Fullscreen HTML5Med Res (??MB)HTML4

DIGITAL F In this chapter, we look at the circuits· that make up the various logic elements and discuss · their operation. We also look at the various logic families; and ·we .test your knowledge. LESSON 3: DIGITAL CIRCUITS By Louis E. Frenzel, Jr. In the previous lesson, we introduced the basic digital logic elements, such as the inverter, AND gate, OR gate, and NAND and NOR gates. These are the basic logic elements that process binary signals in digital equipment. We discussed their operation in terms of the logic functions they perform and the operation of each was expressed in Boolean algebra, truth tables, and timing diagrams. Only logic symbols were used to illustrate those devices. In this lesson though, we want to take a look inside the logic symbols. There are two basic methods of implementing digital circuits: discrete and integrated. Discrete component circuits are those made up of individual transistors, resistors, diodes, capacitors and other components wired together on a printed circuit board. The integrated circuit has all the components together on a tiny chip of silicon. Today, most digital circuits you will encounter will be of the integrated circuit form. Occasionally, however, you will run across a discrete component circuit in an older piece of equipment or in one requiring some special or simple function. We will discuss both ICs and discrete component circuits in this lesson. Inverters Let's begin our discussion with the circuit used to make a logic inverter. We will talk about simple discrete-component circuits first and that knowledge will easily translate to integrated circuits. For our discussion here, zero volts or ground represents a binary O and + 5V DC represents a binary 1. The main element in an inverter circuit is a switch as shown in Fig. la. The switch is connected in series with a resistor to the supply voltage. A binary input 86 SILICON CHIP +5V ,_:~ INJ!UT---l\ ~ /OUTPUT ✓ " -- R1 OUTPUT (a) Fig.1: a logic inverter operates like a shunt switch (a) in parallel with the output. A transistor (b) operated at cut-off and saturation functions in the same manner. signal controls the operation of the switch and the binary output appears across the switch. When the input is binary 0, the switch is open. The output, therefore, is + 5V or binary 1. If the input is binary 1, the switch is closed. Current flows through the switch and resistor Rl. The output is 0 volts or binary 0, because the resistance of the switch is near zero. A common bipolar transistor can be used as the switch to form a simple inverter as shown in Fig.1 b. A transistor works well in this application because it can be turned off so that no current flows through it, or it can be turned on to let current flow through it. Recall that there are three basic operational states of a bipolar transistor: cut off or non-conducting, conducting in the linear region, and saturation. Those states are achieved by biasing (see Review of Diode and Transistor Biasing). When the transistor is cut off, it is non-conducting and acts as an open circuit. With the proper bias on the transistor, it conducts d. Fig.2: a MOSFET inverter (Qt) using a MOSFET (Q2) as a load in place of a power-consuming resistor. flJ•--.::.1•1,-:.m INPIIT .. in the linear region. This means that the collector current is directly proportional to variations in the base current. That permits a small base current to control a large collector current and thus cause amplification to take place. The linear conducting mode is not generally used in digital electronics. If sufficiently high bias current is applied to the base of the transistor it will conduct hard and act as a very low resistance. During that time both the emitterbase and base-collector junctions are forward biased. The voltage drop between the collector and emitter at that time is extremely low; therefore, the transistor appears to be a virtual short circuit. When in saturation, the transistor acts as a closed switch. Using those principles, the operation of the inverter is easy to understand. If the input to the inverter in Fig.1 b is 0V or ground, the base-emitter junction of the transistor will not be forward biased. No current will flow through Rl or the transistor. The transistor thus appears to be an open circuit. Therefore the output is + 5V or binary 1 as seen through Rl. When a binary 1 or + 5V signal is applied to the input, the base-emitter junction is forward biased. The value of resistor R2 is made low enough so that the base current is high enough for the transistor to saturate. During this time, the transistor acts as a very low resistance. A typical output voltage between collector and emitter might be 100 millivolts (100mV). This is sufficiently low so as to represent a binary o. Metal oxide semiconductor field-effect transistors (MOSFETs) can also be used to form an inverter as shown in Fig.2. Here N-channel enhancement mode MOSFETs are used. Ql is the inverter switch while Q2 acts as the load resistor. This technique is widely used in N-channel MOS (metal oxide semiconductor) integrated circuits. Transistor loads are easy to make in integrated circuit form and also take up much less space than an integrated resistor. An enhancement mode MOSFET may also act as a switch. When its gate voltage is below some threshold voltage (about + 1.5 volts in common N-channel MOSFETs), the transistor is cut off. It acts as an open switch. When a positive logic signal above the threshold value is applied to the gate, the transistor conducts; therefore it acts as a closed switch. The operation of the inverter in Fig.2 is simple. When the input voltage is binary 0 (near 0V), transistor Ql does not conduct. Q2, however, is conducting because it is biased on. The output voltage is, therefore, + 5V as seen through Q2. When the input voltage is a binary 1 level or approximately + 5V, Ql conducts. It acts as a very low resistance; therefore, little voltage is dropped across it. The output voltage is near 0V or binary 0. AND and OR Gates AND and OR gates can be constructed with diodes and resistors. For example, a simple OR gate is illustrated in Fig.3. If both inputs to the OR gate are binary 0 or ground, neither diode conducts and no current flows through resistor Rl. The output, therefore, is at 0V or ground as seen through Rl. If both inputs are binary 1, both diodes D1 and D2 conduct. Current flows through resistor Rl. The output, therefore, is a binary 1. Most of the voltage applied to the inputs will appear across Rl except approximately 0.7V which is dropped across each diode. With a + 5V input, the output would be approximately +4.3V. If one input is binary 0 and the other binary 1, the output will also be binary 1. For example, if input A is + 5V and input Bis 0V, diode D1 in Fig.3 will conduct. The output will be approximately + 4.3V. That will cause diode D2 to be reverse biased and it will be cut off. A simple discrete-component AND gate is shown in Fig.4. If both inputs are binary 0 or ground, both diodes D1 and D2 conduct. Current flows through Rl. The output voltage at that point is the voltage drop across the diodes. For a typical silicon diode, the voltage drop will be approximately 0.7V. That is a lowlevel voltage and represents a binary 0. If one input is binary O and the other is binary 1, the output will also be binary 0. For example, if input A is binary 0 and input Bis binary 1 or + 5V, diode D1 conducts. The output will be approximately + 0.7V. This means that diode D2 will be reverse biased and, therefore, cut off. If both inputs are binary 1 or + 5V, both diodes conduct. The output will be + 5V less the voltage drop D1 e--------c INPUTS 02 A B OUTPUT R1 C ,, ,, ,,, 0 0 0 0 0 ... Fig.3: simple diode OR gate and its truth table. +12V R1 01 A 8 A INPUTS C OUTPUT 02 r... B Fig.4: diode AND C ,, ,, , 0 0 0 0 0 0 0 gate and its truth table. JANUARY 1988 87 across the diodes. If the inputs are + 5V, then the output will be + 4.3V or binary 1. If both circuits are as shown in Figs.3 and 4, additional diodes may be added to accommodate more inputs as needed. Furthermore, those simple circuits can be combined with inverters to implement almost any logic function. However, discrete component circuits take up a lot of space and are inconvenient to construct. Their performance is also generally poor. For that reason, they are used only where simple noncritical circuits are needed. +5V INPUTS OUTPUT C = A.ii 00 Digital Integrated Circuits Virtually all pieces of digital equipment are built these days using integrated circuits. An integrated circuit is one in which all the components - including transistors, diodes, resistors and capacitors - are fully formed and interconnected on a tiny silicon chip. Many inverters, logic gates, flipflops, and other logic and linear circuits can be contained within a small area. A typical silicon chip is roughly square, with sides of approximately 2.5 to 6mm. The smaller chips contain several gates or inverters while the larger chips might contain a complex circuit such as a microprocessor. The chip is encapsulated in a special package with leads that can be plugged into a socket or soldered to a printed circuit board. The most popular form of package is the dual-in-line package, or · DIP (see Fig.5), which may have 8 to 64 pins. Digital integrated circuits are generally divided into four basic categories: SSI, MSI, LSI and VLSI. Those designations, described in Table 1, show how digital Fig.5: dual in-line package (DIP) for integrated circuits. OUTPUT :::::[3o---c ~ = TI NANO TRUTH TABLE A B C 0 0 D 1 1 1 0 1 1 1 1 D ~ Fig.6: simplified TTL NAND gate circuit (a), and its schematic symbol (b) and truth table. !Cs are classified according to size, density and function. Digital !Cs are also classified by the type of transistors used in their circuitry. The two basic types are bipolar and MOSFET. IC manufacturers have developed a wide range of digital IC families using both types of transistors. Typical bipolar families include resistor-transistor logic (RTL), diode-transistor logic (DTL), transistor-transistor logic (TTL), emittercoupled logic (EGL), integrated-injection logic (I2L) and several others. RTL and DTL aren't used any more in new designs, but you may occasionally find them in older equipment. TTL and EGL are widely used today while I2L circuits are common in LSI and VLSI designs. MOS logic families include P-channel and Nchannel MOS and complementary MOS (CMOS). Because bipolar circuits are larger and consume more power, they take up more space on a silicon chip and, therefore, are used primarily for SSI and MSI circuits. Most LSI and VLSI circuits are MOS or CMOS. In this lesson we are going to talk about the most popular forms of logic today, TTL and CMOS. Transistor/Transistor Logic TABLE 1 LEVEL OF ICs BASED ON CIRCUIT DENSITY SSI MSI LSI VLSI 88 Small-Scale Integration: Chips containing 12 or less gates, inverters, flipflops etc. Medium-Scale Integration: Chips containing 12 to 100 gates, inverters, tliptlops etc, usually connected as functional circuits that do something such as counters, registers, decoders, multiplexers, and many others. Large-Scale Integration: Chips with 100+ gates, tliptlops etc, often forming complete circuits such as microprocessors, pro.gram and control circuits, and many others. Very Large-Scale Integration: Chips with 1000+ gates, flipflops and other circuits such as 32-bit microprocessors, data acquisition systems, gate arrays and much more. SILICON CHIP Probably the most popular form of SSI and MSI digitial ICs is transistor/transistor logic (TTL). TTL is used in everything from personal computers to the most advanced avionics equipment. TTL circuits use bipolar transistors and operate on a power supply voltage of + 5V DC. The basic TTL logic circuit is illustrated in Fig.6. That particular circuit performs the positive logic NAND function. However, other versions are available to perform the AND, OR and NOR functions . A single input version of the circuit is used as an inverter. The circuit has three parts: a multiple emitter-input transistor fQl), a phase splitter transistor (Q2), and the output stage (Q3 and Q4}. Refer to Fig.6a. Transistor Qt and Rt function as a simple diode AND gate where the emitter-base junctions of Qt are diodes. The main purpose of phase splitter Q2 is to furnish complementary logic signals to the two output transistors Q3 and Q4. Q4 is the output switching transistor and performs the function of an inverter while Q3, along with Dl and R4, forms an active pull-up stage. It is similar in operation to the MOSFET load resistor described earlier. Some TTL circuits are available without the active pull-up stage. Q3, Dl and R4 are eliminated and the collector of Q4 is brought out to one of the DIP pins. An external load must be connected. Open collector circuits are useful for driving components such as LED indicators, relays and other external circuits. In most cases the active pull-up circuit is preferred, because it represents a very low impedance when the output of the gate is binary 1. That permits the circuit to quickly charge any external capacitance, thereby greatly reducing the rise time and speeding up the circuit. The logic levels for the typical TTL circuit are 0V to + 0.8V for binary 0 and + 2.4V to + 5V for binary 1. Now let's see how the TTL circuit functions. Remember that it is a NAND circuit. You can refresh your memory about how it works by referring to the truth table in Fig.6b. Assume that either or both inputs A and Bare at ground or binary 0. The corresponding emitter-base junctions of Ql then conduct through Rl. When Qi is conducting, it pulls the base of Q2 to almost 0V and so Q2 is cut off. As a result, base current is supplied through R2 to Q3 which conducts. Transistor Q4 is cut off at that time. The output voltage will be + 5V less the voltages dropped across R4, Q3 and Dl. This output voltage is typically in the + 2.4V to + 3.6V range. If both inputs are binary 1, the base-emitter junctions of Ql do not conduct. However, the basecollector junction of Ql does conduct and provides base current to Q2 and through Q4. Transistor Q4 saturates and effectively brings the output to near ground level. The most popular form of TTL is the 7400 series which provides of all of the commonly used logic functions. The individual ICs are usually labelled with the manufacturer's logo, the device type number, and a REVIEW OF DIODE AND TRANSISTOR BIASING Diodes and bipolar transistors are made by combining N- and P-type semiconductor materials (silicon, germanium, gallium arsenide) to form junctions . A PN junction creates a diode. CATHODE ® If the base-emitter junction is forward biased and the base-collector junction is reverse biased, the transistor conducts. This is the normal condition for a transistor operating in the linear region for amplification. In logic and switching applications, the bias arrangements shown below are used . Here the transistor is used as a switch to turn on an incandescent bulb. ANODE The diode symbol (above) is used in schematic diagrams. The current flowing in a diode depends on its bias, an externally applied voltage. The circuits show the two ways to bias a diode. 01 n + T T I I ....L. 'I .J.. 'T' I I Rl ....L. REVERSE BIAS (NO CURRENT FLOW) FORWARD BIAS (CURRENT FLOWS) C )l (a) ""T"" I I Rl .L + (b) (a) If the cathode (N-type material) is made negative with respect to the anode as shown at A, the diode is forward biased and it conducts. The amount of current flowing is controlled by the resistive value of R1 . If the cathode is made positive with respect to the anode as shown in B above, the diode is reverse biased. No curr-ent flows in the circuit. Transistor biasing follows similar rules . An NPN transistor is illustrated below. EMITTER C BASE (8) I ! ) ~---tt-·----111 ~---1 + (b) ... If the input is grounded as in A, the base-emitter junction is not forward biased. Therefore , the transistor does not conduct and the bulb does not light. If the base is made positive as shown in B, the base-emitter junction is forward biased . The base-collector junction is reverse biased so the transistor conducts and the bulb lights. If the base current is made high as determined by Rb, the transistor will conduct hard. Its collector-to-emitter resistance will be very low and only a small voltage will appear between the collector and emitter. The collector may only be +0 .1 V with respect to the emitter at ground . With the base-emitter junction forward biased and conducting , the voltage across it will be the same as a conducting diode or about +o. 7V. If the base is +0. 7V with respect to the emitter or ground and the emitter is +0.1 V, then the base is positive with respect to the collector. This means that the base-collector junction is forward biased also. This condition is caused by high base drive. When both junctions are forward biased, the transistor is said to be saturated. Saturated operation is typical in bipolar transistor logic circuits . JANUARY1988 89 date code. Each of the TTL manufacturers such as TI, Fairchild, Signetics, National Semiconductor, and others, has its own special company symbol. The part number designates the specific device. For example, a 7430 is a single 8-input NAND gate. INPUTS OUTPUT A ~ A+B B~ INPUTS NOR TRUTH TABLE Propagation delay A B C Propagation delay is the time that it takes a logic change at the input to propagate through the device and appear as a logic-level change at the output (see Fig. 7). For TTL circuits the propagation delays are generally in the 2 to 30 nanosecond range and operating frequencies up to 125MHz are possible. 0 0 1 1 0 1 0 1 1 0 0 0 INPUT---- Ip Fig.7: propagation delay is defined as the time offset between input and output logic level transitions. Power dissipation is another important parameter. The lower the power consumption, the better. However, the faster the circuit, the more power it consumes. Most common TTL circuits have a power consumption in the 1-25 milliwatt (mW) range per gate. Going faster Some TTL circuits use Schottky diodes to speed up circuits while reducing power consumption. Essentially, each transistor in the circuit has a Schottky diode connected between the base and collector as shown in Fig.B. OR Fig.8: schematic diagram of a Schottky transistor. When saturated transistors are used in an IC, it takes a finite amount of time for the circuit to turn off. That condition, known as charge-storage puts a limit on the speed of operation. However, if a Schottky diode is used, saturation does not occur and there is no charge storage problem. As a result, switching speeds are faster and propagation delays are lower. However, standard Schottky TTL circuits have relatively high power consumption. The most popular TTL circuits today are the socalled low-power Schottky devices that have propagation delays as low as two nanoseconds. Those ICs are designated by an LS in their part number (ie, 74LS20). 90 SILICON CHIP W 00 Fig.9: schematic diagram of a CMOS NOR gate (a), and its schematic symbol (b) and truth table. Complementary MOS Ip DIOOE PREVENTS SATURATION S = SOURCE G = GATE D = DRAIN Another popular family of SSI and MSI logic circuits is complementary MOS or CMOS. CMOS circuits use both P-channel and N-channel MOSFETs, thus the prefix C for complementary. The power supply voltage is typically + 5V, although most CMOS circuits can operate reliably with supply voltages in the + 3V to + 18V range. Fig.9a shows the basic CMOS logic gate which performs the NOR function as indicated by the symbol and truth table in Fig.9b. Note that Qt and Q2 are Pchannel MOSFETs while Q3 and Q4 are N-channel MOSFETs. All four transistors are enhancement-mode MOSFETs meaning that the transistor is normally off until the threshold gate voltage is exceeded with a logic input signal. Now let's see how that CMOS NOR circuit operates. Assume logic levels of 0V and + 5V for binary 0 and binary 1, respectively. Keep in mind that in order for an N-channel MOSFET to conduct, its gate voltage must be positive with respect to its source. Usually the threshold value is approximately + 1.5V. Any input voltage greater than that will cause the transistor to conduct. Otherwise, the transistor will be off. In P-channel MOSFETs, the gate must be made negative with respect to the source. Again, the threshold value must be observed. Referring to Fig.9a, assume that both inputs A and B are at 0V or at ground potential. Since ground is more negative than + 5V, the gates are negative with respect to the sources so both Qt and Q2 conduct. Q3 and Q4 will be cut off at that time, because their gates are at 0V and below the threshold level. As a result, the output will be + 5V as seen through Qt and Q2. If either input A or B is binary 0 while the other is + 5V or binary 1, then either one but not both transistors Qt and Q2 will conduct. For example, if input A is 0V and input Bis + 5V, Ql will conduct but Q2 will be cut off. If input Bis binary 1, Q2 will be cut off but Q4 will conduct. With Q4 conducting, the output will be binary 0. With both inputs binary 1, both Ql and Q2 are cut off. However, both Q3 and Q4 will conduct, keeping the output at binary 0. The truth table sums up all possible conditions of inputs and outputs of the NOR gate. Naturally, additional inputs and transistors may +5V +5V +5V 1k IN (b) (a) Fig.11: using a 7401 open-collector 2-input NANO integrated circuit as a NANO (a), and as an inverter LED driver (b). (b) .,. (al Fig.to: schematic diagram of a TTL quad 2-input NANO gate (a) and a logic circuit made from the quad sections of a 7400 IC (b). be added to create gates with 3, 4 or 8 inputs. CMOS ICs are very popular because of their very low power consumption. The only time current really flows in the circuit is while the output switches from one state to the another. The power dissipation of a typical gate is in the 10-nanowatt range. This is very low power consumption and makes power supplies simpler and cheaper, and heat dissipation from the IC practically non-existent. Even though low power consumption is the primary virtue of CMOS circuits, that does not mean that they are necessarily slow. They are typically slower than TTL circuits, but fast enough for many applications. Typical propagation delays are in the 10 to 50 nanosecond range. Another advantage of CMOS circuits is their high noise immunity. That means they essentially ignore any extraneous signal, pulse, glitch or undesirable input. As a result, CMOS is excellent for use in industrial and automotive applications where high noise is common. The two most popular lines of CMOS circuits are the RCA 4000 series and the Motorola 14000 series. Both have a wide variety of gates, flipflops, inverters and functional logic circuits such as counters, registers, multiplexers, decoders and others. Using Logic Gates Fig.10 shows how TTL gates are used. Fig.10a illustrates a common TTL IC, the popular 7400 quad SHORT QUIZ ON DIGIT AL FUNDAMENTALS - LESSON 3 7. Logic circuits using both N-channel and P1 . In logic circuits, the transistor is used as a: a. resistor c. switch b. diode d. capacitor 2. When both junctions of a bipolar transistor are forward biased, the transistor is said to be._ _ __ channel MOSFETs are called _ _ __ _ MOS. 8. An enhancement mode N-channel MOSFET has a threshold of + 1 .5V. The gate voltage is +0. 7V with respect to the source. The MOSFET is: a. cut off b. conducting 3 . To save space on an integrated-circuit chip, a ____________ is used as a pull-up or load in a MOS inverter. 9. The most popular form of TTL has high values of circuit resistors and uses diodes between the base and collectors of the transistors to prevent saturation . This kind of TTL is called _ _ _ . 4. The basic TTL gate performs which logic function: b. NANO a. AND 10. The primary advantage of CMOS is: a. low cost b. low propagation c . low noise margin delay d. low power consumption c. OR d. NOR 5. The inputs to a 7 400 TTL gate are +0 .1 and +3.8V. The output will be: a. binary O b. binary 1 c. no change d. not enough information 6. Which of the following is not a type of bipolar logic? b . ECL a. NMOS d . DTL c . RTL 11 . TTL is slower than CMOS. a. True b. False AJBU!q ·q ·g as1ei ·q · ~ ~ UO!tdwnsuoo J8MOd MO( ·p ·0 ~ A}IHOlfOS J8MOd MO( .6 JJO lOO '8 AJBlU8W8fdWOO .L SOv'JN ·e ·g ON\fN ·q ·v (JOlS!SUBJl l08ij8 Pl8!! JOlOriP,UOO!W8S ap1xo 1etaw) 1 3.:ISOv'J ·8 pateJntes · c 40l!MS ·O . ~ S~3MSN'1 JA N UA RY 1988 91 2-input NAND while Fig. lOb shows a typical logic circuit using it. Note that unused inputs should be connected together and to the supply voltage via a resistor to avoid extraneous input signals. Fig.11 shows another IC circuit application, using a 7401. This is similar to the 7400 in that it is a quad 2-input NANO. However, the outputs are all open collector, meaning that they require an external load or pull-up resistor. The pull-up resistor is shown in Fig.lla. Fig.llb shows how the gate is used as an inverter and LED driver. The output load is a LED with a resistor to set the current value. When the input is low (binary 0), the output is high and the LED is off. If the input is high (binary 1), the output is low and the LED turns on. A CMOS circuit application using a 4001 quad 2-input gate is shown in Fig.12. With two of its inputs wired together a gate becomes an inverter (Fig.12b). Two gates wired as inverters can be connected to form a simple astable multivibrator, usually called a clock circuit (Fig.12c). The output is a continuous rectangular pulse train with frequency determined by the values of resistance and capacitance in the circuit. With the values shown, the output frequency is F = Did you • llllSS +5V ~=~ 2 (b) JUUUl f = 1/2.2RC (C) (a) Fig.12: a CMOS 2-input quad NOR integrated circuit is shown in (a). With the two inputs tied together (b) the gate becomes a simple inverter that can be used for the pulse generator shown in (c).The numbers indicate the pin termination in (a). 1/2.2RC, where R = lOk0, and C = l000pF, and the frequency is 45,454Hz or 45.454kHz. ~ Reproduced from Hands-On Electronics by arrangement. Gernsback Publications, USA. © these issues? LIMITED NUMBERS OF BACK ISSUES ARE AVAILABLE SO DON'T DELAY ISSUE HIGHLIGHTS ~---------I I I I I I I I I I I ........_ ·.:',: ... -------, Please send me a back issue for D November 1987 □ December 1987 Enclosed is my cheque or money order for $ .... .... or please debit my □ Bankcard □ Visa Name .... ... ...... ... .. ..... .......... ... ... ..... ... ... .......... .... .... ....... ......... ....... . . Address ........ ....... ... ....... .. .. ... .. .... ......... ..... .. ... ...... ............... .. ....., .. Suburb/town .. .... .... ..... ........ ..... ............ ..... ..... .. Postcode ......... ...... . Card No ... ...... ... ... ..... ....... ......... ....... .... .. ........ ....... .... .. .... ..... .. .... .. . Signature .... .. .... .. .... .. ...... ........ ........ .Card expiry date ....... / ....... /... ... . I I November 1987: Your House Wiring Could Kill You; 1GHz Digital Frequency Meter; Car Stereo in Your Home; Capacitance Adapter for DMMs December 1 98 7: High-Power Amplifier Module; Building an AT-Compatible Kit Computer; Passive lnfrared Sensor for Burglar Alarms; Universal Speed Control and Lamp Dimmer; 24V to 12V Converter. Price: $5.00 each (incl. p&p). Fill out the coupon at left and send to: s,ucoN CHIP, PO Box 139, Collaroy Beach, 2097. I----------------- - ------------------ 92 SILICON CHIP