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So I'm a hobbyist mostly working with micro-controllers and simple low voltage digital circuits and I always thought the TIP120 transistor is a useless one for me because it has a Vce(sat) of around 2V. But yesterday I was testing it in a circuit and I realized that the Vce is actually way below that. So I got curious and read the datasheet again thinking maybe I had made a mistake and then I realized that Vce values are contradictory in it.

In the Electrical Characteristics table it states that Vce(sat) @ Ic=3A, Ib=12mA is 2 Volts (in the Max. column):

TIP120 Vce characteristics

But further down in the Typical Performance Characteristics section you are presented this:

Vce,Vbe,Ic curve for TIP120

Here even the hfe is the same so at 3 Amps you have the same 12mA of base current that was specified in the table. But as you can see Vce is 1V here as opposed to the 2V stated in the table. And at 5 Amps it's ~1.3V as opposed to 4V(!) stated in the table.

So what's going on here exactly? 4V compared to 1.3V is a lot of difference. Is it because the table is stating the maximum? If so, what factor can cause Vce to change so much given that Ib and hfe are the same?

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    \$\begingroup\$ The figure may be typical value while the table as you have mentioned is the maximum value. \$\endgroup\$
    – AJN
    Jul 28, 2020 at 6:05
  • \$\begingroup\$ @AJN Yes it's the "Typical Performance Characteristics" section. What's the difference? \$\endgroup\$
    – Pouria P
    Jul 28, 2020 at 7:03
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    \$\begingroup\$ If you bought, say, 100 of these transistors, most of them will follow the curve (or close to it). But a few of them will have VCEsat up to 4V due to variations in manufacturing. Manufacturer doesn't guarantee the typical value for any single transistor sold to you. If you want to guarantee a low value of VCEsat in your circuit, choose another part number or buy this part in bulk and discard all the large VCEsat ones yourself (not economical). \$\endgroup\$
    – AJN
    Jul 28, 2020 at 7:12
  • \$\begingroup\$ @AJN Ah I see. I just read somewhere that typical characteristic curves are "marketing BS" and should be disregarded. But what you said makes more sense. Thanks. \$\endgroup\$
    – Pouria P
    Jul 28, 2020 at 7:25

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Is it because the table is stating the maximum?

Yes. That's what 'Max' means.

If so, what factor can cause Vce to change so much given that Ib and hfe are the same?

Hfe isn't necessarily the same. Internally the Darlington is a concatenation of two transistors. Due to process variations the current gain of each transistor can vary widely, and is strongly current dependent (dropping rapidly at high current). Total Hfe variation may be even greater because it is the product of the individual Hfe's, which have different curves.

In saturation Hfe becomes much lower as the Collector-Emitter voltage drops. Total saturation voltage may be determined by the first transistor (whose current gain drops to 1 in full saturation) or the second transistor if its gain is too low or resistance too high to support the desired current. All of these factors are process dependent.

The reason for the large maximum saturation value is that due to process variation a few individual transistors may approach that figure. The manufacturer doesn't want to throw away those marginal parts because it would raise the price (even just having to thoroughly test every unit for that parameter could make them too expensive).

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