On Wed, 14 Dec 2005 20:58:40 -0600, Pete Holland Jr. wrote:
> Hey, Rich Grise!
> A question related to your response, and then a general question on the
> subject of transistors.
>> Then there's the kind of Earth ground that's a counterpoise for an
>> antenna. When the radio wave induces a current on the antenna, at
>> the end where the receiver is, the other input terminal is ground,
>> because for any current to flow, you have to complete a circuit.
>> In this case, the circuit is completed by the capacitance between
>> the antenna and ground; or maybe just the EM field - I drank too
>> much when I got to that class. ;-)
>> And then there's "circuit ground", which is just a common reference
>> point for the power supplies and signal return paths, and doesn't
>> have to be grounded, they just use the term by convention.
> I built one circuit. It was to show capacitors at work, storing and
> releasing electicity. I decided to test if the battery could be a regular
> ground by attaching the positive terminal on the charged capacitor to the
> LED and the negative to the negative terminal on the battery. As I'm sure
> won't surprise you, the LED didn't light up. I'm figuring both poles of
> the power source have to be in the circuit, you can't divide the polarity
> between two different sources. So I was still learning the limits when all
> the responses hit.
Well, the operative word here would be "circuit". There has to be a path
for the electrons/charge to go all of the way around, back to the source.
The reason "ground" works in this case is that that "ground" provides
a "return path".
In the water hose model, it looks like just cutting the hose lets all
of the water flow out, but the part of that analogy that's missing is
that all of the water flows out of the ground, down to the water table,
and is sucked up again by your well pump.
For electrons, you have to provide a path for them, because when you
cut a wire, it's more like pinching off a hose.
> But that brings me a question that has puzzled me since I was a kid trying
> to understand the concept of RF ground--as a kid, I had a little transistor
> radio (this was before IC's and stuff, around the early 1980's). Ground
> was supposed to be important, but where was it on the transistor radio? It
> was completely self contained, and encased in plastic, too. I took one
> apart, and couldn't figure it out. How do they do it?
Open the transistor radio again, and notice the "loopstick antenna". It's
a ferrite slug, either a cylinder about 3/8" (1 cm) diameter x 5" (12 cm)
long, or a rectangular solid, about 2.5" (6 cm) long, about 1/2-3/4" (12
- 20 mm) wide, and about 3/16" (~4mm) thick. Whichever of those it is,
you'll see that it's wound with hundreds, maybe thousands of turns of
wire. In this case, the "return" is simply the other end of that coil:
it's actually, conceptually, acting like the secondary of a very large,
nebulous transformer. (the ferrite slug picks up the H field - you'd
have to study some antenna physics, or maybe just a textbook: my
explanation falls way short.)
> The other question concerns what makes transistors necessary. One of the
> projects connects the two batteries, LED, and the collector and emitter of
> one transistor (the symbol shows the arrow on the emittor line pointing
> away from the base, not towards) in series in one part of the circuit. In
> the other, it is one of the batteries, the key (cheap but effective
> momentary switch), and the base of transistor in series. The LED doesn't
> light up unless the key is pressed. Thanks to Big Mouth Billy Bass, I
> completely understand why the circuit works the way it does, the question I
> have is why is a transistor necessary? Take a powered radio, for example.
> Wouldn't it be more efficient to simply attach the battery in series with
> the radio signal, boosting it? Or does the transistor act more as a safety
> barrier (receives the signals and passes them to the next part of the
> circuit without the two parts overloading or interfering with each other)?
It's because it can amplify. If all you want to do is light a bulb, then,
yes, just flip the switch. But to use a radio signal, the signal needs to
be amplified, because by the time it reaches your antenna, (with those
50KW distributed throughout millions of cubic miles of space), you're
picking up a signal that's in the microvolts range.
Here's a practical experiment: Connect a speaker to a lamp. Yell at the
speaker. See if it lights the lamp. Now, get a power supply, and put the
+ terminal to one lead of the lamp, put the other lead of the lamp to the
collector of your NPN transistor, and connect the emitter of the NPN to
the - lead of the supply. Connect the speaker directly from the emitter
to the base of the transistor. Now shout at the speaker. Please report
your results. :-)