Using CMOS logic for amplifying small signals

Absolutely! Works great. You can even make band pass, hi, low cut filters etc. Biasing is just a single high value resistor from output to input. with an AC coupled input.

Really useful where you are wanting a square wave output and have an analog input - like zero crossing sensing, converting an optical signal back to digital, input to a phase lock loop, etc..

Probably not what you'd want for hi fidelity audio.

Using TTL and CMOS logic IC as the linear amplifiers used to be a textbook issue 20 years ago. Keep in mind that the cross conduction is quite high in the linear area: several mA per a common gate at +5V supply. The stability is the problem, too.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I never had any stability issues. But you must use them near the bottom of their specified supply voltage range or the quiescent current will cause them to become too toasty.

Joerg Schulze-Clewing's previous post on this topic:

Yes, with UB gate, you just need a relatively high value resistor from input to output, and capacitively couple to the input, but I would suggest not using a buffer like the 4049UB since the quiescent current will be higher than it needs to be. Something like a 4069UB (unbuffered) gate operated at a few volts... in fact that kind of application is pretty much why the UB gates are still around. You can put a bunch in series and get more amplification, but of course you have to capacitively couple the input of each stage from the previous stage. With a buffered gate you will probably make a high frequency oscillator if you try to bias it with a resistor.

Best regards, Spehro Pefhany

I've used the buffered ones as amps. But one must follow RF guidelines, this ain't your old audio amp no more even if used as such. Also, sometimes unorthodox supply tricks need to be used, for example current limiting the supply so you can set a desired quiescent current regardless of process tolerances. Those can be quite high and aren't always fully spec'd.

Of course if you limit the supply current then you get an unknown limit to output voltage swing... nothing comes for free..

Best regards, Spehro Pefhany

It certainly could. I'm looking at a National Semiconductor databook called CMOS Integrated Circuits, March 1975.

It includes "AN-88, CMOS Linear Applications". Search for it, I found a Fairchild equivalent on their web site.

I like the note on the first page, "Application note, July 1973, revised April

Well, sure, they aren't meant to drive the speakers during a Stones concert :-)

But resist the temptation to use the 74C series devices in there. Those seem to be on the way to lalaland.

Maybe they changed the resolution settings for the PDF scanner in 2003 :-)

Looks pretty much like National AN-88. The one in my book says July

Is a microchip application note on using their line of pic products for encoding and decoding X10 home automation signals.

On page 5 of the schematics they show a 4069 used as a single channel AM receiver. Two inverters are used as tuned RF amps, two others used as untuned RF amps, and 1 for a detector stage.

A little tweaking of values and you'd have your broadcast band radio.

Since you can also parallel inverters on the same chip for greater current drive, maybe another chip could be used for the speaker driver?

Wireless power transmitter operating at 13 mhz, Cmos inverter operates the oscillator buffer and driver for a pair of mosfets on the PA stage.

If I were doing it, I'd look at the unbuffered version, like 74HCU04, which is essentially 6 MOSFET inverting amplifiers.

Good Luck! Rich

Yes, it's possible. CD4007UB is intended to be used that way (and has some extra connection points that you won't find in an inverter).