In a common transistor switch configuration, what is the typical VCE(sat) voltage when the transistor is saturated?

• A. Approximately 0.2 V
• B. Approximately 0.6 V
• C. Approximately 1.2 V
• D. Approximately 5 V

Answer: (A)

When a transistor is used as a switch and is fully on, it reaches saturation where both junctions are forward biased. In this state the collector-emitter path behaves like a very low resistance, so the voltage across them is kept very small. For silicon devices this VCE(sat) is typically about 0.2 volts, which is why the switch dissipates little power at a given load current. The exact value depends on how much base drive you give the transistor and the collector current; with heavier currents or less-than-adequate base drive it can rise a bit, but around 0.2 V is the standard target. This is not 0.6 V, which is the approximate base-emitter drop, nor as high as 1.2 V or 5 V, which would indicate the transistor isn’t in saturation.