Vcesat

Vce(sat), or Vce(sat) for collector-emitter saturation voltage, is the voltage across the collector-emitter terminals of a bipolar junction transistor (BJT) when the device is in saturation. In this state, both the base-emitter and base-collector junctions are forward-biased, and the transistor is being driven as a switch rather than as an amplifier. Vce(sat) represents the minimum voltage drop that occurs across the transistor when it conducts its rated current under saturation.

Measurement of Vce(sat) is specified in datasheets under defined conditions, typically with a specified collector current I_C and a defined base drive I_B (or a forced beta, β_f). The value is device- and condition-dependent, varying with current, temperature, and the amount of base drive. In practice, Vce(sat) is a key metric for switching applications because it directly affects conduction losses and efficiency.

Typical values vary widely: small-signal transistors may have Vce(sat) around 0.1 to 0.2 V at modest currents when saturated, while power transistors can exhibit higher saturations, often in the 0.2 to 0.4 V range at higher currents. The exact figure for a given device is found in its datasheet and depends on the intended operating point and the magnitude of base drive used to ensure saturation.

Design considerations include ensuring sufficient base current to force saturation (often described by a forced beta, β_f = I_C/I_B, chosen to keep the transistor well into saturation) to minimize Vce(sat). Temperature and aging can shift Vce(sat) as well, so designers consult datasheets for conditions close to their operating environment.