On Fri, 15 Oct 2004 14:23:24 -0500, "Steve Nosko"
wrote:
While not necessary for understanding this circuit, I'll fill-in this bit
here. A "clamper" is a diode and cap circuit which will clamp a particular
point on a waveform to a specific voltage (kinda like being clamped in a
vice on the bench), but NOT change the wave's wiggling *shape*. Sometimes
this is needed. In the coupling circuit in question, it is the POSITIVE
PEAK of the signal that gets clamped to +0.7 volts. The wave's SHAPE is
un-changed, but the whole thing is shifted in its DC component.
What you are thinking of is called a "CLIPPER" because it CLIPS *off*
part of the waveform like barber's scissors.
Gotcha, Steve. I won't forget that distinction in a hurrq! It\s
amazing after nearly 2 years of studying this subject that I didn't
tumble the true meahing of clamping. Duh!
you state (and the spice progs agree) that what *actually* happens in
this case is that the whole AC waveform gets shifted south into
negative territory. It's still a full wave, but it's way down into
the negative and only the highest peaks just creep above zero volts.
Is this effect *solely* attributable to the steady build-up of
negative charge on the cap's RHS?
YUP !
I think what's really freaking me
out here is the fact that the signal source is grounded on the neg.
side and yet we have that same signal that after going through a cap
can end up going fully negative *below* ground. It just seems like any
such voltage beneath zero/ground potential is breaking the laws of
physics. Ground should be the 'absolute zero' of the potentials in any
circuit and here it is being violated. I need some help to get my
thick head around the concept! :-(
Oooooo. BEWARE! GROUND IS NOT ABSOLUTE ! ! ! Nope, nope, nope. This is
going to take some time to explain and more experience/study will be needed
for you to really _get it_.
The bad news for you may be that ground, I must sadly inform you, is
relative. There is no one, solid, never varying, absolute thing which is
ground, except in our imaginations. Many hams believe there is, but there
isn't.
That being said, let's start out simply and build.
Here is a very applicable analogy:
Voltage, also called "potential difference", is a lot like altitude --
height. We can talk about the height above the street level. We might
consider the street level to be "ground". In Physics, moving some object to
a higher level gives it the "Potential" to do damage if it falls on your
head, so the "potential energy" of it is greater. Holding it three feet
above your head gives it a certain potential, right? Voltage is just like
this.
HOWEVER, what about standing on the roof of a building THEN moving the same
object three feet above your head. You must agree that it has the SAME
potential to do damage TO YOUR HEAD that it did in the first example, right?
In this case, the roof of the building is our ground. So ground is relative
and *we* get to pick it. It is usually a known point in our circuit and we
use special symbols to show it. Note that this is why voltage is also is
called potential *DIFFERENCE*.
Unfortunately, this analogy will fall apart when trying to use it for
negative voltage, if we put this negative voltage "Below" our ground, but
for the "relative" concept, I hope it worked.
Now I'll try *negative*.
Okay, Steve, I've snipped your explanation about negative voltages as
that wasn't quite what I was getting at. I'm familiar with the
analogos you used (which I'm sure will help ohters in future via
Google). I've done quite a lot of experimentng with opamps using split
supplies, hence I don't have a problem envisaging below ground
voltages where the supply is say +15V - 0 -15V. The problem I was
having was with below ground voltages in a circuit with only one
ground and one V+ supply! I'm gojng to have a good think about this
before posting back for further clarification. I'm not completly sure
that its all down to the cap alone. Mebe the diode has an effect on t
producing his sub-ground signal as well?
Steve
--
Fat, sugar, salt, beer: the four essentials for a healthy diet.
|