EMRFD Message Archive 3768

Message Date From Subject
3768 2009-11-13 19:04:28 popmedkoh Great Learning Site
This may be old hat to most of you but I just stumbled across a terrific animated diagram showing the flow of electrons and holes in a bipolar junction transistor while you interactively vary the base voltage:

For the first time I understood why swapping the emitter and collector doesn't work out even if the twin back to back diode model suggests otherwise - and why the twin diode model is so inadequate. I'll bet I'm not the only one who tried to get two 1n34 diodes to amplify! Great animation of a JFET and very detailed views of practical transistor construction as well. The site is run by Eric Coates (not a relative or acquantance) a Brit who capped off a multi-decade career in tech and vocational electronic work by earning advanced web degrees. The website has been up for just over a year. Judging by his semiconductor material, I can't wait to see what he comes up with as he fills out his list of topics. Apologies for posting also in Bitx20 but I know there are some who are members of one group but not the other and I wanted to spread the word. This is one website worth perusing!

David KE1LY
3769 2009-11-14 08:07:18 Alan Melia Great Learning Site
Hi David, transistors DO actually work the other way up!! the major problem
is that they have a gain of less than unity.....so they are not a lot of use
in most applications. They do however make quite good avalanche switches for
producing very short fast pulses, the limitation is a fairly low supply

Zetek make transistors specifically for this service. I have a memory of a
Ham Radio (mid 1970s) article on using reversed NPN transistors at 12v
rather than purpose-made transistors at 100v. In the early days of
transistors this format was used to generated the short sampling pulses for
sampling oscilloscopes. The only way one could look at and measure nsec rise
times in those days.

Alan G3NYK
3770 2009-11-14 08:51:00 popmedkoh Re: Great Learning Site
3771 2009-11-15 05:16:43 g3oth Re: Great Learning Site
3773 2009-11-15 08:55:28 k5nwa Re: Great Learning Site
At 07:16 AM 11/15/2009, you wrote:
3774 2009-11-15 11:37:20 g3oth Re: Great Learning Site
> This is interesting, even today the FET switches are a weak point on
> a switch based mixer. How does operating them in reverse affect the
> Ft of the transistor?

Hi Cecil

For what it is worth, I have just measured the Hfe of three 2N3904 NPN transistors on a cheap LCD Multimeter and the range varied between 147 and 160 in normal mode and all measured as 3 in inverse mode when plugged in reverse.
A similar test on three 2N3906 PNP transistors resulted in a range of 212 to 216 in normal mode and all measured as 5 in inverse mode.
You can draw your own conclusions as to what the Ft might be !
I think the switching speed is the all important factor, I don't have any info on this but Alan might me able to expand on this further.
73 de Russ G3OTH
3775 2009-11-15 11:57:47 g3oth Re: Great Learning Site
Cecil, if you Google this subject a wealth of info comes up at:


Have fun using "Inverse Mode" with Russ G3OTH
3776 2009-11-15 17:23:34 k5nwa Re: Great Learning Site
At 01:57 PM 11/15/2009, you wrote:
>Cecil, if you Google this subject a wealth of info comes up at:
>Have fun using "Inverse Mode" with Russ G3OTH

I have looked at hundreds of articles and the information is very vague;

1. A good percentage of the articles are basically copies of each
other where in one paragraph they talk about "reverse" active mode
and just mention the decrease in beta. I've done the testing and
gotten the collector and emitter switched by mistake and observed
this effect. A few articles mention lowered breakdown voltage which
makes sense due to the doping concentrations in the new collector to
base region.

2. In a couple of articles it mentions that the beta and Ft is
degraded but no explanations.

3. In a couple of articles it mentions that the flow of minority
carriers is improved due to the high collector doping, which
translates to a faster device.

If used to achieve high speed switching then 1 and 3 must be in play,
eventually I need to do some testing.

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