Extra Class question of the day: Digital integrated circuits

September 11, 2012 Dan KB6NU

Integrated circuits (ICs) are now an integral part (pun intended) of amateur radio electronics. The two main technologies used to manufacture IC are transistor-transistor logic, or TTL, and complementary metal-oxide semiconductor, or CMOS.

CMOS is arguably the most common type of digital IC. An advantage of CMOS logic devices over TTL devices is that the have lower power consumption. (E6C05) CMOS digital integrated circuits also have high immunity to noise on the input signal or power supply because the input switching threshold is about one-half the power supply voltage. (E6C06)

TTL is the other common digital logic IC technology. 5 volts is the recommended power supply voltage for TTL series integrated circuits. (E6C01) The inputs of a TTL device assume a logic-high state if they are left open. (E6C02)

BiCMOS logic is an integrated circuit logic family using both bipolar and CMOS transistors. (E6C12) An advantage of BiCMOS logic is that it has the high input impedance of CMOS and the low output impedance of bipolar transistors. (E6C13)

Tri-state logic devices are logic devices with 0, 1, and high impedance output states. (E6C03) These devices can be made with either TTL or CMOS technology. The primary advantage of tri-state logic is the ability to connect many device outputs to a common bus. (EC604) When a device’s outputs are in the high-impedance state, they act as if they are disconnected.

Digital Logic Schematic Symbols

When working with digital ICs, it is important to recognize the various symbols for the different types of logic gates. In Figure E6-5, 1 is the schematic symbol for an AND gate. (E6C07) In Figure E6-5, 2 is the schematic symbol for a NAND gate. (E6C08) In Figure E6-5, 3 is the schematic symbol for an OR gate. (E6C09) In Figure E6-5, 4 is the schematic symbol for a NOR gate. (E6C10) In Figure E6-5, 5 is the schematic symbol for the NOT operation (inverter). (E6C11)

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