CD4009 is a buffer. If you connect its input to its output, then you get a bit of SRAM, not an amplifier.
CD4049 is an inverter. If you do the trick above then you indeed get an amplifier, nonlinear and with poorly-controlled gain but an amplifier nonetheless. This isn't some kind of Easter egg; an inverter is just a high-gain amplifier that's usually allowed to saturate, so it fundamentally just does that.
Such amplifiers are not very good, but they're fast-ish and cheap. They're often used for crystal oscillators. The preferred logic series these days is 74HCU. That's "unbuffered" logic, where your inverter really is just one CMOS inverter, and not a string of three like usual. That makes the gain more stable, since the three inverters wouldn't match perfectly, and would each end up biased somewhere different.
> CD4049 is an inverter. If you do the trick above then you indeed get an amplifier, nonlinear and with poorly-controlled gain but an amplifier nonetheless.
The CD4049 hex inverter chip is a popular amplifier chip in some guitar distortion pedals due to all the reasons you mentioned (non-linear and poorly controlled gain). There were a few designs based on connecting several of the inverter stages in series.
The wiring of the MOSFETs inside has superficial similarity to an AB class dual pentode push-pull tube power amplifier and has similar qualities in the sound it produces.
Oops; you're totally right. I read the datasheet title, and didn't read to the subtitle. In any case, the trick is alive and well with 74HCU logic, good in to the tens of MHz whenever exact gain and distortion don't matter.
> In any case, the trick is alive and well with 74HCU logic, good in to the tens of MHz whenever exact gain and distortion don't matter.
But you don't get the voltage tolerance with 74HCU (6V limit).
This was one of the interesting things about the old 4000 series because they had metal gates and thick oxide--they tended to work from less <1V (probably not for analog, though ...) the whole way to 20V (convenient for 2 9V batteries).
Old 4000 series were also notoriously vulnerable to static discharge, so I suspect that they didn't have much in the way of ESD protection (if any at all).
You put a feedback resistor in place from input to output to bias it and then capacitively couple the input and output.
Metal-gate CMOS was particularly good for this as it had an operating voltage from <3V to about 18V.