Science universe: Physics articles

Combinations of Gates Very often it is convenient to be able to perform a logical function using a different kind of gate. AND Followed by NOT If an AND gate is followed by a NOT gate (see Fig.12) the output of the circuit will be . Therefore, the combination acts like a NAND gate.   AND Preceded by NOT If all of the inputs to an AND gate are inverted, see Fig .13, the output of the circuit will be F = Ā.Ḃ. The truth table for this equation is given by Table 8.10. If this table is compared with the truth tables of the various gates it will be seen that the NOR logical function has been performed. Therefore, Similarly NAND Preceded by NOT The truth table for the circuit of Fig. 14 is given by Table 11. This shows that the output of the circuit is 1 whenever any one, or more, of its inputs is at 1. This means that the circuit performs the OR logical function. The Boolean expression for the circuit is NAND Followed by NOT When ...

The NAND Gate The NAND gate. Fig .6, performs the inverse logical function to the AND gate. The output of a NAND gate is at 0 only if all of the inputs to the gate are at 1. The truth table of 2-input and 3-input NAND gates are given by Table 8,6. The Boolean expressions for the two NAND gates are given by equations (8.6) and (8.7) respectively. The NOT function can be produced using a NAND gate in either of two different ways. Referring to the truth table of the 2-input NAND gate it can be seen that: (a) if the inputs A and B are connected together so that A = B, Fig . 7( a ), the output will always be inverted, and (b) if either input is held at the logical 1 voltage level, Fig. 7( b ), the output will always be NOT the logical state of the other input. There are a number of NAND gales available in the various TTL sub-families: 7400 quad 2-input NAND gates 7410 triple 3-input NAND gates 7420 dual 4-input.NAND gates 7430 single 8-input NAND gate. The NOR ...

Combinational Digital Circuits Modem electronics make ever increasing use of digital electronic circuitry which responds only to signals that can only take up either one of two logic levels. Either the signal is HIGH or it is LOW. In most circuits the HIGH state is used to represent binary 1 while the LOW state represents binary 0. For example, in the TTL logic family binary 0 is indicated by a voltage in the range 0 V to + 0.8 V and binary 1 is indicated by a voltage in the range + 2 V to + 5 V. For CMOS devices logic 0 is less than VDD /3 and logic 1 is greater than 2KDD/3, where VDD is the power supply voltage. The Binary Code In digital electronic systems, the devices have two stable states, ON and OFF, and for this reason the binary number system is used. In the binary system only two digits 0 and 1 exist. Larger numbers are obtained by utilizing the powers of two. The digit at the right-hand side of a binary number represents a multiple (0 or 1) of 20; the next digit to the l...

Digital Integrated Circuits Digital integrated circuits are all classified as being one of the following: (i) a small-scale integrated circuit (SSI), (ii) a medium-scale integrated circuit (MSI), (iii) a large-scale integrated circuit (LSI) and (iv) a very-large-scale integrated circuit (VLSI). These categories are based on the number of transistors within each 1C. An SSI 1C has up to 10 transistors, an MSI has between 10 and 100 transistors, LSI ICs have between 100 and 5000 transistors and, lastly, a VLSI circuit contains more than 5000 transistors. Digital ICs are manufactured using different technologies and are members of various logic families. The three basic logic families are (a) transistor-transistor logic (TTL), (b) complementary metal oxide semiconductor (CMOS) and (c) emitter-coupled logic (ECL). All three logic families contain sub-families which have their relative advantages and disadvantages making one or another particularly suited for different applications. TTL ...

Linear and digital integrated circuits Most integrated circuits (ICs) are classified as being either linear or digital devices. A linear circuit is one whose output signal has a linear relationship with the input signal. Linear ICs are also known as analogue ICs because the signals that they handle may vary continuously over a range of values. Linear ICs include operational amplifiers (op-amps), audio-frequency power amplifiers and various radio and television circuits. Most linear ICs employ bipolar transistors throughout, although a few types of op-amp have either a JFET or a MOSFET input stage. Digital ICs operate with binary digital signals that are always in either of two possible states; namely logic 0 and logic 1. Digital ICs include gates, counters, memories and micro­processors. Some ICs include both analogue and digital circuitry; three examples of these ICs are timer, analogue-to-digital converters (ADC) and digital-to-analogue converters (DAC). Linear Integrated Circu...

The fabrication of a complete integrated circuit The main differences between integrated and discrete circuits which perform the same function are that the integrated circuit uses transistors and diodes as liberally as possible. This is because resistors and capacitors occupy more space in the chip than do transistors and they In the fabrication of a complete integrated circuit all the components, active and passive, required to make up the circuit are formed at the same time. The components are then interconnected as required by means of an aluminium pattern which is deposited on the top of the silicon slice. As an example, suppose the simple circuit shown in Fig.15(a) is to be integrated. Fig. 15(b) shows the three components of the circuit diffused into a p-type substrate. The components are each isolated from the substrate by a reverse-biased p-n junction and are connected together in the required manner by an aluminium pattern which is deposited on to the surface of the chip. ...

Integrated Circuit Capacitor Integrated capacitors can be fabricated in two ways: either the capacitance of a reverse-biased p-n junction can be utilized, or the capacitance can be provided by a layer of silicon dioxide separating two conducting areas. The construction of a junction-type capacitor is shown in Fig. 14( a ). The p-n junction is formed at the same time as either the emitter-base or the collector-base junction of a transistor. Provided the p-n junction is held in the reverse-biased condition, a capacitance of about 0.2 pF/mil can be obtained. Since the area of the chip available for a capacitor is limited, values of up to about 100 pF are available. Fig. 14(b) shows a MOS capacitor; one electrode of the capacitor is provided by an aluminium layer that is deposited on to the top of the silicon layer and the other electrode is produced by the diffused n + region. The capacitance provided depends on the thickness of the silicon dioxide layer and the area of the aluminium p...