Transistor datasheets min-typical-max - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: August 23, 2018, 04:57:55 am
content: 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 ?

author: hamster_nz
author_rank: Super Contributor
timestamp: August 23, 2018, 05:36:47 am
content: 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.Seehttps://www.electronics-tutorials.ws/amplifier/emitter-resistance.htmlfor a better discussion than I can type in a few minutes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2018, 09:49:17 am
content: 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

author: Audioguru
author_rank: Super Contributor
timestamp: August 23, 2018, 11:06:59 am
content: Quote from: hamster_nz on August 23, 2018, 05:36:47 amNo. 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2018, 11:48:45 am
content: Quote from: Audioguru on August 23, 2018, 11:06:59 amNot 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

author: hamster_nz
author_rank: Super Contributor
timestamp: August 23, 2018, 12:05:08 pm
content: Quote from: Audioguru on August 23, 2018, 11:06:59 amIn my defence, I did have a "maybe" in there, and 100mA & 5mA are the Base-Emitter Saturation Voltage test conditions for the BC546's datasheet...But one thing is sure: with a current gain of 100, to switch 100mA, 1mA of base current isn't going to cut it. You need many times tha t current.

author: Audioguru
author_rank: Super Contributor
timestamp: August 23, 2018, 12:44:39 pm
content: Most little European "BCxxx" transistors have their saturation voltage listed when the base current is 1/20th the collector current.American little "1Nxxxx"transistors have their saturation voltage listed when the base current is 1/10th the collector current.The hFE current gain is listed when there is plenty of collector to emitter voltage so itis notsaturated.