Using CMOS logic for amplifying small signals

Something has occurred to me, and I was wondering if anyone knew more about this. Could a CMOS logic inverter, like a 4049, be used as a very high gain amplifier if it were properly biased? I figured that the DC offset should be set at just about half the supply voltage, so that the gate was always between a '1' and a '0'.

I am not sure if the gate would behave linearly in that region, or if thermal noise might cause it to reach a valid logic state and not amplify the signal, but if it would work, it might be a cheap way to make an amplifier for long wave or AM radio.

Reply to
Mr. B
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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.

Reply to
default

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

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Reply to
Vladimir Vassilevsky

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.

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

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*-semi-analog-use-of-CMOS-logic-chips+Philips-*-excels-here+CD4007UBE+not-HC04+zzz+74HCU04+lot-to-*+*-specsmanship+cross-currentsnews:3Y2ag.76890$ snipped-for-privacy@newssvr13.news.prodigy.com

Reply to
JeffM

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

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
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.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
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.

Reply to
Gary Peek

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Reply to
Joel Koltner

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

2003." Whaddaya think they changed? I suspect that's just when they turned it into a PDF file.
Reply to
Joel Koltner

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

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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

1973, revised June 1986. Whatever they might have revised back then. I could go look in the real old App Databook. I keep them all, for good reason. While you can still get stuff for the Plymouth Fury some semi mfgs have a habit of dropping docs off the web server the millisecond something goes obsolete.
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Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

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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.

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whole article (~65kb) Schematics:
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transmitter circuit (45kb)
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receiver circuit (35kb)

Reply to
default

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

Good Luck! Rich

Reply to
Rich Grise

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).

Reply to
whit3rd

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