why use a resistor across an inverter?

[Peter]

I saw a circuit where a 74AC04 invertor had a 1Mohm resistor across it.


I've looked this concept up in my electronics books, but haven't had any
luck.


Does anyone know why?


thanks in advance

 

[Peter]

Yep. It basically turns the inverter into a high-gain "linear"
amplifier that will respond to small signals (and, of course, large
ones!) at its input. Typically used with other components to make an
oscillator.

How it works is fairly simple - the resistor applies feedback so that
the output will settle at around half the supply voltage and so will
the input. This region of operation, at neither "1" or "0", isn't
normal for a digital circuit and isn't covered by the specification.
So actually using it as a linear, analogue, amplifier isn't a good
idea - but it works well in oscillators and threshold detectors.

This still confuses me a bit.

How is this considered high gain if the output is half what it should be
(2.5 volts basing the chip to be exactly 5 volts)?

How do you konw the output will be "around" half?

Sounds like the output will either be 0 or 2.5 volts instead of 0 or 5
volts.

 

[The Real Chris]

Hello,

It may be just to load the ouput during "no load" conditons that might make
the inverter unstable. Some inverters do not work without any load and just
fly up and down in an erratic way so the output votlage cannot be measured
without a load.

Chris