> Simply, the t-rise and fall are determined by the capacitive load the on
the
> transistor. Depending on the amplifier configuration (common emitter) ,
the
> 'Miller effect' can increase the effective capacitance. So your t-rise and
> fall depend on frequency, external circuit components and amplifier
> configuration.

Is this a commercial or hobby project?  I wouldn't just hack something like
this on a commercial product, but I once built a small transmitter from
"parts bin" transistors for which the specs where rather unclear.  The basic
trick was to have a common emitter transistor drive one in the common base
configuration.  In other words, pulse goes into base of Q1.  Emitter to Q1
goes to ground, and the collector drives the emitter of Q2.  The base of Q2
is tied to a solid low voltage reference with good bypass caps.  The emitter
of Q2 drives the load.  The point of all this is to greatly reduce the
Miller effect.

This transmitter was built and worked very well.  This was many years ago in
a college dorm.  The signal was fed into the power wires, but you could pick
it up with a hand held radio up to about 30 meters from the building.  I'm
sure it was totally illegal, but we had great fun with it on Sunday nights.
One time we got everyone in the dorm to flush their toilets at the same
time.  (Don't do this with sinks or whatever.  Toilets usually had a "soft
off").  Apparently the higher elevations of Troy New York had an unexplained
water outage for up to 10 minutes.  Surprisingly, we never got into much
trouble about that.  We found out later that the signal hopped a few power
transformers and in one case we heard about was picked up over a mile away.
Good thing this ended soon afterwards when the semester ended.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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