Re: PMOS in parallel with NMOS

Tomás Ó hÉilidhe wrote:

Let's say I have a microcontroller pin that will have one of three
states: 5 V, 0 V or high impedence.

Let's say I have a PMOS and an NMOS, and I connect their gates
together. I connect the microcontoller pin to the gates.

Now, when my pin is high, the NMOS will turn on. When the pin is low,
the PMOS will turn on.

What will happen though when I set the pin to high impedence? I'm
hoping that neither transistor will be on, but I'm concerned that
maybe there might be some sort of voltage on the microcontroller pin
(it might be behind a large resistance, but it's still a voltage) that
will switch one of them on.

In my current design, I'm using bi-polar transistors, and it works
great. I'm thinking of switching to MOSFET's though because they let
current flow in both directions.

The MOSFET's will be used to turn on LED's.

Spice is your friend, or you can look at this data ***:

http://www.standardics.nxp.com/products/hef/data***/hef4069ub.pdf

It shows P/N fet interaction with input swing quite nicely.

-jg

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