Confused about how output of transistor gets turned into AC

[Yonatan]

I'm working on my second build and have been doing a lot of reading to understand how a transistor circuit works, but I'm confused about how the output of the transistor gets turned into an AC signal.  What I understand is:

- Signal sent from guitar into transmitter base is AC
- But current can only flow in one direction through transistor
- So transistor is biased to get current flowing through it
- When AC current hits base of transistor, it lets proportionally smaller/greater amounts of current flow through transistor (collector -> emitter).  This change in current becomes the amplified signal

But it isn't changing direction any more (like the input signal was), though of course it does have a waveform in it.  So here is where I'm confused:  I think that capacitors block DC, so the output cap is now meant to filter out the AC current and block the DC current.  But isn't the output only DC, because the amplified signal isn't reversing direction?  Or is it still considered to be "reversing direction" relative to some new reference?

[defaced]

You may find the explanations in this thread helpful: http://www.diystompboxes.com/smfforum/index.php?topic=94883.0

I believe is a fundamental error in your thinking, AC just means an oscillating signal, though it often is, it does not necessarily have to be positive and negative with regard to zero volts (ground).  You can have AC but centered around a different voltage than ground.  Note the "vRef" in many schematics, that's setting up the whole circuit to balance the AC signal around the 4.5v vRef "reference voltage".  Why 4.5v?  It's half way between our 0 voltage ground and 9 voltage rail, so the signal can swing +/- 4.5v before it hits into a voltage rail.  If you had a 100v system, you'd likely want to use a 50v vRef for the exact same reasons. 

In an attempt to distill down R.G.'s excellent reply in that thread (which solidified my understanding of the topic, thanks again R.G.!) Basically, when you DC bias a transistor you turn it on to some degree.  When you pass AC into it, the transistor is turned more on (more current flows) and more off (less current flows) from that center bias point.  Cool, so current is changing in the transistor.  Now if you look at transistor amplifier circuits, there is usually a resistor in series with the transistor and the output signal is taken from the junction of the transistor and the resistor.  So what happens when current changes through a fixed resistance?  Ohm's Law says voltage has to change, V=I*R.  The change in current through a fixed resistance causes a change in voltage, and presto, your tiny change at the input of the transistor makes a big change at the output of the transistor and you have amplification - all because of Ohm's Law! 

[frank_p]

http://www.diystompboxes.com/smfforum/index.php?topic=75652.msg616803#msg616803

[Yonatan]

Thanks all for the replies!  I think that this is kind of where I was heading when I said "Or is it still considered to be reversing direction relative to some new reference?"  But I'll take a look at those posts.

[Gurner]

the current in your example is always heading in the same direction, it just gets modulated....ie less current ,more current, less current etc. I guess you could say the amount of current is reversing (in comparison to when the current  was max), but the direction doesn't reverse.

Put your finger over the end of a hose & turn the tap (a faucet for those who don't speak English) on halfway.....it might squirt 20ft...turn the tap full it'll squirt 40ft, turn the almost off, it'll dribble on your toes. Essentially, the water distance here is alternating  (relating to how much the tap is turned on/off) just like an AC Signal....yet the water always heads in the same direction.

[PRR]

I just dug a ditch across my land.

Water flows one way, down slope.

If I block and un-block the ditch, I get waves on top of the flow.

If I could block/un-block very fast (hundreds per second), they would be audible in the water.

If I stuck my ear in the water, my eardrums would block the steady flow of water from going right through my head, but I could still sense/hear the wave vibrations.

[Jdansti]

Tube (triode) and transistor amplification concepts are similar. Here's a good tutorial using a "water amplifier" analogy that helped me understand how tubes and transistors amplify:

Part 1

Part 2

Part 3

Edit: added pt2 & pt3