Hello everybody,

I'm trying to make an AND gate with two MOSFET N resistors (see image below).



Are there any risks having the source of the transistor T1 floating. It happens when T2 is 'opened'.

When the T1's source is floating, should T1 be 'closed' when its gate is at 5V?


This circuit is for educational purposes, I don't plan to implement it in a real product.
Thanks for your help.

Edit: I'm not sure I have created this thread in the right section.

 

Welcome to AAC!

Are there any risks having the source of the transistor T1 floating. It happens when T2 is 'opened'.

No - assuming you aren't touching things and causing ESD events.

When using transistors in digital circuits, we refer to them as on or off, not opened or closed.

When the T1's source is floating, should T1 be 'closed' when its gate is at 5V?

T1 can't turn on if it's source is floating.

Why did you draw the circuit with so many unnecessary wire bends?

 

You could do this a little "better" if you need to use N-Channel MOSFETs. Ground both sources, and connect the drains an one end of a pullup resistor together. You just made a very nice NOR gate without the worry of a floating source (should'nt be a problem in any case).

Using DeMorgan's first theorem (oh, it has been a long time!) you can treat the NOR gate to as an inverter NAND by considering that the output will be high when both inputs are at ground.

 

Which you can turn into an AND gate by inverting both inputs.

Bob

 

You do know that the output voltage at S will be at the MOSFET's threshold voltage below the AB input voltage, since the MOSFETs are acting as source followers (?).

 

You do know that the output voltage at S will be at the MOSFET's threshold voltage below the AB input voltage, since the MOSFETs are acting as source followers (?).

This AND gate wouldn't be able to drive a similar AND gate!