TTL and CMOS Circuits:

TTL is the short form of transistor transistor logic. As the name recommend here transistors are utilized for realization of digital logic or circuits. For our cause, it shall be necessary to know how the bipolar junction transistor (BJT) behaves as a switch. It shall be a digression from the main topic. Nonetheless, the following topic may not be understood without having knowledge of this. There are two kinds of BJTs viz. npn and pnp and their corresponding symbols are illustrated in Figures (a) and (b), respectively. Both the BJT has terminals called as base, collector and emitter as also illustrated in the same Figure. Let us see how they act as an electronic switch. When the voltage between the base and emitter is zero, the switch is open, there is no current among collector and emitter junction and the transistor is off. While the voltage between the base and emitter is non-zero, the switch is closed, there is current among collector and emitter junction and the transistor is on. Note that the voltage applied between the base and emitter should be positive for npn transistor whereas it must be negative for pnp transistor. For npn transistor the direction of current is out of the transistor while it is into the transistor for pnp transistor.