Wednesday, April 01, 2009

How are you to switch negative voltages?

I've been wrestling with the problem of key switching again. Because the key driver circuits switch a negative voltage to ground it creates a bit of a problem.

When I first looked at the j-Omega MPT8 I thought it could switch negative voltages but after thinking again and e-mailing then it seems not.

What's all the fuss about though? I can use a solid-state relay, optocoupler or CMOS switch package right? You don't even have to think to hard to get it working. The issue here is that I have 49 keys and very limited space. I'd really just like a transistor and maybe one or two resistors per switch. CMOS switches only come in quad packages at most so I'd need 12 and all the trracking back and across each other to get everything wired. If I must have a PCB at least I'd like it to be simple.

Generally switching negative voltages to ground is not something you get a lot of talk about when looking up these things. Everything is geared to positive voltages and how to bias your transistor that way. It's not impossible just less usual and if you wan to use a simple +nv/0V logic level your options are limited.

The reason for this is not that you can't do it (just switch from n-channel to p-channel FET) but that transistors that switch negative voltages themselves need a negative voltage to switch. which takes you back to square one.

Well, If a CMOS switch can switch a negative oltage with just a +nV power rail how does it do it? I've been wondering.

Google books have a Modern CMOS Circuits Manual online and chapter four has the answers