Water and Plumbing Anology to Describe Voltage and Current Thru a Transistor by Grey Rollins

Graphic Representation of Voltage and Current

Link provided by Zen Mod- https://www.bcae1.com/


All right, let's get the quickie out of the way first. The output cap on the Zen blocks DC and lets AC pass. We'll come back to that in a moment.
As for water, we'll start with an ordinary garden hose. It's got one of those pistol grip thingys on the end. (Don't you just love it when I talk technical?) Turn on the water at the faucet. Assuming that you're not squeezing the handle, no water flows, right?
Let's evaluate the situation as it stands right now: We have pressure (aka voltage), but no water is actually flowing. In electrical terms, we have no current. There's water in the hose, ready to flow if only you'd let it.
So what are you waiting for? Let it flow.
Squeezing the handle part-way allows water to flow. You now have pressure/voltage and flow/current. Be careful with the term current because water can have currents too, and it'd be easy to muddy the waters (ahem).
Let's look at this in a little more detail. If you were to use a meter to measure the water pressure inside the hose at this moment, you'd probably find that the pressure has dropped by a small amount. To put this in test bench terms, you'll generally find that the rail voltage drops a bit when you hook a circuit to the power supply. It's pretty much the same thing.
The brass pin that moved when you squeezed the handle went from stopping the flow entirely (infinite resistance) to a more moderate resistance. The flow rate (current) is determined by how much the valve restricts (resists) the pressure (voltage).
So you can have voltage without current. It's the potential to flow. If someone in the house flushes the toilet, the potential...the water pressure in the hose will drop. If the resistance remains the same, the current will fall too--just like in electronics.
Your hand on the handle controls the water flow. You need not get your hand wet. The handle is analogous to the Gate. The voltage at the Gate controls the current between the Drain and the Source without ever getting its fingers wet, if you follow what I'm saying.
It's easier to visualize this in electronics with a tube or a JFET.
You know how your hair sticks to your comb on a dry winter morning? That's static electricity. Like charges repel. Opposites attract. You've heard that. Let's take a look inside a tube for a moment.
The cathode (think: Source) emits electrons. They float around the cathode in a cloud looking for something to do. They're bored. They want to go somewhere. We'll give them a destination by giving the plate (think: Drain) a positive charge. Opposites attract. The electrons, which are negatively charged, start towards the plate/Drain.
But wait...
The grid (think: Gate) stands in the way. And it's got a negative charge. Like charges repel. The electrons are repelled.
So we vary the negative charge on the grid and by doing so we allow the plate to draw more or fewer electrons to itself. Ta da! We have managed to control the electron flow and create a signal at the plate. The operative point here is that it was done with a static charge. No current flows though a grid. Lacking electron "flow" i.e. current, it has effectively infinite resistance.
A JFET works pretty much the same way, but it's easier to visualize electrons flowing through space than through a solid. At least for me it is. So I used a tube for the example.
Back to the capacitor, but keep the static charge thing in mind. You're got two plates staring at each other across a gap. Put a charge on one side (let's say it's negative) and add our magic formula (like charges repel) and Presto!...electrons (the negative guys) flee from the other plate, out into the wire and on towards their destiny. Now if things stop right here (in your mind, play a sound effect of brakes screeching)...you have a DC charge on one side and a single, sharp pulse coming out the other side of the cap. Once that pulse (the amp might go BUMP at turn-on, for instance) is gone, no further current flows. Why? Because those bad-boy electrons on one side scared off all the electrons on the other side. There aren't any more to scare. Well, okay, there are, but let's leave that alone for the moment.
So in other words, caps block DC.
Now, if you change the first plate's charge to positive, then all the electrons come running back and current flows again. And then you change the first plate to negative and they run away, positive and they come back, and so an AC signal can flow through a cap, but DC can't excepting that BUMP at first, which if you think about it, changing from 0V to some steady voltage like, say, 10V, does constitute change, hence AC, at least for an instant. Grey

JFets

By the way, if you want an image of how a JFET works, just squeeze the garden hose with your hand. (Yes, I know it's difficult...eat your Wheaties, you wimp.) As you squeeze and release the hose, you modulate the water flow. Simple, eh? Grey

Question:

Ok, so the Gate voltage doesn't control the Drain to Source voltage directly. The Gate voltage opens and closes a "valve" in the transistor that in turn controls the Drain to Source voltage. Is that correct?
Is this the misunderstanding where people think electrons come from the positive side, when in fact they are coming from the negative side and run to the + side?

So, as the electrons flow from the Source to the Drain; the lower Gate voltage (AC?) "controls" the higher DC voltage, raising and lowering it. Does this turn Source to Drain into a AC pulse to simulate a Sine wave?

You reverse the whole thing with P-channel Fets or PNP, but there's only N-type tubes, right?

Also, like in the Zen amp, 35v DC is the potential voltage, if you opened up all the way. Correct or not? The resistor then, as a current source, just controls the amount of current you want that part of the circuit to have. Is this correct or not?

Answer:

1- Yes. Note, however, that bipolars function differently. In a bipolar, current actually flows through the base. This bothers me greatly because deep down inside I'm programmed to think that grid (think: base) current is a bad thing. If you've got grid current flowing, you're on the verge of disaster--your plate glows bright red and the whole thing goes POOF very quickly. The fact that it's entirely normal for a bipolar just doesn't sit well with me. That's one reason that you haven't seen a lot of bipolars in the circuits I've posted here. It just ain't natural for current to flow through the control pin of a gain device.

2- It's kinda like hot and cold. Hot exists. It refers to a material object with a lot of thermal energy. Cold doesn't really exist, per se, it's just a convenient word we use to describe something that is "less hot."
Likewise, there's negative, which means that there are electrons available, and positive--which isn't really a separate thing; it's just a convenient word to say "less negative."
(Yes, pendants could argue about this for ages, but it's not an inaccurate way to frame things, seeing as how protons aren't mobile in the electrical sense. Plasma isn't part of our audio-related game plan.)
The whole thing about which way current flows gives me a headache. You want to know the truth? I ignore all those silly arrows on schematics, at least in terms of current flow. To me they are just abstract symbols with no real meaning. If I start paying attention to them I get confused. I think of it in the purely physics sense of where the electrons actually are and where they're actually going. I can't bother my poor feeble brain with some arbitrary fiction about current flow. It just bothers me to no end.

3- If it helps, you can think of the signal as varying DC.
(To pendants: SHADDUP!)

4- Right. Tubes only come in the N flavor, because of the aforementioned lack of mobility of protons. Solid state goes at things a little differently and so you can have P-ch devices which act like upside-down N-ch devices.

5- I'm holding a bottle in one hand trying to get Wyatt fed and typing with the other. Don't have a Zen schematic handy. I'll try to get to this one later.

Wiki posted by Vince Di Nenna

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