[sdiy] Simple discrete Unity-Gain Follower ?

[ASSI]

On Wednesday 30 April 2003 17:11, Michael Buchstaller wrote:
> OK, i can just use an Opamp with the "-" input tied to the output to
> buffer the signal, but i would like to make a discrete transistor
> output stage. An Emitter follower won´t do because of it´s Offset and
> unequal push/pull currents.
>
> Any ideas or pointers to a simple solution ?

By far the simplest solution would be to go for power opamps, 
preferably with fully protected outputs. Make sure they are internally 
compensated for unity gain already.

If you have lots of discretes around and don't mind to mix and match, 
look at the internal circuits in some datasheets, lot's of opamps have 
an explicit output buffer stage that you can try to mimic with discrete 
transistors and diodes. Just boosting the output with complementary 
external transistors is also possible if you put the feedback around 
the entire opamp+booster structure _and_ independently provide fast 
feedback from the opamp output to prevent oscillation. I would also add 
emitter degeneration resistors. To make the circuit sufficiently linear 
you will probably want to bias into mode A operation from the supplies 
and add some diodes for levelshifting. Looking at some discrete power 
amplifiers should get you the right ideas or dig up the "OpAmps for 
Everyone" from http://www.ti.com (slod006a.pdf) and crack open chapter 
11 and 14. The "700 Series Design Manual" that you can download from 
http://www.arraydesign.com also shows a few simple opamp output stages 
in chapter 5. 

> It needs not to deliver lots of current, but should be capable of
> driving several 600 Ohm inputs in parallel. (say: 150 Ohms input
> impedance)

The bigger problem will be the possibly large capacitive loading, so 
you better make sure you compensate the buffer correctly for this. 
There is a short discussion of the technique in chapter 13 of "OpAmps 
for Everyone", but by far the best reference on that IMHO is in the 
book "Operationsverstärker" by Jiri Dostal.


Achim.
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