[sdiy] simple transistor buffers?

[David G. Dixon]

> So i am working on a circuit where there are a bunch of outputs. to save
> space/opamps I decided it would be nice to use transistor buffers (emitter
> followers)

If they are precision CV outputs, then opamp followers are really the only
defensible option.  For audio circuits emanating from opamps (provided they
are not coming from a circuit which has compromised the normally very low
output impedance of the opamp, e.g. an ideal diode or rectifier, etc), the
output is already buffered.

> however when i switched to what i THOUGHT was a usable buffer, the output
> gain (versus opamp followers) dropped drastically.
> 
> let me see if i understand this properly. ( i dont think i do)
> 
> if i want to buffer an lfo that is running between + and - 12 volts, I was
> putting the oscillator wave through a 1k resistor into BASE. I was then
> putting the collector to +v. What confuses me is what to do with the
> emitter other than
> this is where i take the output and a resistor to ground or -v should
> connect here.
> 
> I am not sure if i should put emitter to GROUND or to -v through say a 10k
> resistor.

The emitter follower output will only go as low as whatever is connected to
the emitter resistor.  Hence, if you ground the emitter, then follower
output will not go below 0V.  This is because the transistor (presuming it
is NPN) can only source current, so the voltage at the emitter must always
be higher than whatever is connected on the other side of the emitter
resistor.  If you want the emitter follower to give output over the entire
range between the rails, then you must connect the emitter resistor to the
negative rail.  However, even then, the output will still ride 0.6V to 0.7V
below the input, which means that you will get clipping distortion if your
signal goes right to the negative rail.

Plus, if you take the base very negative (more than about -6V) while the
emitter is grounded, the breakdown voltage of the base-emitter diode is
exceeded, which degrades the transistor.  If you use grounded-emitter
followers (for positive-only signals), then you should probably put a diode
between emitter and base (pointing toward the base) to protect the
transistor from negative base swings.

(Ain't Horowitz and Hill grand?)