>If the transistor  has a beta (Ic/Ib) of 100, and the collector current is
>300mA, then the base current is 3mA, NOT 30mA.   I'm not sure where the
"10% of
>the collector current" rule came from, but I think it's wrong.

You will not get a transistor to saturate if you use the published beta
value to get the base current. Transistors in saturation always have the
base overdriven to force the device right into saturation. In the case of
bipolar transistors this is known as "forced beta", as the common way of
doing it is to drive the base with a current that is many times what is
normally expected for the same collector current in linear operation.

The general rule of thumb is to use a forced beta of 10 to get a transistor
saturated to the point where the C-E voltage is really at it's minimum
possible. If one worked on a forced beta of 25, I think you would find the
voltage would go up to 0.3 to 0.4. If you relied on the published beta value
the saturation will rise to about the B-E voltage, in other words about 3
times what is achievable.

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