Coupling caps- can you explain them to me?

[Chuck H]

In reality I'm just a hack that builds amps. I've never studied electronics and I'm not even able to read calculus formulas because I never learned the language (though I see that changing in my future). And I just want to say to any greenie who has ever been frustrated trying to figure out just what the $%#& is going on inside a tube amp that I wish I had a post like this to read seventeen years ago when I was just starting out. Even with good mechanical and conceptual skills it's been a long road to being able to think outside the box on issues like 'electrons aren't water', the differences between DC and AC voltages and impedance vs. resistance. Some of the fundamental issues here could have saved me alot of time and effort (and painful shocks)

Thank you to Enzo and Steve who are our resident experts on this particular post. I look foreward to your corrections (even if I don't always handle it gracefully).

Chuck

[Steve Conner]

Don't get frustrated. I've got an engineering degree and was tearing my hair out just a couple of months ago trying to figure out transistors and having this same problem: what's moving in what direction?

I don't blame you! When it comes to junction transistors, they really are confusing. For a start, they use two kinds of charge carriers - electrons and "holes" - that carry opposite charges and flow in opposite directions at once. This is why they're called "bipolar" transistors, as opposed to MOSFETs and vacuum tubes, which only use one kind of carrier.

To make matters worse, they come in two varieties, NPN and PNP, that are exact mirror images of each other, with the electrons and holes changing places. (To be pedantic, the "N" parts of the device contain electrons, and the "P" regions contain holes. MOSFETs also come in P-channel, where the charge carriers are holes only.)

And as the final insult, schematics of old transistor circuits don't always follow the modern practice, of drawing the circuit so that more positive voltages are towards the top of the page, and conventional current flows from the top of the page towards the bottom.

According to this drawing convention, PNP transistors should always have their emitters pointing towards the top of the page, and the ground rail in a PNP transistor circuit should be at the top of the page, with the battery "dangling" from it by its positive terminal. I find schematics that don't strictly follow this practice almost impossible to understand, and often have to redraw them before I get it.

But, to mentally model the operation of a transistor in a circuit, all you really need is Horowitz and Hill's "Transistor Man". Except for PNP you have to glue his feet to the ceiling, as it were.