[Dr. Shane] asks the question: what happens if you connect the output of an inverter logic gate back to the input? In theory, it doesn’t make sense, but depending on the gate’s physical construction, you’ll get into a strange state. The transistors within the gate will behave differently than they normally would, and you’ll wind up with an amplifier or an oscillator. You can see the results in the video below. In the second video, you can see what the odd connection does to the thermal properties of the inverter, too.
The CMOS inverter becomes biased in the active region, so it makes sense that it settles at the halfway point. The TTL inverter is slightly different, but the delay through the gate isn’t enough to produce a good oscillation. However, an odd number of inverters connected in a ring like this is one way to create a simple oscillator.
While we don’t suggest this method, it is interesting to see what happens when you bias different gates this way. We’ve certainly seen it done using a variety of different inverter types ranging from discrete transistor circuits to inverters on the die of an IC.
We’ve seen ring oscillators in a number of forms. Inverters can also make the phase-shifting part of a crystal oscillator.
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