PN2222 vs BC547 Datasheet Question - Page 1
Forum section: electronics / beginners

author: MikeTannel
author_rank: Contributor
timestamp: July 29, 2025, 05:48:10 pm
content: Hi Everyone, I have a question. Picture this: Take a 2222 jelly bean transistor, tie the base and emitter to ground and apply a voltage at the collector via a 1k resistor.I found that at 8v the transistor is off as I would expect, but at 9v current starts to flow out the base (~0.25 mA - but only out the base, no collector current).Now do the same thing with a BC547. I went to 20v and still there was no current out either the base or the collector.The 2222 surprised me, but I thought - aha! let's check the datasheet. So I compared the datasheets for both transistors hoping to find some difference that would allow me to predict the behavior. Nothing was obvious to me.So my question is - What in the data sheet allows to predict this difference in behavior? I would think some breakdown voltage, but none were obvious to me how they might apply.I attached two sample datasheets as examples.Thanks in advance,Mike

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2025, 06:07:31 pm
content: Are you sure you've not got the collector and emitter reversed with PN2222? The collector and emitter connections are opposite to the BC547.Once the reverse breakdown voltage (normally around 7V) is exceeded, current will flow, even without any base drive.

author: MikeTannel
author_rank: Contributor
timestamp: July 29, 2025, 06:57:58 pm
content: Aw man, that's all it was - just inserted backwards. Doh. Well thanks much zero999!

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2025, 07:01:37 pm
content: Planar silicon BJTs have base-emitter breakdown voltages that are much lower than the collector-base breakdown: 7 V is typical.This is quite different from older germanium BJTs. The 2N404A has CB breakdown at 40 V, BE at 25 V.As noted elsewhere, pinouts of small plastic versions of older metal-case transistors can be different from the original TO-18 2N2222 (triangular) and each other (CBE, BEC, etc.) depending on prefix.

author: mawyatt
author_rank: Super Contributor
timestamp: July 29, 2025, 07:27:22 pm
content: Jim Williams told at the ISSCC long ago about using a reversed biased BE Si NPN Junction as a cheap voltage reference in a product. Later in production discovering that many were oscillating at a very low frequency due to the reversed biased BE negative resistance region working with the decoupling capacitor forming a relaxation oscillatorHere's an example from a DSO/AWG Curve Tracer:Besthttps://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4423303/#msg4423303

author: mawyatt
author_rank: Super Contributor
timestamp: July 29, 2025, 10:00:16 pm
content: Sorry the missing above post was removed and asked a question about Jim Williams use of the reversed biased BE junction as a "Zener".We'll leave the explanation below if that's OK.Actually he was using the EB as a Zener/Avalanche Diode and the BC junction as a temp compensation. The BE reversed bias TC can be around ~ +2mv/C for certain NPN Si transistors while a forward bias PN or junction is ~ -2mv/C.As we've shown in the link above when connected with the NPN Emitter as the + for the overall "Zener" and the Collector as - or ground, leaving the Base floating, then the overall "Zener Voltage" becomes the voltage of the reversed biased EB junction in series with the forward biased BC junction. In this configuration the EB current flows thru the BC junction and the NPN is acting in the "Inverted Mode" and the two junction have opposite TCs.Because of this "Inverted Mode" and Avalanche effect the overall "Zener" can show a negative resistance over a small bias current range as shown and produce a relaxation oscillator with parallel capacitance!!Best

author: MikeTannel
author_rank: Contributor
timestamp: July 30, 2025, 12:13:19 am
content: Absolutely.I deleted my question because I realized the response was probably going to be too nuanced for my basic understanding of things and I didn't want to waste peoples time. I was right. :-)Never the less - thank you - for taking time to answer. It's good to know I can count on folks here when I need to.Best Regards

author: mawyatt
author_rank: Super Contributor
timestamp: July 30, 2025, 12:40:36 am
content: Don't worry about asking questions, simple or complex, there are lots of resources here to help answer. Over time you'll learn those you can trust, those you can't, and those that provide lots of mis-information.For some reason anything associated with a DSO seems to bring out all sorts of flaming!!Best

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2025, 07:29:45 am
content: Quote from: MikeTannel on July 30, 2025, 12:13:19 amPlease avoid deleting things. It does get a little annoying when someone responds to a question and then no one knows what the question is.Note that reverse breakdown can damage a transistor, especially if it's long lasting and at higher currents. There are odd applications when it's useful, such as a reverse bias oscillator.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 30, 2025, 04:23:50 pm
content: Quote from: Zero999 on July 30, 2025, 07:29:45 amDefinitely this. If I find myself wanting to delete a post that has already been answered, I use the "Modify" button to edit it. Once open for editing I write the word "EDIT:" underneath the original text and then a "Please ignore" request, or whatever else you want to say.Another trick, if you just want to alter the original wording slightly, is to "Modify' and then use thestrikethroughfeature on the unwanted text, and then add the correct text.