A (hopefully) Simple Input Capacitor Question

[swinginguitar]

Consider this schem:



The textbooks always say that caps block DC but pass AC. In reality (as I understand it), no current passes through a capacitor, but AC swings around it (charge/discharge).

That being the case, in the schematic above, does the AC current (guitar signal) actually alternate between the terminals of C1 via the ground rail (sleeve of input jack to ground, tip lug to the cap)? I don't quite follow how the AC signal travels through the circuit. Same for C2 on the output...how does AC get through there?

Further: As I understand it, an amplifier stage (a transistor in this case) has DC running through it (a 9v battery for instance), and is biased halfway (4.5v for instance), and the input signal modulates that DC up and down (between 0 and 9v for instance). Right? So we're actually "alternating" in the sense that the voltage/current rises and falls, but not actually "swinging" back and forth in the circuit i.e. changing directions physically...

[earthtonesaudio]

Current moving on one side of a cap causes current to move on the other side, but in between something called "displacement current" is happening.  It has to do with moving charges generating a magnetic field within the vacuum or dielectric of the cap.

[merlinb]

The textbooks always say that caps block DC but pass AC. In reality (as I understand it), no current passes through a capacitor, but AC swings around it (charge/discharge).

Electrons do not pass through the capacitor, but as far as practical electronics is concerned, AC current (charge) does flow through the capacitor. Subatomic physicists can keep their cake to themselves...

Further: As I understand it, an amplifier stage (a transistor in this case) has DC running through it (a 9v battery for instance), and is biased halfway (4.5v for instance), and the input signal modulates that DC up and down (between 0 and 9v for instance). Right? So we're actually "alternating" in the sense that the voltage/current rises and falls, but not actually "swinging" back and forth in the circuit i.e. changing directions physically...

Correct. The current in the transistor only flows in one direction, but the amount of current can change. The collector voltage swings between a high and low value, in sympathy with the base voltage (ok, in 'inverted' sympathy if you prefer). In electronics theory we can imagine this to be an AC signal superimposed on an unchanging DC signal, so on paper we can break the circuit into two 'equivalent' versions. One showing just the DC operation, and the other just the AC operation, depending on what we want to to figure out. After a while you will get used to thinking about circuits in these two ways, and continually swapping between them.

[swinginguitar]

the bulb over my head is now dimly lit...i'm getting it.

so my visualization of current changing directions physically in the circuit was wrong...it sounds like what you guys are saying is although electrons don't move through a dialectric, when an AC signal hits one side of a capacitor, the other side gets "pushed" as well (but it's not electrons moving through the circuit)...aka displacement current..?

maybe analgous to striking a drum head and displacing air molecules on the bottom of the drum? (compression/rarefaction)

[swinginguitar]

The collector voltage swings between a high and low value, in sympathy with the base voltage (ok, in 'inverted' sympathy if you prefer).

Why inverted?

[CynicalMan]

Further: As I understand it, an amplifier stage (a transistor in this case) has DC running through it (a 9v battery for instance), and is biased halfway (4.5v for instance), and the input signal modulates that DC up and down (between 0 and 9v for instance). Right? So we're actually "alternating" in the sense that the voltage/current rises and falls, but not actually "swinging" back and forth in the circuit i.e. changing directions physically...

The input current and output current change direction, but you are correct about the current for the rest of the circuit always going from ground to power. The amount of voltage and current change but it's all in the same direction, except for the input and output. For a good visual representation, check this out: http://www.falstad.com/circuit/e-ceamp.html

[merlinb]

Why inverted?

I just mean that in this circuit when base voltage increases, collector voltage decreases, so the output signal appears inverted,

[swinginguitar]

CynicalMan - thanks for the link - the animation is what I was looking for.

A final question along these lines with respect to the animation and conventional current versus electron flow:

the animations show electrons moving from the (+) side of the power source, into the collector, and out of the emmitter to ground. in reality, are the electrons flowing from the (-) side of the power source and up, or is what is shown the reality of electron flow with the polarities shown backwards for the sake of math, or....

[CynicalMan]

In reality, yes, current flows from negative to positive. But conventional current flow, from positive to negative, is often used. Firstly, it's a historical convention from before it was known that current flowed from negative to positive. Secondly, it's often easier to think of current as flowing from power to ground, and most power supplies are positive with respect to ground.

[merlinb]

the animations show electrons moving from the (+) side of the power source, into the collector, and out of the emmitter to ground. in reality, are the electrons flowing from the (-) side of the power source and up, or is what is shown the reality of electron flow with the polarities shown backwards for the sake of math, or....

The animations are showing current, not electrons. Electrons are just theoretical, subatomic equations- they are not particles in the sense of grains of sand. Quantum probability I think is the term. Neither waves no particles, just like light. There is no point trying to work out how a circuit works in terms of electrons.

What matters is charge, and you can imagine charge flowing in either direction, it makes no difference. Ben Franklin decided over 100 years ago that current would be positive charge flowing from positive to negative, and there is no reason to change this for an equally correct system of negative charge flowing in the opposite direction.

[swinginguitar]

...but doesn't direction matter with components like tubes, diodes and transistors? wouldn't they behave differently if the 'current' were reversed?

i understood everything up to that last one...charge/current/electrons...grrr

i always pictured electrons as particles being exchanged (a chain of atoms exchanging electrons from outer shell to outer shell)

and i'm breaking down on the positive convention thinking about 12ax7's for instance - heat applied to the filament causes thermionic emiision ("up") through the tube. How does the positive current analogy work there...current flowing ("down") from plate to cathode...?...whew!!!

[CynicalMan]

i always pictured electrons as particles being exchanged (a chain of atoms exchanging electrons from outer shell to outer shell)

For our purposes, we don't really need to worry about quantum physics. That model is good enough, and we can think of electrons as flowing from negative to positive.

i understood everything up to that last one...charge/current/electrons...grrr

From wikipedia: "A flow of positive charges gives the same electric current as a flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention for the direction of current which is independent of the type of charge carriers is needed. Therefore the direction of conventional current is defined to be the direction of the flow of positive charges.

In metals, which make up the wires and other conductors in most electrical circuits, the positive charges are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron motion in a metal conductor is in the direction opposite to that of conventional (or electric) current."

[merlinb]

i understood everything up to that last one...charge/current/electrons...grrr

The Wikipedia explanation is a good one. The point is, its not the electrons that matter, its the charge. Since charge is just a number, not a real 'thing' or particle, we can either label it positive and say it flows in one direction, or label it negative and say it flows the other way, it makes no difference. Just pick one and stick to it. If you have any sense you'll want to pick the same one as everyone else, i.e., conventional current!

(Another reason why trying to think of what the electrons are doing is kind of a waste of time, is that they travel at different speeds in different parts of the circuit! From snail's pace in a piece of wire, to kilometres per second in a triode!! How much more confusing is that...)

[fpaul]

For a good visual representation, check this out: http://www.falstad.com/circuit/e-ceamp.html

Thanks for posting this, good stuff. 

As a chemist I have to say I vote for the negative to positive flow since it's the electrons that are actually moving.  Just my opinion, but if Ben had picked that from the get go there probably wouldn't be the need to have two different conventions with explainations of why. 

[swinginguitar]

For a good visual representation, check this out: http://www.falstad.com/circuit/e-ceamp.html

Thanks for posting this, good stuff. 

As a chemist I have to say I vote for the negative to positive flow since it's the electrons that are actually moving.  Just my opinion, but if Ben had picked that from the get go there probably wouldn't be the need to have two different conventions with explainations of why. 

Agreed - I have to visualize something to understand it....and it's not working for me here 

Those animations were very helpful though...anyone used that tool for your own circuit design/analysis?

[CynicalMan]

If you want a sim for designing and analyzing circuits, go for LTSpice. That sim is only really useful for seeing the current.

[PRR]

> is what is shown the reality of electron flow

Show me an electron. I've never seen one, much less seen which way it was moving.

There's a whole book There Are No Electrons. Some of his logic is doubtful and some of his illustrative interludes are over-blown, but it is still a very good book.

BTW, I have done the experiment where you float charged mist between electrodes and adjust electricity against gravity. The result is steps of charge, experimental evidence for an indivisible low-mass charged particle. I accept "electrons" as a useful theory, with possible truth. The action of a thermionic (vacuum tube) cathode is explainable with loose electrons. Accepting this idea, it is clear that in a Vacuum tube the electrons move from negative cathode to positive plate. BUT when I sketch I always draw the arrow top to bottom, + to -.

Electrons are Negative. If you do the math using electrons, there's "-" signs EVERYWHERE. This is real brain-pain, and invitation for error. Whereas if you assume positive charge going the other way, "it makes no difference" except the math is far easier.

There is no electron (or protron) flow through a capacitor. But you sure can have current flow. You just can't have sustained current flow. You can push more electrons onto one plate, which repels electrons off the other plate and around the circuit. There's enough loose electrons in any conductor for a large current, but not for very long.

> we're actually "alternating" in the sense that the voltage/current rises and falls, but not actually "swinging" back and forth in the circuit i.e. changing directions physically...

Your guitar string, speaker cone, or air molecule has a "rest" position, and swings BOTH ways from there.

Tubes, BJTs, FETs do NOT "swing both ways". They only swing one way. Tube plate more positive than cathode, etc.

How do we pass a bidirectional wave through a unidirectional device? We add the bidirectional wave to a larger unidirectional "bias".

To level a site for house or road, the surveyor holds hand up or down to tell the stickman what to do. "up or down" relative to what? Below ground is awkward and can't be seen, so relative to shoulder. But what if there is a hill in the way? The surveyor can hold up a rod which can be seen over the hill. He may hold it at 10' for "OK", 8' for "low", 12' for "high". The stickman observes the 10' and ignores it, watches for the +/-2' deviations from 10'.

You have a signal swing +1V/0V/-1V. Add 4.5V to this. You now have +5.5V/+4.5V/+3.5V. All unidirectional.

At the output, the next box may not like the 4.5V bias. So subtract 4.5V. You have your +1V/0V/-1V bidirectional swing.

If you knew the "4.5V" was exact, you could use a 4.5V battery for subtraction. But that's awakward, and we never have exact bias. If you use a capacitor, at turn-on it passes a thump from the 4.5V bias, but only as many electrons (charge) as it will hold, then the cap charges-up to whatEVER the average DC voltage is. On the plan at the top of the thread, C2 has the left end at average 4V or 5V DC. The resistor VR1 drains the right end of C2. After infinite time the DC voltage here will be infinitely close to zero. In much less than one second it will be close-enough to zero DC that it won't much bother the next box. (After about 10*C*R seconds it will drain to 1% or about 0.044V DC.)

> As a chemist I have to say I vote for the negative to positive flow since it's the electrons that are actually moving.

Yes, but "reality" (or theoretical approximation) is often awkward for engineering. When air and gasoline burn, the "true" pathway is hundred of ephemeral fire-molecules. Each mediated by electron-cloud entanglement, but you don't write-out the electrons. For most practical cases we only want the long-term result CO CO2 HO H2O N O NOx + HEAT. While that is (in practice) a one-way reaction, many reversable reactions are complicated yet often handled with simple notions.

[merlinb]

As a chemist I have to say I vote for the negative to positive flow since it's the electrons that are actually moving.  

That only works if you're willing to imagine electrons as little particles, like ball bearings. In reality we know they are not particles, nor are they waves, they just act like one or the other some of the time. In fact, in a few years we may find string theory completely overwrites the idea of particles! Like the book says, there are no electrons...

Additionally, I still want to know how anyone who thinks in terms of electron current gets around the mind-bending idea of the electrons travelling at massively different speeds and densities in different parts of a circuit, and that for AC they do not flow at all, they simply vibrate. How can that possibly make it easier to visualise circuit operation

if Ben had picked that from the get go there probably wouldn't be the need to have two different conventions with explainations of why

We already had the 'why'! Positive charge moves from positive to negative- simple! The confusion only arose when someone discovered some annoying negatively charged particles (or is it waves  ). You only prefer electrons because you think they are like little marbles, which in reality they are nothing like at all 

[fpaul]

You only prefer electrons because you think they are like little marbles, which in reality they are nothing like at all 

Actually I prefer electrons because in a metal structure they are free to move.  It helps me to visualize them flowing in the circuit like marbles , especially since a lot of textbooks use that simple model.   I am familiar with clouds and uncertainty.  That helps me visualize bonding, not really helpful with current.  I'm simply stating my preference.  If you don't like it, don't use it.

[merlinb]

Actually I prefer electrons because in a metal structure they are free to move.  It helps me to visualize them flowing in the circuit like marbles

But the electrons are just a picture you have created in your imagination, an analogy for something we don't yet understand. So why not imagine some other made-up particle, or hole, or whatever, flowing in the other direction? That way your mental image would agree with all the current arrows on the schematic.