Hi all,
having tried to figure out in some circuit (part of circuit is below) i came across to following:
"The 1MQ resistor across the first inverter puts it into a linear mode, making it act like a very high gain inverting amplifier." I dont get all this sentence.

My questions
1. i cant find what "linear mode" means with regard to inverter.
2. how "it act like a very high gain inverting amplifier"

I would be grateful for clarifications
Thanx

 

hi dm,
Welcome to AAC,
The 1meg effectively biases the Unbuffered type inverter IC input to approx Vsup/2, so it acts like a linear amplifier.

Do you follow OK?
E

 

74HC04 already behaves as a high gain inverting amplifier.
When the input voltage is below ½Vcc, the output is at Vcc.
When the input voltage is above ½Vcc, the output is at 0V.



The resistor from output to input introduces negative feedback. This reduces the gain of the inverting amplifier.
If the feedback resistor is low enough, the output will sit at ½Vcc, a stable linear bias region.

 

Thank you so much, guys. Looks like i got it.

 

hi dm,
Your circuit in LTSpice simulation.
Could you post the full circuit?
E

 

"The 1MQ resistor across the first inverter puts it into a linear mode, making it act like a very high gain inverting amplifier." I dont get all this sentence.

reality is that everything is really analog but in a digital world, it is just that digital responses tend to be much easier to see as discrete states (which is by design). in digital world circuits usually drive outputs so that they snap to either high or low state (near supply rail). you cannot get output to get to some "in-between" values and stay there.
well you can if you can add non-digital components, it is possible to abuse some digital circuits to operate outside design parameters. but this is getting away from digital world and closer to the analog world.

for example by using sufficiently small resistor across inverter, you can soften its response to get output that is not simply either one or zero but could be set to some other values like in an analog world. this process is called linearization because response curve is less of a "Z" and closer to a line or "/".

this can be done on many devices. consider diode. it can work as a switch because of pretty sharp knee in the VI curve. but if you add series resistor, that knee is not as sharp. by changing resistor value you can get response that is more and more linear and less knee shaped.

 

Thank you for detailed description. everything fell into place. you helped me so much.

 

There is an old but very useful application note here:
http://www.bitsavers.org/components/national/_appNotes/AN-0088.pdf
One thing to bear in mind is that CMOS gates are start to consume power when used in linear mode, and can get quite warm.

 

There is a concern about operating the digital devices in the linear mode is that they will draw more current. I am not sure if heating will be an issue, but it was with some devices back around 1970. So I suggest after running the device that way for a few minutes, 5 or 10, feel the IC and see if it is getting hot.