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
Using CMOS logic for amplifying small signals
Started by ●January 4, 2008
Reply by ●January 4, 20082008-01-04
On Fri, 04 Jan 2008 11:48:46 -0500, "Mr. B" <un@dis.closed> wrote:>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. > >-- BAbsolutely! 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 by ●January 4, 20082008-01-04
Mr. B wrote:> 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
Reply by ●January 4, 20082008-01-04
Vladimir Vassilevsky wrote:> > > Mr. B wrote: > >> 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. >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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●January 4, 20082008-01-04
Mr. B wrote:>[...]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.electronics.design/browse_frm/thread/280042b4010789fd/69eead0b1d41407f?q=*-semi-analog-use-of-CMOS-logic-chips+Philips-*-excels-here+CD4007UBE+not-HC04+zzz+74HCU04+lot-to-*+*-specsmanship+cross-currents news:3Y2ag.76890$H71.59901@newssvr13.news.prodigy.com
Reply by ●January 4, 20082008-01-04
On Fri, 04 Jan 2008 11:48:46 -0500, "Mr. B" <un@dis.closed> wrote:>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. > >-- BYes, 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 -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●January 4, 20082008-01-04
Spehro Pefhany wrote:> On Fri, 04 Jan 2008 11:48:46 -0500, "Mr. B" <un@dis.closed> wrote: > >> 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. >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 by ●January 4, 20082008-01-04
On Fri, 04 Jan 2008 10:35:48 -0800, Joerg <notthisjoergsch@removethispacbell.net> wrote:>Spehro Pefhany wrote: >> On Fri, 04 Jan 2008 11:48:46 -0500, "Mr. B" <un@dis.closed> wrote: >> >>> 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. >> > >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's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●January 4, 20082008-01-04
Mr. B wrote:> 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.
Reply by ●January 4, 20082008-01-04
