Technology of digital components

The same logic function can be built from different underlying circuit technologies, known as logic families, and each family makes its own trade-offs between speed, power consumption, noise immunity and cost. Within an integrated circuit the gates are formed from transistors arranged in a characteristic way, and the family determines the supply voltage, the input and output behaviour and how chips of the same type can be connected together.

Bipolar families use bipolar junction transistors. Transistor-transistor logic (TTL) became a widespread standard, with the well-known 7400 series, offering good speed and reasonable power use. Emitter-coupled logic (ECL) keeps its transistors out of saturation to switch very fast, at the cost of higher power consumption, and was used where speed mattered most. Field-effect families use MOS transistors: NMOS uses n-channel devices, while CMOS uses complementary pairs of n-channel and p-channel transistors so that almost no current flows except during switching, giving very low static power.

Output stages also differ. A normal gate drives its output actively high or low, but a tri-state output adds a third high-impedance condition that effectively disconnects the gate, allowing many devices to share a common bus. An open-collector (or open-drain) output can only pull its line low and relies on an external pull-up resistor, which lets several outputs be wired together and is useful for level shifting and for combining signals. Mixing families requires attention to voltage levels and how much current an output can supply.

Frequently asked questions

Why does CMOS use so little power?

It uses complementary transistor pairs so that one of each pair is always off in a steady state, meaning current flows mainly during switching rather than continuously.

What is a tri-state output for?

It adds a high-impedance state that disconnects the gate from the line, letting several devices share one bus without their outputs fighting each other.

Why is ECL faster than TTL?

Its transistors never fully saturate, so they switch more quickly, though this speed comes with noticeably higher power consumption.