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?