits XI: Transistor Switching and Device Regions Step by Step

Saturation creates the low output in a transistor inverter. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

A high input supplies enough base current for saturation

The base path supplies 1 A; beta makes a 3 A drive limit.

\beta I_B=3\cdot 1\ \text{A}=3\ \text{A}

Saturation pulls the output low

The load demand is also 3 A, so the transistor saturates and the output is 0 V.

I_C=3\ \text{A},\quad V_\text{out}=0\ \text{V}\Rightarrow \text{LOW}