"Steve Evans" wrote in message
...
On Tue, 12 Oct 2004 17:10:41 -0500, "Steve Nosko"
wrote:
[snip]
IF we had 5 volts peak (10 volts peak-to-peak) the cap left side, the
base
voltage will swing from +0.7 volts to -9.3 volts.
WHEW !
Did this work

Sure did, Steve! I ran it through a spice program and you're right in
every detail. So there's obviously some basic flaw in my understanding
of caps. The textbooks say to treat a cap as a short circuit at AC

C O O L !!! So my explanation did it...sorry for the gloat folks. Had
me scared there for a minute...30+ years doing and teaching electronice not
for naught after all.

This is super, Steve Evans, Now you're learning.
Now, look at the Spice circuit _after a bunch of input cycles_ and
monitor the voltage on BOTH sides of the cap and you will see that they are
both the same **AC wise **, but there will be a constant, or DC difference.
You can use the difference probe and you will only see a constant (DC)
difference in potential across the cap. If you look at each side
individually, you will see that the AC signal (yes I know "AC Signal" is
redundant, but I think it helps clarify) is identical on both sides, though
shifted by that DC amount. You can even change the waveform. Put *TWO* AC
sources in series, one less voltage than the other, and different
frequencies. The two cap sides will still have the same waveform.

The concept you quote of "cap as a short circuit at AC" is true, but
more easily seen in the steady-state. This is what we call it when all of
the transient effects have died out. These transient effects are usually
capacitors getting charged up (sorry Ratch, but I gotta say it). The AC
component will "pass through" the cap un-impeeded, BUT you can also have
this DC offset from one side of the cap to the other.
In the case we have focused on (coupling cap & NO inductor) you have a
little more complexity throwing in a monkey wrench. The Base-Emitter
junction/diode is rectifying the AC and charging up the cap _just like it
does_ in the common power supply rectifier circuit--but in this case it
makes trouble for us by reverse biasing the base.
I am glad to help, but as you see, you have sort of jumped into the
middle of circuit theory armed only with the "AC short" part of the
capacitor's characteristics. My bet is that you'll remember this very well.

They say: "we learn from our mistakes". Well, BOY! ARE WE LEARNIN' NOW!

73,
--
Steve N, K,9;d, c. i My email has no u's.