Emitter follower - hows that work?

[ethrbunny]

Im wanting to put together a simple 2 channel mixer. In order to avoid loading down the inputs I was thinking of putting an 'emitter follower' at each and the combining the outputs in a pot to make a 'blend'. Simple enough? Is it the right direction? dunno.

In any event - Im reading what I can about the emitter follower and am wondering what is the purpose of the resistor from collector to base or from a voltage source to the base via a resistor. Is this to compensate for the diode drop across the gate->emitter? Often there is another resistor from emitter to ground. Is this to remove the DC added in the previous step?

Any pointers as to how to calculate these resistances would be appreciated. I think I have a notion as to where to start but it would be nice to get there in some reasonable time.

Be well.

[brett]

Hi.
I'm no expert, but then again emitter-followers aren't that complicated.
First, the principle is that if the transistor is operating as you want it, the emitter voltage will "follow" the base voltage.  "Follow" is used instead of "equals" because there is a constant 0.7 volts difference.  For a properly designed emitter follower, the voltage at the emitter is always 0.7 volts less than at the base.

Now, lets take an NPN transistor as an example.  Those resistors between the supply voltage and base, and base and ground do two things.
1.  They set the base voltage (so that the transistor is turned on), and
2.  They set the input impedance (so they usually should be big, like >200kohms)

For most applications, a base voltage around 3 to 6 volts is good, so that the emitter can swing a couple of volts either way without hitting 0V or 9V.  So, the supply to base and base to ground resistors should be approximately equal.  Appropriate values are in the range - 100k, 220k, 330k or 470k.  The main reason that they're not usually smaller is to keep input impedance up, and the reason why they're not too large is to keep noise down (larger resistors inject more noise).

You also mentioned the emitter resistor.  This keeps the resistor operating at a satisfactory current.  With a 9V supply, a value of 10k is often used, so that the idle current is about 1mA (a nice amount).  1uA would be too little and 1A would be far too much.

I hope that helps somewhat.

[Paul Perry (Frostwave)]

http://www.eee.bham.ac.uk/collinst/walkthroughs/index.html

[ethrbunny]

Thank you both v much! I am enlightened!

[ethrbunny]

One question - which NPN would be good to try? Typing "npn transistor" into mouser gives more responses than typing "quickie" into google.... :shock:

[R.G.]

Keen's Second Law: When in doubt, whip in a 2N5088.

[ethrbunny]

Id love to see a list of the other "Keen laws". That would be a sticky fer sure.

[zachary vex]

for many years (in my youth) my go-to npn transistor was the 2N2222A.  then it became the 2n3904, and then the 2N5088.  now i automatically reach for an MPSA18. 8^)  it's like a sharp-looking black suit... good for any occasion.

[R.G.]

Keen's First Law: Get it hot first.

[Satch12879]

There's a really good design example of a emitter follower for audio frequencies in The Art of Electronics.  Sadly no such a detailed runthrough for a JFET source follower...

[T-rask]

Hi all, first post on here, cause for some reason, I've come here for a while but never registered.

So in response, R.G, I agree with Ethrbunny and would like to know if there is a list of Keen laws out there anywhere, I've already stumpled on some in my travels, but to have them in one place would be nice.

If not one, start one?