Can you help me understand the direction current flows in PNP circuits?

Started by Herr Masel, November 15, 2005, 10:05:23 AM

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Herr Masel

I'm not sure this is for this board, if not I'll move it. I'm doing more reading on electronics (though I've had less time since I've started studying). I've read some things before, now I started reading the technology of the fuzz face. What I don't understand is this:

Some beginner's electronics essays depict the flow of current as water going through the plumbing, which is how I've always seen it. In a NPN circuit, I would see the current coming from the input, coming in through the positive lead of capacitors, coming out the negative, coming in the base of transistors and coming out through the emitter and collectors. Then to the positive of the battery, out the negative and back to the input. But in a PNP circuit like a fuzz face things go "wrong". Say a fuzz face, from GEO: http://www.geofex.com/Article_Folders/fuzzface/fftech1.gif. You have to look at it starting from the negative side of the battery, but then for example how would the voltage feedback resistor pass the current from Q2's emitter to Q1's base, if the C is positive and the B is negative?? Shouldn't the input cap be set the other way around, to allow the signal to pass through, or are current direction and signal direction unrelated? My question is pretty simple and pretty complex at the same time, I hope I made it clear.

Pedal love

Quote from: Herr Masel on November 15, 2005, 10:05:23 AM
Shouldn't the input cap be set the other way around, to allow the signal to pass through, or are current direction and signal direction unrelated?

Current flows from ground up. Voltage is simply applied. Signal is a by product of current and voltage.

R.G.

QuoteShouldn't the input cap be set the other way around, to allow the signal to pass through, or are current direction and signal direction unrelated?
Current direction and signal direction are indeed not related. Signal is AC - alternating current - and it just wiggles back and forth. The signal MODULATES a DC current flow in the active device, so that the DC flow is always through the device, but signal flows in and out at right angles to the DC in the main device. the capacitors are a kind of grill that blocks the DC flow but lets the back and forth wiggling come through. Think of a cap like a plastic sheet separating two water tanks. No water passes from tank to tank, but you can push on the plastic and the push goes through.

In both NPN and PNP transistor circuit, the DC current flows in the direction of the emitter arrow on the schematic symbol.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

Pedal love


Herr Masel

Ok, I'm not sure I can visualize how signal can run alternating, ignoring circuit polarity, but logically it makes sense. Back to reading now. Thank yous.