Transistor Hard Saturation -- Rule of Thumb

For hard saturation, the general rule-of-thumb that engineers employ is known as the 10:1 rule (Rb:RL), where we make the base resistance Rb ten times load resistance RL. Hence, simply put, Rb = 10 × RL. In this case, for the rule to work, the supply voltage Vcc must be the same as the switching voltage at the base Vi.

TTL Switching Circuit Example

For example, in a digital TTL circuit, where the supply voltage is +5 V and the switching voltage is also +5 V, if the collector resistor were to be 1 kΩ, then the base resistor would have to be ten times collector resistance, hence it would be 10 kΩ. When you see this 10:1 rule, you can tell at a glance that the transistor is saturated.

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