I higher is hfe in 2N3904? - Page 1
Forum section: electronics / beginners

author: Jasonbit
author_rank: Regular Contributor
timestamp: July 20, 2018, 02:30:14 pm
content: Maybe I don't see right the all spefectaion in this datasheet (https://www.onsemi.com/pub/Collateral/2N3903-D.PDF). Supposedly this transistor on model 2N3904 should be HFE=40 but when I measure with my multimeter (on function hfe) I get a value 212. Why? (see this table please:https://snag.gy/a9SM8T.jpg)Thanksheyyy, after I posted my question I see which this list has two lines to 2N3904. Ok, but what line should I look? (I need a transistor with a hfe very lower. Which model do you suggest I use? And where and how can I search about transistor with hfe below 100?Thanks

author: rstofer
author_rank: Super Contributor
timestamp: July 20, 2018, 02:38:37 pm
content: The values in the table are mostly Minimums but the Maximum might be as high as 300. In other words, the gain could be all over the map. The manufacturer just isn't characterizing Typical and Maximum. That a safe thing to do because we would design with Minimum gain and provide feedback to control the amplifier if we had very high gain.

author: drussell
author_rank: Super Contributor
timestamp: July 20, 2018, 02:41:43 pm
content: A 2N3904 has an hFE of between about 25 and 400, depending on the operating conditions. It says so plainly in the data sheet characteristics charts. (The gain of any transistor will vary widely depending on the exact circuit conditions).Perhaps if you told us a bit about what you are trying to do, what your circuit is, etc. we can provide some more helpful guidance.

author: Jasonbit
author_rank: Regular Contributor
timestamp: July 20, 2018, 02:45:09 pm
content: But which line should I look? I repair ob "Note2". (2. Pulse Test: Pulse Width < 300us; Duty Cycle < 2%) This value are for when I will use transistor on this conditions?The purporse of the circuit is see three stage on transistor (cutting, active, saturation). For this I thought to use an transistor with hfe very lower. What do you think?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: July 20, 2018, 03:20:52 pm
content: Why not design the circuit so it works with any transistor, with a gain higher than 40?Designing a circuit so it only works with transistors with a narrow Hfe range, is extremely bad practise and will only cause problems, later on.

author: rsjsouza
author_rank: Super Contributor
timestamp: July 20, 2018, 05:19:02 pm
content: Exactly. A transistor hfe changes too much with the bias, therefore you will always have to work with approximations.I strongly recommend you to search around on the internet about "bipolar transistor bias" or "transistor amplifier design" to get some useful references about the subject.I just found a practical guide online in the link below, with various sections that talk about fundamentals.http://www.learnabout-electronics.org/Amplifiers/amplifiers10.php

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2018, 08:19:26 pm
content: Hfe depends on diffusion during fabrication so it is not well controlled and it is very temperature sensitive. At best the manufacturer will grade parts into separate groups but even then, hfe varies considerably from unit to unit.For switching applications, hfe is what matters but for linear operation, transconductance which is very consistent is more important. If controlled switching is necessary, then there are various anti-saturation circuits like Baker clamps which can be used to handle a wide range of hfe.Quote from: Jasonbit on July 20, 2018, 02:45:09 pmThe datasheet for the transistor will include hfe specifications at different collector currents. For transistors intended for switching applications this will be very brief but more specialized amplifiers may list hfe specifications at several collector currents. If necessary, pick a transistor intended to operate at the collector current you have chosen to get a better specification.If you want to explore cutoff, active, and saturation, then adjust the circuit to the transistor instead.

author: Jasonbit
author_rank: Regular Contributor
timestamp: July 21, 2018, 11:03:48 pm
content: Thanks to your advice'sExist any book or article where can I identify all parameters in datasheet? For example, the parameter Vebo - Emitter-Base Voltage and Vceo - Collector-Emitter Voltage I can't identify what means the last letter (o). I know this parameter means Voltages applied between emitter-base and between collector-emitter, respectively. But what means a last letter?Thanks

author: tggzzz
author_rank: Super Contributor
timestamp: July 21, 2018, 11:17:54 pm
content: Quote from: Jasonbit on July 21, 2018, 11:03:48 pmSeehttp://lmgtfy.com/?q=Vceo

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2018, 03:12:18 am
content: Quote from: Jasonbit on July 21, 2018, 11:03:48 pm"o" in this case just means that the other terminal is open. Vce has several ratings depending on if the base is positive, negative, shorted to the emitter through a resistance, or open. Vceo is the lowest of these so the safest to design with.