[sdiy] 4049/4069 linear mode

Derek Holzer derek at umatic.nl
Sun May 31 12:33:34 CEST 2009


Hi everyone and thanks for the feedback on this particular IC!

Scott, you mentioned biasing the input of the gate for use as an 
amplifier, but all the single supply circuits I see using this technique 
such as the Anderton "Tube Fuzz" (and I *think* in the Wasp filter, 
although I'm not sure...) do not bias it at all.

To be more specific, I'm using this in small battery powered synths 
based on 4093 gated square wave oscillators, and I'm trying to get the 
best use of it as both gain control and overdrive. Should I worry about 
biasing in this case? Would I get better performance out of it if I do?

Hadn't considered driving an LED with it, was using discrete transistors 
for that as I have a ton of 4069 rather than 4049.

best!
Derek

Scott Nordlund wrote:
> As far as I know, the 4049 can source/sink more current, so it's suitable for driving things like LEDs.
> 
> Forget looking at the schematics that use the 4049 or 4069, look at the datasheet (for the 4069UB, not the buffered 4069B, since it's got additional stages that ruin the fun).  An input voltage of Vdd will turn the N MOSFET on (Vgs = Vdd) and the P MOSFET off (Vgs = 0), thus giving a low output voltage.  Conversely, an input voltage near Vss will turn the N MOSFET off (Vgs = 0) and the P MOSFET on (Vgs = -Vdd), producing a high output.  So it works as a digital inverter.  An intermediate voltage will result in both MOSFETS being partially turned on and operating in a more or less linear region, though they weren't designed with this in mind.  This isn't always a good thing, as it can allow a rather high current to flow from Vdd to Vss (though these chips seem to be able to handle it).  If you've got it biased at 1/2 Vdd, a small signal superimposed on top of this will be amplified and inverted.  Thus it's also an amplifier, though not a very good one.  It uses a lot of powe
r!
>   and isn't particularly linear, especially for large input amplitudes, but sometimes this is what you want.  The very high input impedance is an added bonus.

-- 
::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
http://www.vimeo.com/macumbista :::
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