[sdiy] Simple discrete Unity-Gain Follower ?

[Magnus Danielson]

From: Ingo Debus <debus at cityweb.de>
Subject: Re: [sdiy] Simple discrete Unity-Gain Follower ?
Date: Thu, 01 May 2003 14:31:05 +0200

> Magnus Danielson wrote:
> >>I do not want an output that easily overshoots or oscillates, or behaves
> >>strange. So feedback shall be avoided, if possible.
> > 
> > 
> > The benefit of feedback should not be underestimated just because there is
> > additional concerns, it's only about appropriatly applying the technique.
> 
> Is it possible at all to make a transistor unity-gain buffer without 
> feedback? Somehow you have to get the gain down to 1.
> 
> An emitter follower sure does have feedback. The voltage drop across 
> the emitter resistor is in series with the input voltage.

I think the important difference here is the distinction between inherent
feedbackloops and explicitly designed feedbackloops.

The emitter follower has a form of inherent feedbackloop which you can control
throught the emitter resistor, it also has a capacitive feedback look from the
collector to the base (called quite simply, the Miller capacitor).

These forms of feedback loops is also something you have to learn to deal
properly with. For instance, there is a temperature feedback loop which you
should be aware of since you can get thermal runaway problems if your emitter
resistance is too low.

But sure, your point that there already are feedback loops in place is correct,
and this only proves my point that you need to apply the technique
appropriately. Feedback loops is a mixed blessing, they can solve alot but they
can create a even greater mess if you don't pay attention to details. So, like
all tools, use with insight and care, be it by template designs and streamlined
design guidelines or by analysis and calculation.

You can even fluke things up with a resistor if you don't understand it
(think 0.1 Ohm resistor into the poweroutlet).

Cheers,
Magnus