>15V Avalanche BJT? - Page 1
Forum section: electronics / projects

author: EEVblog
author_rank: Unknown
timestamp: June 14, 2017, 10:51:15 am
content: Anyone know of any BJT that has a typical reverse B-E avalanche mode (BVEBO) of greater than 15V?The typical value is about 6-8V and it's often not specified at all.Thanks.

author: EEVblog
author_rank: Unknown
timestamp: June 14, 2017, 11:31:48 am
content: I found a couple:http://www.mouser.com/ds/2/315/DSC2A01_AED-26036.pdfhttp://rohmfs.rohm.com/en/products/databook/datasheet/discrete/transistor/bipolar/2sd2704k.pdf15V and 25V BVebo respectively.

author: Zero999
author_rank: Super Contributor
timestamp: June 14, 2017, 12:02:18 pm
content: Just one question: why?

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 14, 2017, 03:22:02 pm
content: I seem to recall that if Vebois around 5-6V that the breakdown mechanism is zener-like electron/hole tunneling, while above 15V it is almost exclusively from avalanche. The former type of transistor makes an excellent noise source, while the latter type can be used as a really fast/low-pF clamp on (high impedance) signal lines.Hopefully this isn't asecret squirrelproject, as I am curious about it, too.

author: Zero999
author_rank: Super Contributor
timestamp: June 14, 2017, 09:52:23 pm
content: Yes, it seems like an odd requirement. It's usually bad practise to subject the base-emitter junction to reverse bias. In circuits which will reverse bias the BJT, such as an astable multivibrator, the usual way round this is, to put a diode in series with the base and a high value base-emitter resistor if you're that worried about the tiny leakage current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 15, 2017, 04:12:37 am
content: I've seen a few types from time to time, but never took note of them, sorry...I've managed to avoid needing that parameter in an application. Guessing your application is a little less forgiving.Tim

author: BrianHG
author_rank: Super Contributor
timestamp: June 15, 2017, 08:20:24 am
content: I believe this is for something special for EEVBlog video #1000?Remember, we are now at #999.....We will most likely have to wait for the video.

author: kalel
author_rank: Frequent Contributor
timestamp: June 15, 2017, 09:06:08 am
content: Quote from: Hero999 on June 14, 2017, 09:52:23 pmThat's the only way I've seen them used so far (I mean a single transistor LED blinking circuit), but there are surely some other uses out there. I'm also interested in what the intended use is, if it can be shared.

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2017, 09:49:54 am
content: Quote from: kalel on June 15, 2017, 09:06:08 amThat's one use for this but it wasn't what I was referring to. I was talking about the standard astable multivibrator, made with two transistors. When each transistor turns off, its base is momentarily subjected to the inverse of the power supply voltage, minus a diode drop. If the power supply voltage is below the reverse base-emitter rating plus a diode drop, then no diodes are required, but for higher voltages, diodes can be put in series with the base connections to protect the transistors from the reverse voltage spikes.https://en.wikipedia.org/wiki/MultivibratorLow voltage astableHigh voltage astable

author: kalel
author_rank: Frequent Contributor
timestamp: June 15, 2017, 09:57:12 am
content: Quote from: Hero999 on June 15, 2017, 09:49:54 amThank you for clarifying.Yes, that is a more common circuit than the single transistor thing.

author: David Hess
author_rank: Super Contributor
timestamp: June 16, 2017, 09:33:56 am
content: Transistors intended for "chopper" applications like the old MPS404/MPS404A which is currently available as the CMPT404A from Central Semiconductor have high Vebo. These transistor are also useful for audio muting applications like the example from Rohm and the Toshiba 2SC2878.Besides having high Vebo, these transistors also have high reverse current gain. Before JFETs became available, these were what you used in chopping applications and they still have advantages at low voltages.Lateral PNP devices built on NPN integrated circuit processes also have high Vebo which is why they are often used for differential input stages and cascodes to provide high differential input voltage capability like with the 324/358, 4136, 301/301A, 741, 833, etc.