Threshold Boundary Cuts Off Base Drive - Electronic Circuits XI: Transistor Switching and Device Regions Step by Step

At the stated base-emitter drop there is no excess voltage. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

At the stated drop there is no remaining resistor voltage

Here the input and training drop are both 1 V, so the base resistor has zero volts left.

1\ \text{V} - 1\ \text{V}=0\ \text{V}

Zero excess voltage gives zero base current

The book's convention makes the boundary a cutoff case with 0 A of base current.

I_B=0\ \text{A}