Transistor Logic

TTL (Transistor-Transistor Logic) is a name for type of logic circuits which are constructed using the bipolar transistors.

In general the bipolar transistors such asthe NPN and PNP transistors are current operated devices. In these types of transistors, when used for digital logic circuits, typically the input voltage is appliedacross the Base-Emitter junction. As a result of this a small base current is produced in that junction. This small base current is magnified by the transistor's current gainfactor B(called Beta), resulting in a large amount of collector-emitter current. This is basically how a TTL switch works.

The TTL family of circuits are generally veryfast, but this speed comes at the cost of higher power dissipation. Also, a typical TTL logic gate circuit requires multiple transistors and other passive components suchas resistors. This results in lower number of gates that can be put on an IC built using TTL family.

The CMOS(Complimentary Metal Oxide Semiconductor) family of logiccircuits are buit using Metal Oxide Semiconductor(MOS) Field Effect Transistors (FET). These types of transistors are voltage operated devices. A typical CMOS logic gatecircuit can be constructed using as little as two MOSFET transistors (A CMOS Inverter) resulting in a much higher density of logic gates that can be put on an IC.

Also,CMOS gates use much much lower power to operate compared to the TTL gates because no resistors are required to build such circuits.

CMOS logic also allows much higheroperating voltages compared to the TTL family.

One drawback of CMOS family is that they are slower than the TTL family, however these days this may no longer be true.