On Thu, 26 Dec 2013 16:07:49 +0000 ct1dmk <ct1...@gmail.com> wrote:
> The target is 4ns, while ideas seemed to be clear at some point, now I'm > having doubts if better to use a MOSFET or a bipolar transistor > as the switch element. Experiments with MOSFETs presented me some > difficulties charging the gate capacitance having some trouble to > achieve something in the 4ns region. Well 4ns seems hard whatever device > anyway. First idea that comes to mind is using a bipolar transistor in avalanche mode. There are specialized transistors for this, like the ZTX415[1,2], but you can use 0-8-15 tranistors like a BC548 as well. If using a general purpose transistor, then the avalanche voltage will vary quite a bit from transistor to transistor. Easiest way to get around this is to use a "small" resistor from base to emiter to drain any charge generated, then insert a fast rising spike into the base. Using this, you can enter the range of the avalanche voltage while surpressing the effect until you want to trigger it. With this you should be able to get into the low ns range, if not even below 1ns. The limiting factor is usually how fast the base voltage can rise. The avalanche voltage for BC548 in my experiments was in the vincinity of 200V. I took the idea from [3], which contains a description of such a pulse generator with a general purpose transistor on page 32. Almost the same circuit can be found in [4]. Attila Kinali [1] ZTX415 datasheet http://www.diodes.com/datasheets/ZTX415.pdf [2] ZTX415 avalanche mode transistore application note 8, by Neil Chadderton, 1996 http://www.diodes.com/_files/products_appnote_pdfs/zetex/an8.pdf [3] "A seven-nanosecond comparator for single supply operation", by Jim Williams, 1998 http://cds.linear.com/docs/Application%20Note/an72f.pdf [4] "Slew rate verification for wide band amiplifier", by Jim Williams, 2003 http://cds.linear.com/docs/Application%20Note/an94f.pdf -- 1.) Write everything down. 2.) Reduce to the essential. 3.) Stop and question. -- The Habits of Highly Boring People, Chris Sauve _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.