Re: quick emitter follower question

John Larkin wrote:
On Thu, 14 Sep 2006 18:50:39 +0100, Eeyore
<rabbitsfriendsandrelations@xxxxxxxxxxxxxxxxxxxxxx> wrote:



John Larkin wrote:

On Wed, 13 Sep 2006 20:20:16 +0200, "Ban" <bansuri@xxxxxxxxxxxx>
wrote:

tempus fugit wrote:
Hi;

I'm thinking about using a simple 1 transistor emitter follower
in an audio design (the audio signal will pass thru it). Does the
noise figure of the transistor need to be taken into account, or
will there be no noise added since there is no amplification
happening (sort of like the signal just 'passes thru' the
transistor untouched)?

Thanks


Not only will the transistor add some noise, it will also add a
lot of distortion. around 1% for 600mV peak to peak. The
distortion raises proportional to the input level and is
independent from the bias point. The noise is completely
irrelevant compared to this phenomenon. If you want the signal
"untouched", you should use an opamp as buffer, OP27 comes to mind.

What's the mechanism? With enough DC voltage across the emitter
resistor and a light load, one can approach constant emitter
current. All that's left is Early voltage effects, pretty small
usually.

To drive the output load the stage needs a varying current, which in return
requires a slightly different Vbe. The smaller the load, the more the
variation in Vbe, which has an exponential characteristic. The inherent
feedback helps linearizing, but for lower R_load and higher capacitive
loading distortion shows up immediately.


You could also use a constant current load too.

Graham

That would be a cure, but then any load impedance will annulate the effort.


Even better. At 10 mA emitter current, output impedance will be under
3 ohms, so a reasonable load will hardly pump the b-e junction
nonlinear.

John

The output impedance is (1/gm + Rg/Hfe)||Re. With 50k input impedance we
will hardly come below 100R.
So even if the simple follower has a very high input and low output
impedance, its properties cannot be used simultaneously. Those disadvantages
can be overcome with some more sophisticated circuitry like the
White-follower or this differential input stage:
+---+------o
| | +Vb
/ \ |
( 1m) |
\_/ |
| |
| |/
+-|
| |>
| |
.---)---+ out
| | +------o
|/ \| |
o---| |-+
|> <| |
| | |
'-+-' |
| |
.-. |
| | / \
| | (10m)
'-' \_/
| | -Vb
+-----+------o
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
But for driving long cables I would recommend rather the opamp buffer
solution with a complementary output stage.
--
ciao Ban
Apricale, Italy


.

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