Using CMOS logic for amplifying small signals

[Mr. B]

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.

-- B

 

[default]

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.

-- B

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.
--

 

[Vladimir Vassilevsky]

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?

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
http://www.abvolt.com

 

[Joerg]

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.

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.

 

[JeffM]

[...]Could a CMOS logic inverter, like a 4049, be used
as a very high gain amplifier if it were properly biased?

Joerg Schulze-Clewing's previous post on this topic:
http://groups.google.com/group/sci....4HCU04+lot-to-*+*-specsmanship+cross-currents
news:[email protected]

 

[Spehro Pefhany]

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.

-- B

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

 

[Joerg]

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.

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.

 

[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

 

[Gary Peek]

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?

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.

 

[Joel Koltner]

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

www.fairchildsemi.com/an/AN/AN-88.pdf

 

[Joel Koltner]

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

 

[Joerg]

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

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

 

[Joerg]

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.

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

 

[Joerg]

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.

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.

 

[default]

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.

-- B

http://ww1.microchip.com/downloads/en/AppNotes/00236a.pdf

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.

http://www.edn.com/article/CA6501085...dustryid=44217 whole article
(~65kb)
Schematics:
http://www.edn.com/contents/images/6501085f1.pdf transmitter circuit
(45kb)
http://www.edn.com/contents/images/6501085f2.pdf receiver circuit
(35kb)

--

 

[Rich Grise]

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.

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

Good Luck!
Rich

 

[whit3rd]

...Could a CMOS logic inverter, like a 4049, be used as a very high
gain amplifier if it were properly biased?

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).