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Jul30-07, 04:44 PM
berkeman's Avatar
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Quote Quote by chroot View Post
Well, actually berkeman, there's a huge issue left -- one that I thought you were alluding to in your last post. That issue is: power.

When you provide a middle-of-the-road input to a CMOS gate, you'll actually turn on both output transistors simultaneously. These two transistors will fight each other, conducting away your precious battery power. Everytime a gate changes state, the two transistors briefy conduct simultaneously, and this is the only time that a CMOS gate uses any appreciable power besides leakage.

If you apply a very slowly-rising signal to a CMOS gate, you're going to burn up a ton of power until the input rises high enough to turn off the PMOS devices.

Again, this isn't the kind of thing you're going to notice or worry about while bread-boarding in an introductory EE class, but it's a very, very real concern in industrial-quality design.

- Warren
Absolutely, but that's on part (a) of the two circuits. I was referring to the part (b) LED circuit in my previous post. Also, all the gates in these first two circuits are labelled "TTL", so the power issue in (a) is less important than if they were CMOS.

So to summarize where we are, we need the fundamental problem still to be identified in (a), and how can we reconfigure (b) to be more practical and support some amount of fan-out?