Role of output series resistor in output buffers

[shredgd]

Hi everyone,

I always see a small series resistor (from the typical 100 ohm of the TS808 to 1k in some Boss pedals) at the emitter output of bipolar output buffers or at the output of opamp buffers in guitar pedals, but I can't find any information about its role in the circuit, or its implications tone-wise. On the contrary, every educational schematic that I've come across about emitter followers or opamp-based circuits omits that series resistor.

Does anybody know anything about that?

Thanks in advance,

Giulio

[anotherjim]

It's just there to limit any fault current to a safer value than it would be without it. 100R is small enough to have zero effect on level or tone in normal use.
With op-amps, it also has another use. An op-amp is doing it's best to keeps it's output following what the input and feedback network tell it to do. The capacitance of a cable connected to the output can change the amplifiers control response, enough that it gets unstable and bursts into oscillation. The 100R slows the reaction to the cable capacitance enough for the amp to stay in control.

[antonis]

There are some different settings but - despite of them - it's a current limiting resistor..

If it's placed before a shunt resistor (maybe with a series capacitor..) they form a voltage divider with the impedance of whatever is pluged on...
(or else, emitter's resistor would be straightly dominated by following stage's input impedance..)

For op-amps, it "protects" them from oscillation due to stray capacitance of whatever they are connected to...

edit:
Jim, grrrrrr...

[PRR]

http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

Hmmm. That does not actually explain the output resistor. Maybe R.G. had it in notes that didn't get on the page.

I had a case which may not have crossed R.G.'s path. I built a line driver. In a worst-case bench-test, I could reliably kill the transistor. A few Ohms added made it bullet-proof.

[DavidRavenMoon]

I built a line driver. In a worst-case bench-test, I could reliably kill the transistor. A few Ohms added made it bullet-proof.

It also protects the some op amps from having their output shorted.