Mohamed M. Texas Instruments introduced the series TTL family in Transistor—transistor logic uses bipolar transistors to form its integrated circuits. Since the transistors of a standard TTL gate are saturated switches, minority carrier storage time in each junction limits the switching speed of the device. Variations on the basic TTL design are intended to reduce these effects and improve speed, power consumption, or both.
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Noise Immunity Power Dissipation Speed: Speed of a logic circuit is determined by the time between the application of input and change in the output of the circuit. Fan-out: Determines the number of circuits that a gate can drive.
Noise Immunity: Maximum noise that a circuit can withstand without affecting the output. Power: When a circuit switches from one state to the other, power dissipates. TTL Logic In transistor-transistor logic TTL , logic gates and other digital circuits are made using bipolar junction transistors and resistors. The term transistor-transistor is because both logic function and amplification is done by transistor.
Using TTL logic families, many logic gates can be fabricated in a single integrated circuit. For logic gate built using TTL logic families, input are given to the emitters of the input transistor. There are two transistor stages in the circuit, a multi-emitter input transistor and output transistor. Function of a multi-emitter transistor is same as that of a two parallel transistor with common base and collector terminals. Operation of TTL NAND Gate When the two emitters of the input transistors are connected to high voltage, then emitter-base junction of the transistor is reverse biased that means, transistor is in reverse active mode.
In reverse active mode, less magnitude current flows in the opposite direction. This current reaches base of the output transistor, allowing it to conduct and pulling down the output voltage to zero. When any one of the input terminal is low, the current through other branch flows out through this terminal. Now no current reaches the base terminal of the output transistor, so output remains at high state.
Transistors are not allowed to go into deep saturation thus, eliminating storage delays like in TTL logic families.
Transistors are driven either in cut off or in active region. This is achieved by using voltage values close to each other. For logic one it is In the active region, charge stored in the base region of transistors is kept to minimum. Difference between these two logic states is very small. This improves the speed of operation at the expense of noise margin. Propagation time for an ECL gate is 0.
But the disadvantage of ECL logic families is that it uses a negative power supply such that the logic levels are not compatible with any other logic family and makes analysis and measurement inconvenient. ECL logic families requires large currents therefore power dissipation is 3 to 10 times higher than that of TTL logic families. Because of its large power consumption and high requirement of silicon area, CMOS logic gates are preferred over ECL logic families in large scale integrated circuits.
Figure shown below is a CMOS inverter. So in both states, there is no direct connection between power supply and ground, thereby reducing static power loss of a transistor. Now there is a direct connection between output and ground through transistor T3 and T4. Output is pulled down to zero. When any of the input is low, either transistor T3 or T4 will be off. This breaks the connection between output terminal and ground.
Now output is pulled up to logic high. This makes a direct connection between power supply and output terminal. Now the output is raised to logic high value. In other cases output will remain at logic low pulled down by the pull-down resistor. When output is at logic low, static power dissipation is very high. When both inputs are at logic high, both transistors will be ON establishing a connection between output terminal and ground. Thus output is pulled down to ground voltage.
When any one the input is at logic high then the transistor will be OFF, cutting off the path between output terminal and ground. Thus output will remain at high voltage, pulled up by the pull-up resistor.
Digital Logic Design: IC Families Explained
This IC is usually referred to as a monolithic IC first introduced in The digital ICs are categorized as, 1. Medium scale integration MSI 12 to 99 no of gates 3. Large scale integration LSI to no of gates 4. Digital IC can be further categorized into bipolar or unipolar IC. Bipolar ICs are devices whose active components are current controlled while unipolar ICs are devices whose active components are voltage controlled. IC Packaging 1.
DIGITAL IC FAMILIES DTL TTL ECL MOS CMOS PDF
Noise Immunity Power Dissipation Speed: Speed of a logic circuit is determined by the time between the application of input and change in the output of the circuit. Fan-out: Determines the number of circuits that a gate can drive. Noise Immunity: Maximum noise that a circuit can withstand without affecting the output. Power: When a circuit switches from one state to the other, power dissipates.
Digital Logic Families
This is the maximum current which the gate can sink in 0 level. To realize the logical functions, both P-type and N-type transistors are used. Several techniques and design styles fxmilies primarily used in designing large single-chip application-specific integrated circuits ASIC and CPUs, rather than generic logic families intended for use in multi-chip applications. This is the minimum voltage available at the output corresponding to logic 1. Analog Integrated Circuits Not shown are some early obscure dmos families from the early s such as DCTL direct-coupled transistor logicwhich did not become widely available. TTL ytl become the standard logic circuit in many application for a number of years.