Finding Beta (B)?! - Page 1
Forum section: electronics / beginners

author: shanecy
author_rank: Contributor
timestamp: July 17, 2013, 02:05:04 pm
content: SITUATION:I'm doing a small LAB for class on a CE Amplifier for a 2N3904. I'm not given Beta (B) in the schematic.QUESTION:I pulled up the specification sheet for it and couldn't find (B). However, hfewas from 100 - 400. How exactly can I determine Beta (B) so I can find IBand evaluate my equivalent reequivalent model?I know that B = IC/ IBso would I find a sufficient (Q) by employing KVL and solving for the two extremes when IC= 0mA and VCEis 0V?I'm still trying to get the hang of figuring out how to solve for these unknowns. The homework problems were a snap because (B) was actually given to me in the schematic. If it's relevant it's calling that I set up my Function Generator with a level of 20mVpp@ 5kHz. Somehow I'm telling myself I can't do this unless I know what (B) is. Any help is greatly appreciated.Here is the link to the pdf of the datasheet i'm looking at fyi.http://www.parallax.com/Portals/0/Downloads/docs/prod/oem/2N3904.pdf

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 02:23:11 pm
content: hFEis the large-signal gain, and hfeis the small-signal gain. Usage of beta varies, but it's typically "gain" in general.So, this is pretty tedious, but you're going to need to solve this very, very complex equation:beta = hFEorbeta = hfedepending on what you're calculating. The datasheet goes into further detail of the gain at different operating points in the plots. Keep in mind that variation from one transistor to another is significant; don't design too tightly around gain.

author: darko31
author_rank: Regular Contributor
timestamp: July 17, 2013, 03:59:26 pm
content: Or stick the transistor you're using into multimeter, those ECBBCE (or whatever is the arrangement) holes are designed for exactly that.

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 04:03:04 pm
content: And they're completely useless unless they happen to test the transistor under the same conditions as you will have in your circuit. There's a reason it's typically only cheap-ass crappy multimeters that have them.It shouldn't matter, though. A well designed, relatively modern circuit will not depend on beta so strongly. As long as the transistor meets the minimum gain that you've designed for (which should be the minimum gain from the datasheet, not the typical, or worse, what you've measured on one individual transistor), the circuit should work. As long as you have an emitter resistor to help configure the voltage gain of the system, dependence on beta should be minimal.

author: mariush
author_rank: Super Contributor
timestamp: July 17, 2013, 04:19:40 pm
content: Yeah they're kinda useless but still nice to have a general idea.I bought a Uni-T UT602 LR meter (yeah that's right, it doesn't have the C part) some time ago, mainly for measuring inductors. It's cheap, from about 30$ and up:http://www.ebay.com/itm/UNI-T-UT602-Modern-Inductance-Capacitance-Tester-Multimeter-Meter-/310683381662?pt=LH_DefaultDomain_0&hash=item48562c2f9eBut it also has the transistor socket function.. BC337 was one sale at Farnell a few days ago so i bought a strip of 150 of them and i tested one randomly.. datasheet says minimum 100, max 600.. the one i tested said 360.

author: c4757p
author_rank: Super Contributor
timestamp: July 17, 2013, 04:31:02 pm
content: ...an LR meter with no C but with transistor, diode and continuity test. What are they doing, just rolling dice to pick features? Why not a 10 kHz oscilloscope and a 500 W power supply as well?

author: mariush
author_rank: Super Contributor
timestamp: July 17, 2013, 05:12:58 pm
content: Quote from: c4757p on July 17, 2013, 04:31:02 pm) Yeah, i know...They have three of them, the last one being more complete:UT601 : R , C , D, TUT602 : L , R , D , TUT603 : L, C, R, D, TIt's still average specs wise.. i mean it has no autoranging and no indicator regarding if you're overloading or not... i can test a 22uH inductor on the 20uH scale and it would happily say it measures 20uH... but it's good enough for my needs and it's enough to just switch between several ranges when measuring to get an accurate reading.