[sdiy] Buffers on the LM13x00

[Ray Wilson]

I don't use the buffers because they are too low in impedance. The TL082's 
or TL084's are way way higher than the buffer inputs on the LM13x00s. So you 
get more accurate integrators by using the far higher impedance input op 
amps instead of the buffers. In VCAs it is not as critical because there is 
no capacitor/integrator involved so the buffers are very handy in that case. 
I would bet that the schematic drawing in the LM13x00 data sheet is somewhat 
simplified. There are probably more than just two transistors involved 
whereas the schem only shows the 2 trannie darlington configuration. 
(someone chime up if I'm off base... as if I have to ask :-). Typically you 
have to tie the output to -V through a low value resistor (around 5K) to get 
the output down to near ground.

Cheers

Ray

----- Original Message ----- 
From: "Aaron Lanterman" <lanterma at ece.gatech.edu>
To: "synthdiy" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, March 12, 2006 1:09 AM
Subject: [sdiy] Buffers on the LM13x00


>
> I notice in some filter designs, like the Korg MS-20, the LM13x00 buffers 
> are used; in others, like several of Ray's, the designer ops to ignore the 
> built in buffer and uses op amp buffers instead.
>
> What are the Pros and Cons of each approach?
>
> In particular, I can't figure out how some designs deal with the fact that 
> there should be a 1.4 volt drop through the Darlington buffer. The designs 
> just look like they ignore the drop altogether.
>
> - Aaron, currently with a case of insomnia
>
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>
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> and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
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