Transistor Experiments #2

Hi everyone. Welcome to my second post on my series of transistor experiments for my Transistors to Digital Logic course. My goal is to democratize this course and its subjects. I hope that students, by reading these posts in tandem with taking the course, will get a better idea of the non-streamlined trials and tribulations that every engineer and scientist goes through.

With that said, let’s dive in.

After my first attempt at making an AND gate, I realized that the circuit was in a hanging state whenever any of the input voltages were set to 0.

For reference, here is the original circuit I had:

For my second experiment, I wanted to see whether switching the order of the LED and transistors would affect the behavior. That resulted in the following circuit:

When I tested the circuit with various digital values at sites A and B, the LED turning on only depended on whether site B was 1.

That is to say, regardless of whether site A was a 0 or 1, the LED shined ONLY when site B was 1, which is not supposed to happen in an AND gate.

This stumped me because N MOSFET transistors are supposed to be open (disconnected) circuits when they receive a 0. Therefore, A=0 should not be able to deliver any voltage for B.

So how was the LED shining?

When I looked into how N-MOSFET transistors worked, I realized that the answer lied in good old physics.

N-MOSFET transistors operate by creating an electric field when their gates are hooked up to a logical 1 voltage. On its own, this can act like a capacitor, which builds up charge. This charge ultimately creates its own voltage that, when connected to ground, causes a current.

That is why even if only the B transistor receives a 1 logic voltage, the LED will still shine in the second AND gate I designed.

For my next transistor experiment, I need to design an AND gate that guarantees its output gives a 1 only when A and B get a 1 and gives a 0 any other time. To do so, I will need to incorporate a second kind of MOSFET: the P-channel MOSFET.

Until then, see you soon and stay creative!