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A better 4:1 Mux?

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ACF2802

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Hello everyone, this is my first time posting here (not really, I've tried to post this 3 times but the forum keep eating my darn post for some reason.)

I've been studying digital electronics, CMOS logic in particular.

While looking at a diagram for a 2:1 mux I though it was more complicated than it needed to be, being made of 2 transmission gates and an inverter. **broken link removed** Why cant it be made from just two transistors?

Anyway, here is a simulation I made using 3 of these purposed 2-transistor MUXs combined into a 4:1 mux.

**broken link removed**

Seems to work in simulation, is there a fundamental flaw I'm missing?

Thanks.

(PS. I wonder why the first three times I posted this it didn't show up? I have a supposition there is a flaw with the Preview Post button, because the first three times I hit submit after using preview, but this time I skipped the preview.)
 
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Your ingenuity is admirable. The problem is, you're not using both the n-channel and p-channel devices to transmit the signal which will distort the signal if you're trying to send a rail-to-rail voltage, which is what CMOS muxes are typically used for.

For example, when both select lines are low, I believe Input A is selected, which then goes through the two p-channel devices to the output. This works fine for a high voltage signal, but a low voltage signal will be limited by the gate turn-on voltage of the p-channel device which is about 2-3V above ground for the source-drain.

You have the same problem when transmitting a signal near the supply rail through the n-channel devices.

That's why both n and p channel devices are used in a transmission gate--so that the gate has a low impedance for both high and low voltage signals.

If you put a significant load on your outputs (say 1k ohm to ground and a 1k ohm to the plus supply) your simulation should show this (if it's a SPICE type simulator).

As a famous physicist once said "Make it as simple as possible...but no simpler." There's a good reason that CMOS muxes are designed as they are.
 
As a famous physicist once said "Make it as simple as possible...but no simpler." There's a good reason that CMOS muxes are designed as they are.

I figured as much, I just wanted to hear that from someone and with an explanation for why.

Thanks much!
 
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