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Transistor datasheets min-typical-max

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lordvader88:
What does it mean when they don't list all info, and just leave a blank or dash -

Like I'm looking at a BJT because I want a rough idea what gain to use, and they just list the min,  no typical, or max for some versions of the BJT. In this case it's an older BJT and maybe they don't know ?

hamster_nz:
No, it is a case of no value is specified - you should design such that you only assume the minimum gain, and so any higher gain (which might be quite significantly higher) does not make any noticeable impact.

If the min figure is 100, and you are using the transistor as a common emitter switch, then make sure that sufficient base current is available to saturate the transistor - e.g. to switch 100mA, make sure you have maybe 5mA of base current available - this way you are sure that the transistor is 'fully switched' regardless of the part's actual gain.

If you are using it as a simple common emitter amplifier, include a resistor between the emitter and power rail (and usually a capacitor too), so the difference in gain is minimal.

See https://www.electronics-tutorials.ws/amplifier/emitter-resistance.html for a better discussion than I can type in a few minutes.

T3sl4co1l:
On a related note, I never did discover how you're supposed to know what sigma they accept when they give a typical value only.

Tim

Audioguru:

--- Quote from: hamster_nz on August 23, 2018, 05:36:47 am ---If the min figure is 100, and you are using the transistor as a common emitter switch, then make sure that sufficient base current is available to saturate the transistor - e.g. to switch 100mA, make sure you have maybe 5mA of base current available - this way you are sure that the transistor is 'fully switched' regardless of the part's actual gain.
--- End quote ---
No. The datasheets for most little transistors show the transistor saturating pretty well when the base current is 1/10th the collector current, not 1/20th, even if the current gain is very high.

T3sl4co1l:

--- Quote from: Audioguru on August 23, 2018, 11:06:59 am ---
--- Quote from: hamster_nz on August 23, 2018, 05:36:47 am ---If the min figure is 100, and you are using the transistor as a common emitter switch, then make sure that sufficient base current is available to saturate the transistor - e.g. to switch 100mA, make sure you have maybe 5mA of base current available - this way you are sure that the transistor is 'fully switched' regardless of the part's actual gain.
--- End quote ---
No. The datasheets for most little transistors show the transistor saturating pretty well when the base current is 1/10th the collector current, not 1/20th, even if the current gain is very high.

--- End quote ---

Not much is gained by oversaturating the transistor.  There are some very real downsides to the 1/10 rule of thumb.  As is the case for any "rule of thumb".  One must not use it mindlessly.

The 10% loss of efficiency (not necessarily so, but true in a lot of circuits) is a very real downside.  The very long storage time is another.  Low-Vce(sat) types have quite high hFE, even at large Ic and small Vce, and are typically spec'd for saturation at hFE = 50 or 80 (where switching time is acceptable).

Of course, if you must handle a wide range of load currents, base current must be fixed by the maximum collector load current, regardless of load*.  In that case, the transistor will almost always be very oversaturated, and that's just something you have to accept.

*Unless you use a saturation control circuit; a Darlington transistor is an example, though a somewhat costly one, the cost being much higher Vce(sat).  There are other solutions that perform better, but they're mostly relevant to IC design.

Tim

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