As you can see on the datasheet below for the 2n2222a NPN transistor, the "Collector-Emitter Saturation Voltage" and "Base-Emitter Saturation Voltage" are defined respectively as 0.3 to 1.0 and 1.2 to 2.0. I believe I understand transistor saturation, but whats the difference between Collector-Emitter Saturation and Base-Emitter Saturation?

The ubiquitous 2n2222a!

  • \$\begingroup\$ Do you understand what saturation is? It's not two different kinds of saturation, it's two different voltages, when the transistor is saturated. \$\endgroup\$
    – user253751
    Feb 25 '20 at 15:28

Vce(sat) is the voltage measured at the collector relative to the emitter under the specified conditions (say 150mA of collector current flowing, and 15mA of base current applied (forced beta of 10, obtained by dividing Ic by Ib thus 150 over 15 mA).

Vbe(sat) is the voltage measured at the base relative to the emitter under the specified conditions, as above.

So if you are wanting to switch a 150mA load under those conditions with (say) a 10V input, you will use the latter number to determine the required resistor value to get the minimum base current needed (and ensure you don't exceed any maximum if the minimum Vbe(sat) is specified).


What's the difference between Vbe(sat) and Vce(sat) ?

Vce(sat)-Vbe(sat)= Vcb (sat) is defined where current gain drops to 10 as a switch rather than a current source.

It is about 1 volt @ 500mA and ~ 0.9V @ 150mA both at Ic/Ib=10 from table above.

  • using Ic/Ib=10 in most cases ( ~ 10% of worst case hFE)
  • Vbe has heavily doped P-N junction with base spreading R or equiv. series R,

  • ESR(b-e) = ΔVbe/ΔIb = (2.0-1.2)V/(50-15)mA = 23 Ω in this range.

  • Similarily for Rce or ESR(c-e)=(1.0-0.3)V/(500-150)mA= 2 Ω ( or 10x smaller)

  • Actually Vcb becomes forward biased in saturation so the quality of super low Vce depends on how the transistor is doped and processed (patented).

  • While the actual value of Rce depends on the size of the chip inside and its thermal properties.

  • I would expect this transistor to dissipate Pdmax=1/ESR(c-e) = 1/Rce = 0.5W ( give or take 50% )

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