[sdiy] simple transistor buffers?

[lanterma at ece.gatech.edu]

On Nov 19, 2010, at 11:40 AM, Speth, John wrote:

> If I were you, I'd go with a FET and not a BJT.  I think an FET is a little simpler to apply.

With a FET you don't need to worry about the voltage drop; it also has much higher input impedance so it won't load down things before it so much.

The Polyfusion VCF shows examples of FET buffers:

http://www.experimentalistsanonymous.com/diy/Schematics/Filters%20Wahs%20and%20VCFs/Polyfusion%20VCF.jpg

The Buchla Music Easel VCF and and early version of the 292 (version B) use FET buffers, although it should be noted that later versions of the 292 use regular op amps. (Probably his earlier use of FETs had to do with the lack of availability of op amps with really high-impedance FET inputs).

The Buchla 291 has more sophisticated buffers made of FET pairs, although Mark Verbos showed that this worked fine when replaced with op-amp buffers (and one of my students built this as a project on the breadboard and confirmed this).

- Aaron