Some nooby schematic questions

[LukeeeeBennett]

So, as some of you know, I'm very nooby to the electronics world, but I'd love to learn a lot to do with it that my Physics course doesn't supply me with.

So let's kick it off with this:
**broken link removed**

This is what I aim to learn to read and fully understand before my gig at 7ish. (GMT)

I'll start with, which way is the current going throughout this? I'm getting confused because there's an input with is giving the circuit a signal from a guitar for example and there's a connected crossover. It's all a bit confusing for a first-timer. (It'd be nice if someone could draw which way everything is flowing)

Next, I don't really understand the grounding.
So, at the transistor, the emitter is connected to ground? Which surely means that the signal that is collected and then emitted is being emitted into ground where it's lost? When really I want it to go to the output?

Then, I was speaking to the almighty 3v0 and he was saying how he wouldn't really claim the 830k, 100k, 10k and 360Ω resistors in series or parallel. I think that when I understand the flow of the current in the schematic I may be able to understand why, but for now, I'm stumped.

I feel really bad for asking, but a lot of you guys seem to love using your vast knowledge to help people.

Thanks a lot.

 

[Diver300]

Please do not try to relate the names of the transistor pins with what they do. The names are largely historic, and may relate to what happens inside the transistor at an atomic level, and have no relevance to the overall behaviour of the transistor.

The design is a bit marginal.

The 830k and the 100k resistor set the voltage on the base of the transistor to about 1 V. That is the average voltage, and the signal will make the voltage go up and down from there.

All silicon transistors have a base-emitter voltage of about 0.6 V, so the voltage on the emitter is about 0.4 V. That gives about 1 mA flowing though the 360 ohm resistor. That all has to come from the transistor, and in turn from the 10 k resistor.

That would imply a voltage drop of about 10 V in the 10 k resistor, which isn't possible. That is what I mean about it being a bit marginal.

Anyhow, what I have just said implies an infinite transistor gain and no base current, and in the real world it will be less, so there will be a base current and the base voltage will be a bit lower, the emitter voltage will also be lower, the emitter current a bit lower and the voltage drop in the 10 k resistor may be only 8 V, which would be fine.

When there is a signal, the base voltage varies, which changes the emitter voltage. That makes the emitter current change. The collector current is always nearly exactly the same as the emitter current, so the collector current changes as well. The change in collector current results in a change in voltage drop on the 10k resistor, so the output voltage changes.

The gain is about -10000/360 = -30 times, with the potentiometer at its maximum.

 

[LukeeeeBennett]

Ahh! Okay! So, the signal is doing this:
**broken link removed**

Then, however how much the base varies it will take more or less of the signal? Or have I missed it again?

 

[ronv]

Amplifier

Maybe a picture will help with the explanation. The circle is just the input like an IPOD or something.

 

[LukeeeeBennett]

Ahh! That helps a lot! Thanks.

 

[audioguru]

It is for an electric GEEtar so of course the circuit is supposed to be biased wrong so it distorts like mad!
Since the value of the emitter resistor is so small at 360 ohms then each transitor will work differently depending on its hFE.
The low value for the emitter resistor causes even the temperature of the circuit to affect how much distortion it creates.

 

[KeepItSimpleStupid]

Your signal path is not good. Capacitors pass AC (the signal), and not DC (the bias). The signal modulates the base in the form of a current.

A BIGGER current modulates the current flowing from the collector to emitter which causes an AC+DC componect at the collector. the capacitor isolates the DC component.

 

[carbonzit]

Rather than muck things up with explanations that are far over your head (apparently to show off how much the respondent knows, instead of trying to actually help you, god forbid), I've drawn this rather crude sketch of the signal flow through your little fuzzbox or whatever it is. Blue is the input signal path, magenta is the output path:

**broken link removed**

It helps to think of the transistor as a valve here, with the input path acting as a controller of the output path.

This is actually a vast oversimplification, but it's good enough for a first order explanation.

If you're really interested in this stuff, it might actually help to start by finding out how an old-fashioned "valve" (= vacuum-tube triode) works, which is probably easier to grasp than transistor action, which is pretty tricky. Lots of explanations out there on the web. (I'll post some links later after I have time to find one.) That will show you how a small current/voltage (the input signal) can control a much larger current/voltage, resulting in the magic called "amplification".

 

[KeepItSimpleStupid]

Nice work carbonzit.

 

[carbonzit]

Nice work carbonzit.

Thanks. Now you get to explain biasing, operating points, common-emitter, swamping, transconductance, etc., etc., to the O.P.

Jes' kidding ...

 

[KeepItSimpleStupid]

I can skip all of that and explain everything with magic smoke, right?

 

[carbonzit]

Yes:

Rule No. 1: Don't Let Out the Magic Smoke