[time-nuts] fast edge, rise time.

[Attila Kinali]

On Thu, 26 Dec 2013 16:07:49 +0000
ct1dmk <ct1dmk at 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