Why does a resistor from emitter to ground change everything?

[camojoe]

So here is a basic fuzz schematic that I made on a breadboard. When built exactly to what the schematic says to do, its a very synth like fuzz sound with no sustain (When I hold out a chord it cuts off after the volume gets quiet enough and when I play on the higher strings it won't even clip the sound.). So after studying other schematics I saw there was usually a resistor from the emitter to ground and what do you know, when I put one in, it made the sound a lot more like regular over drive and the sustain was much better, ALL FROM A SINGLE RESISTOR. Now me as a curious amateur who has no idea how this works wants to know why, my best guess is that the resistor prevents some of the current from flowing to ground so more of it goes out the collector into the feedback loop? Any thoughts are appreciated.

[antonis]

Now me as a curious amateur who has no idea how this works wants to know why, my best guess is that the resistor prevents some of the current from flowing to ground so more of it goes out the collector into the feedback loop?

Resistors doesn't 'prevent' anything..

Current flows according to VOLTAGE difference, so Emitter resistor simply raises Emitter voltage (in relation to GND) - Ohm's law..

It indeed acts in the form of negative feedback but it doesn't do it the way you describe..
(it raises linearity & input impedance and stabilizes Collector current fluctuation due to h_FE & V_BE variations)

Most important is drastic reduction of stage GAIN (as all forms of negative feedback do..) with all the obtained benefits..
(that particular posted circuit isn't the most appropriate for elementary Common Emitter amplifier analysis..)

You could obtain almost the same Gain magnitude in case of bypassing Emitter resistor via a large cap while taking advantage of DC state(bias) stabilization..
(see slides 22 & 23 below..)
http://www-inst.eecs.berkeley.edu/~ee105/fa07/lectures/Lecture%208.pdf

[Rob Strand]

Current flows according to VOLTAGE difference

For the OP, the voltage difference is the voltage between the base and emitter.

A "this causes that type explanation" is, for a given input voltage, the emitter resistor causes less AC current to flow in the base and so you get less AC current in the collector.   This thinking only gives you the idea not the 100% correct answer.  It's not like adding resistor in series with the base, that just reduces the Vbe voltage.    It's more than that due to feedback as antonis explained.

[mac]

Change the transistor, use a Darlington MPSA13/KSP13.
This circuit has a sweet spot when Vc=Vb, that is, when there is almost no idle current across the diode. But the diode has to feed the base too.
A darlington makes both things possible because of its very high hfe.

Some emitter resistance reduces voltage gain and increases input impedance. A nice addition as a gain control.

mac

[R.G.]

Because transistors.

Bipolar transistors (that is, not FETs) allow current to flow from collector to emitter if and only if the base-emitter is forward biased. How much current flows in the base-emitter affects how much current flows from collector to emitter.

If you put a resistor from the emitter to ground, current through the collector-emitter acts in opposition to the base-emitter current by raising the emitter voltage. So for the same base-to-ground voltage change, the base voltage can change much less; the emitter voltage due to that resistor rises and removes some of the effective base-to-emitter voltage that is pushing current into the base. It's a form of feedback.

In this circuit, the base is driven by a DC current from the diode to the collector and some AC voltage from the input capacitor. The base is allowed to have all the current it wants, until it pulls the collector below the base-emitter forward voltage plus the diode voltage. At that point, the base is lowering the collector voltage so the diode current into the base is starved, so the base can't pull it any lower. This is another form of feedback. The collector voltage balances at one Vbe (about 0.5V) plus one diode drop (at these low currents, probably another 0.5V +/-) and stays there.

An input voltage from the cap either eats some of the diode current into the base or pushes more current into the base. With no emitter resistor, the input can turn the transistor on and off pretty quickly. With an emitter resistor, the effect of the input voltage is "softened" a lot.

[camojoe]

Gotcha, so the resistor from emitter to ground lowers the DC current from entering back into the input which makes it more consistent and linear, but it also lowers the gain because the input voltage is softened since the emitter voltage has risen?

[antonis]

Hmm.. I presume a further analysis should implement small signal transistor equivalent model (T, π or hybrid h..)

But, yes.. You can see Emitter resistor as a "backlasher" to anything happens without its presence..

[PRR]

> further analysis should implement small signal transistor equivalent model (T, π or hybrid h..)

No. For this forum's purposes, the Bus Driver model can explain all.