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Note of digital logic circuits by Magudeeswaran v

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EE6301 – DIGITAL LOGIC CIRCUITS – Two Marks UNIT I - NUMBER SYSTEMS AND DIGITAL LOGIC FAMILIES TWO MARKS 1. Define binary logic? Binary logic consists of binary variables and logical operations. The variables are designated by the alphabets such as A, B, C, x, y, z, etc., with each variable having only two distinct values: 1 and 0. There are three basic logic operations: AND, OR, and NOT. 2. What are the basic digital logic gates? The three basic logic gates are AND gate OR gate NOT gate 3. What is a Logic gate? Logic gates are the basic elements that make up a digital system. The electronic gate is a circuit that is able to operate on a number of binary inputs in order to perform a particular logical function. 4. What is meant by bit? A binary digit is called bit. 5. What is the best example of digital system? Digital computer is the best example of a digital system. 6. Define Radix. It specifies the number of symbols used for corresponding number system. . 7. Define Nibble and Byte. i) In binary number a group of four bits nibble. ii) A group of 8 bits are called Byte. 8. List the number systems? i) Decimal Number system ii) Binary Number system iii) Octal Number system iv) Hexa-decimal Number system 2nd YEAR/ 3rd SEMESTER – Department of EEE - PSNACET

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EE6301 – DIGITAL LOGIC CIRCUITS – Two Marks 9. Which gates are called as the universal gates? What are its advantages? The NAND and NOR gates are called as the universal gates. These gates are used to perform any type of logic application. 10. How to represent a positive and negative sign in computers? Positive (+) sign by 0 Negative (-) sign by 1. 11. What are the applications of octal number system? The applications of octal number system are: i. It is used for entering the binary data and displaying certain information’s. ii. It is very important for the efficient use of microprocessors and other digital circuits. 12. Why is a hexadecimal number system called as an alpha numeric number system? Hexadecimal number system has the base as 16 and therefore it requires 16 distinct symbols to represent the numbers. These are numerals 0 to 9 and alphabets A to F. Since both numeric digitals and alphabets are used to represent the digits in hexadecimal number system, it is also called as an alphanumeric number system. 13. Mention the classification of saturated bipolar logic families. The bipolar logic family is classified as follows:  RTL- Resistor Transistor Logic  DTL- Diode Transistor logic  I2L- Integrated Injection Logic  TTL- Transistor Transistor Logic  ECL- Emitter Coupled Logic 14. Mention the important characteristics of digital IC’s?  Fan out  Power dissipation  Propagation Delay  Noise Margin  Fan In  Operating temperature  Power supply requirements 15. Define Fan-out? Fan out specifies the number of standard loads that the output of the gate can drive without impairments of its normal operation. 2nd YEAR/ 3rd SEMESTER – Department of EEE - PSNACET

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EE6301 – DIGITAL LOGIC CIRCUITS – Two Marks 16. Define power dissipation? Power dissipation is measure of power consumed by the gate when fully driven by all its inputs. 17. What is propagation delay? Propagation delay is the average transition delay time for the signal to propagate from input to output when the signals change in value. It is expressed in ns. 18. Define noise margin? It is the maximum noise voltage added to an input signal of a digital circuit that does not cause an undesirable change in the circuit output. It is expressed in volts. 19. Define fan in? Fan in is the number of inputs connected to the gate without any degradation in the voltage level. 20. What is Operating temperature? All the gates or semiconductor devices are temperature sensitive in nature. The temperature in which the performance of the IC is effective is called as operating temperature. Operating temperature of the IC vary from 0 C to 700 c. 21. What is High Threshold Logic? Some digital circuits operate in environments, which produce very high noise signals. For operation in such surroundings there is available a type of DTL gate which possesses a high threshold to noise immunity. This type of gate is called HTL logic or High Threshold Logic. 22. What are the types of TTL logic? 1. Open collector output 2. Totem-Pole Output 3. Tri-state output. 23. What is depletion mode operation MOS? If the channel is initially doped lightly with p-type impurity a conducting channel exists at zero gate voltage and the device is said to operate in depletion mode. 24. What is enhancement mode operation of MOS? If the region beneath the gate is left initially uncharged the gate field must induce a channel before current can flow. Thus the gate voltage enhances the channel current and such a device is said to operate in the enhancement mode. 2nd YEAR/ 3rd SEMESTER – Department of EEE - PSNACET

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EE6301 – DIGITAL LOGIC CIRCUITS – Two Marks 25. Mention the characteristics of MOS transistor? 1. The n- channel MOS conducts when its gate- to- source voltage is positive. 2. The p- channel MOS conducts when its gate- to- source voltage is negative 3. Either type of device is turned of if its gate- to- source voltage is zero. 26. How schottky transistors are formed and state its use? A schottky diode is formed by the combination of metal and semiconductor. The presence of schottky diode between the base and the collector prevents the transistor from going into saturation. The resulting transistor is called as schottky transistor. The use of schottky transistor in TTL decreases the propagation delay without a sacrifice of power dissipation. 27. List the different versions of TTL 1. TTL (Std.TTL) 2.LTTL (Low Power TTL) 3. HTTL (High Speed TTL) 4.STTL (Schottky TTL) 5. LSTTL (Low power Schottky TTL) 28. Why totem pole outputs cannot be connected together. Totem pole outputs cannot be connected together because such a connection might produce excessive current and may result in damage to the devices. 29. State advantages and disadvantages of TTL Advantages:  Easily compatible with other ICs  Low output impedance Disadvantages:  Wired output capability is possible only with tristate and open collector types  Special circuits in Circuit layout and system design are required. 30. When does the noise margin allow digital circuits to function properly? When noise voltages are within the limits of VNA (High State Noise Margin) and VNK for a particular logic family. 31. What is the number of bits in ASCII Code? What is the need for ASCII Code? ASCII is a 7-bit code; it can represent 27 or 128 different characters. It can represent decimal numbers from 0 to 9 and letters of the alphabet, both upper and lower case. It is also used for controlling computer peripheral devices with the help of the special control function incorporated in the code. 2nd YEAR/ 3rd SEMESTER – Department of EEE - PSNACET

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