[sdiy] 4049/4069 linear mode

Scott Gravenhorst music.maker at gte.net
Sun May 31 05:54:16 CEST 2009


Derek Holzer <derek at umatic.nl> wrote:
>I've been trying to understand how the 4049 and 4069 inverters work in 
>linear mode. Studying the Wasp Filter, Craig Anderton's 4049 distortion 
>(http://www.tonepad.com/project.asp?id=65) and the schematics which 
>Nicholas Collins included in his circuit-bending book, I'm still not so 
>clear on why they work.
>
>First off, are these essentially the same chip (aside from pin outs), or 
>does the 4049 give additional gain while the 4069 only reaches unity?

Either type can be used as a greater than unity gain amplifier.  Gain depends on the
resistors used (the same formula as an opamp).

4049 has an asymmetrical output current characteristic, 4069 has a symmetrical output
current characteristic.

You should know that calling this a "linear amplifier" is a cruel joke because it's
not linear at all unless you look only at the region of activity near 1/2(Vdd-Vss). 
the gain of these amplifiers is reduced when the output nears the rails.  This
characteristic presents as a soft clipping.  It can be rather nice.  (Wasp filter uses
this fact).

>Second, why does this happen? For gain less than unity, I can see that 
>adding the inverted signal to the original signal via a feedback 
>resistor could create attenuation, but how does this circuit produce 
>positive gain??

The same way an opamp does.

>Thirdly, is the amplified or attenuated signal resulting from using the 
>inverter in this way inverted or not?

If you use the "gate" in a standard "linear amplifier" mode, it will invert.  Note
that the ground reference for this is 1/2(Vdd-Vss).

>Sorry if these are super basic questions. I'm coming at this from an 
>artist's rather than an engineer's background.
>
>Best!
>Derek
>
>-- 
>::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
>http://www.vimeo.com/macumbista :::
>---Oblique Strategy # 5:
>"Abandon normal instructions"
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-- ScottG
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