# How does a two input transistor AND gate work?

I'm trying to self study electronics. I started reading a book on digital electronics and I got stuck on the first chapter. It's about the logic gate circuit. There is a two input AND gate circuit using three transistors:

In my physics book, the circuit for an AND gate shown is pretty clearly. It used two transistors and I get that theory, but I don't get this.

In this image, why is it that when either A or B or both are at 0 V, X=0 V? I know that in these cases, at least one of the transistors is off, so current from 5V passes through R and to the base of transistor T3, making it on. The current should also flow through the resistor across R, so why does X have an output 0?

• X will have an output of Vce of T3, which is low when T3 is conducting (think, tha you can roughly approximate it as short to ground). Not zero though. Commented Apr 26, 2021 at 14:53
• The three resistors and two transistors on the left create a NAND gate. The two resistors and transistor on the right are an inverter that convert it to AND. Commented Apr 26, 2021 at 15:26
• Like you said, transistor T3 is on. Being on just means it conducts current and has (almost) 0V over it so X will be 0V. Commented Apr 26, 2021 at 16:53