Fanout Budget Three-Row Scan - Electronic Circuits XI: Transistor Switching and Device Regions Step by Step

The driver can feed only the base-current sum that fits under its limit. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Four half-ampere bases stay below the limit

4 base inputs at 1/2 A each require 2 A from a 3 A driver, so the checked budget is pass.

4\cdot{1\over 2}\ \text{A}=2\ \text{A}\le3\ \text{A}

Six half-ampere bases exactly use the limit

6 base inputs at 1/2 A each require 3 A from a 3 A driver, so the checked budget is pass.

6\cdot{1\over 2}\ \text{A}=3\ \text{A}\le3\ \text{A}

Seven half-ampere bases exceed the driver

7 base inputs at 1/2 A each require 7/2 A from a 3 A driver, so the checked budget is fail.

7\cdot{1\over 2}\ \text{A}={7\over 2}\ \text{A}>3\ \text{A}