Compare the characteristics of different logic families
Answers
A logic family of monolithic digital integrated circuit devices refers to a group of electronic gates constructed using a design with compatible logic levels and power supply characteristics within a family. It can be used as a building block to create systems or to interconnect complex integrated circuits.
A set of techniques used to implement logic within VLSI integrated circuits is also referred to as a logic family. Some of them use static techniques to minimize the complexity of design. While logic families like domino logic utilize clocked dynamic techniques to minimize the power consumption, size and delay.
Three families of logic are commonly used. They are:
1. TTL or Transistor-Transistor Logic
2. CMOS or Complementary Metal Oxide Semiconductor
3. ECL or Emitter-Coupled Logic
The TTL Family
a. The input gate is characterized by a multi-emitter transistor.
b. Each emitter is a input to the logic gate. So a two input gate and a four input gate have two and four emitters respectively.
c. The output stage is a totem-pole circuit.
d. When one input to a TTL NAND gate is low, the bottom transistor of the totem-pole output Q4 is OFF. The top transistor Q3 is ON. This makes the output a logic high.
e. When all inputs to the TTL NAND gate are high, the top transistor of the totem-pole output Q3 is OFF. The bottom transistor Q4 is ON. This pulls the output low.
The CMOS Family
a. It uses a complementary pair of MOSFET devices - one N-channel and one P-channel.
b. It has a very low power consumption and a high input impedance.
c. When the input to a CMOS gate is high, the N-channel device is ON. This pulls the output to Ground.
d. When the input to a CMOS gate is low, the P-channel device is ON. This raises the output to VDD.
e. It is available in NAND or NOR configurations.
f. The devices have to be handled carefully to avoid damage from static discharge.
g. CMOS chips are available in several logic configurations. Some of them are similar to TTL.
h. Some families of CMOS logic are 4000 series, 40H00 series, 74C00 series, 74HC00 series, and 74HCT00 series.
i. The BiCMOS is the latest series of logic which combines the best features of the bipolar transistors and the CMOS transistors.
j. The BiCMOS combine high speed switching with very low power consumption.
k. The BiCMOS is limited to the microprocessor bus interface logic.
Emitter-Coupled Logic
a. It is very fast but consumes more power.
b. It uses a differential amplifier as a basic circuit.
c. The transistors are not allowed to saturate so it is very fast.
d. Its voltage levels are -0.8 V and -1.7 V. These are not compatible to TTL and CMOS logic.
e. Its logic gates have OR and NOR output.
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