Summary SEC. 3-1 BASIC IDEAS A diode is a nonlinear device. The knee voltage, approximately 0.7 V for a silicon diode, is where the forward curve turns upward. The bulk resistance is the ohmic resistance of the p and n regions. Diodes have a maximum forward current and a power rating. SEC. 3-2 THE IDEAL DIODE This is the first approximation of a diode. The equivalent circuit is a switch that closes when forward biased and opens when reverse biased. SEC. 3-3 THE SECOND APPROXIMATION In this approximation, we visualize a silicon diode as a switch in series with a knee voltage of 0.7 V. If the Thevenin voltage facing the diode is greater than 0.7 V, the switch closes. SEC. 3-4 THE THIRD APPROXIMATION We seldom use this approximation because bulk resistance is usually small enough to ignore. In this approximation, we visualize the diode as a switch in series with a knee voltage and a bulk resistance. SEC. 3-5 TROUBLESHOOTING When you suspect that a diode is the trouble, remove it from the circuit and use an ohmmeter to measure its resistance in each direction. You should get a high resistance one way and a low resistance the other way, at least 1000:1 ratio. Remember to use a high enough resistance range when testing a diode to avoid possible diode damage. A DMM will display 0.5–0.7 V when a diode is forward biased and an overrange indication when it is reverse biased. SEC. 3-6 READING A DATA SHEET SEC. 3-8 DC RESISTANCE OF A DIODE The dc resistance equals the diode voltage divided by the diode current at some operating point. This resistance is what an ohmmeter will measure. DC resistance has limited application, aside from telling you that it is small in the forward direction and large in the reverse direction. SEC. 3-9 LOAD LINES Data sheets are useful to a circuit designer and may be useful to a repair technician when selecting a substitute device, which is sometimes required. Diode data sheets from different manufacturers contain similar information, but different symbols are used to indicate different operating conditions. Diode data sheets may list the following: breakdown voltage (VR, VRRM, VRWM, PIV, PRV, BV), maximum forward current (IF(max), IF(av), I0), forward voltage drop (VF(max), VF ), and maximum reverse current (IR(max), IRRM ). The current and voltage in a diode circuit have to satisfy both the diode curve and Ohm’s law for the load resistor. These are two separate requirements that graphically translate to the intersection of the diode curve and the load line. SEC. 3-7 HOW TO CALCULATE BULK RESISTANCE SEC. 3-11 INTRODUCTION TO ELECTRONIC SYSTEMS You need two points in the forward region of the third approximation. One point can be 0.7 V with zero current. The second point comes from the data sheet at a large forward current where both a voltage and a current are given. SEC. 3-10 SURFACE-MOUNT DIODES Surface-mount diodes are often found on modern electronics circuits boards. These diodes are small, efficient, and typically found either as an SM (surface-mount) or an SOT (small outline transistor) case style. Semiconductor components are combined to form circuits. Circuits can be combined to become functional blocks. Functional blocks can be interconnected to form electronic systems. Definitions (3-1) Silicon knee voltage: + 0.7 V (3-2) N RB 5 RP 1 RN Maximum power dissipation Pmax VK < 0.7 V Bulk resistance: P 80 – (3-4) (3-6) Pmax 5 Vmax Imax Ignore bulk: LINEAR CIRCUIT RTH RB RB , 0.01RTH Chapter 3 Derivations (3-3) Diode power dissipation: (3-7) PD I2 PD 5 VD ID Bulk resistance: V2 2 V1 I2 2 I1 RB 5 _______ I1 (3-5) Third approximation: 0.7 V – + V1 V2 RB VD 5 0.7 V 1 ID RB + VD – Self-Test 1. When the graph of current versus voltage is a straight line, the device is referred to as a. Active c. Nonlinear b. Linear d. Passive 2. What kind of device is a resistor? a. Unilateral c. Nonlinear b. Linear d. Bipolar 3. What kind of a device is a diode? a. Bilateral c. Nonlinear b. Linear d. Unipolar 4. How is a nonconducting diode biased? a. Forward c. Poorly b. Inverse d. Reverse 5. When the diode current is large, the bias is a. Forward b. Inverse c. Poor d. Reverse 6. The knee voltage of a diode is approximately equal to the a. Applied voltage b. Barrier potential c. Breakdown voltage d. Forward voltage 7. The reverse current consists of minority-carrier current and a. Avalanche current b. Forward current c. Surface-leakage current d. Zener current Diode Theory 8. How much voltage is there across the second approximation of a silicon diode when it is forward biased? a. 0 b. 0.3 V c. 0.7 V d. 1 V 9. How much current is there through the second approximation of a silicon diode when it is reverse biased? a. 0 b. 1 mA c. 300 mA d. None of the above 10. How much forward diode voltage is there with the ideal diode approximation? a. 0 b. 0.7 V c. More than 0.7 V d. 1 V 11. The bulk resistance of a 1N4001 is a. 0 b. 0.23 V c. 10 V d. 1 kV 12. If the bulk resistance is zero, the graph above the knee becomes a. Horizontal b. Vertical c. Tilted at 45° d. None of the above 13. The ideal diode is usually adequate when a. Troubleshooting b. Doing precise calculations c. The source voltage is low d. The load resistance is low 14. The second approximation works well when a. Troubleshooting b. Load resistance is high c. Source voltage is high d. All of the above 15. The only time you have to use the third approximation is when a. Load resistance is low b. Source voltage is high c. Troubleshooting d. None of the above 16. How much load current is there in Fig. 3-21 with the ideal diode? a. 0 b. 11.3 mA c. 12 mA d. 25 mA Figure 3-21 + VS 12 V – RL 1 kΩ 81 PIN Diodes A PIN diode is a semiconductor device that operates as a variable resistor at RF and microwave frequencies. Figure 5-43a shows its construction. It consists of an intrinsic (pure) semiconductor material sandwiched between p-type and n-type materials. Figure 5-43b shows the schematic symbol for the PIN diode. When the diode is forward biased, it acts like a current-controlled resistance. Figure 5-43c shows how the PIN diode’s series resistance RS decreases as its forward current increases. When reverse biased, the PIN diode acts like a fixed capacitor. The PIN diode is widely used in modulator circuits for RF and microwave applications. Table of Devices Summary Table 5-3 lists all the special-purpose devices in this chapter. The zener diode is useful in voltage regulators, the LED as a dc or an ac indicator, the sevensegment indicator in measuring instruments, and so on. You should study the table and remember the ideas it contains. Summary SEC. 5-1 THE ZENER DIODE This is a special diode optimized for operation in the breakdown region. Its main use is in voltage regulators— circuits that hold the load voltage constant. Ideally, a reverse-biased zener diode is like a perfect battery. To a second approximation, it has a bulk resistance that produces a small additional voltage. SEC. 5-2 THE LOADED ZENER REGULATOR When a zener diode is in parallel with a load resistor, the current through the current-limiting resistor equals the sum of the zener current and the load current. The process for analyzing a zener regulator consists of finding the series current, load current, and zener current (in that order). SEC. 5-3 SECOND APPROXIMATION OF A ZENER DIODE In the second approximation, we visualize a zener diode as a battery of VZ and a series resistance of RZ. The current through RZ produces an additional voltage across the diode, but this voltage is usually small. You need zener resistance in order to calculate ripple reduction. Special-Purpose Diodes SEC. 5-4 ZENER DROP-OUT POINT A zener regulator will fail to regulate if the zener diode comes out of breakdown. The worst-case conditions occur for minimum source voltage, maximum series resistance, and minimum load resistance. For the zener regulator to work properly under all operating conditions, there must be zener current under the worst-case conditions. SEC. 5-5 READING A DATA SHEET The most important quantities on the data sheet of zener diodes are the zener voltage, the maximum power rating, the maximum current rating, and the tolerance. Designers also need the zener resistance, the derating factor, and a few other items. SEC. 5-6 TROUBLESHOOTING Troubleshooting is an art and a science. Because of this, you can learn only so much from a book. The rest has to be learned from direct experience with circuits in trouble. Because troubleshooting is an art, you have to ask “What if?” often and feel your way to a solution. SEC. 5-7 LOAD LINES The intersection of the load line and the zener diode graph is the Q point. When the source voltage changes, a different load line appears with a different Q point. Although the two Q points may have different currents, the voltages are almost identical. This is a visual demonstration of voltage regulation. SEC. 5-8 LIGHT-EMITTING DIODES (LEDS) The LED is widely used as an indicator on instruments, calculators, and other electronic equipment. Highintensity LEDs offer high luminous efficacy (lm/W) and are finding their way into many applications. SEC. 5-9 OTHER OPTOELECTRONIC DEVICES By combining seven LEDs in a package, we get a seven-segment indicator. Another important optoelectronic device is the optocoupler, which allows us to couple a signal between two isolated circuits. SEC. 5-10 THE SCHOTTKY DIODE The reverse recovery time is the time it takes a diode to shut off 181 after it is suddenly switched from forward to reverse bias. This time may be only a few nanoseconds, but it places a limit on how high the frequency can be in a rectifier circuit. The Schottky diode is a special diode with almost zero reverse recovery time. Because of this, the Schottky diode is useful at high frequencies where short switching times are needed. SEC. 5-11 THE VARACTOR The width of the depletion layer increases with the reverse voltage. This is why the capacitance of a varactor can be controlled by the reverse voltage. A common application is remote tuning of radio and television sets. SEC. 5-12 OTHER DIODES Varistors are useful as transient suppressors. Constant-current diodes hold the current, rather than the voltage, constant. Step-recovery diodes snap off and produce a step voltage that is rich in harmonics. Back diodes conduct better in the reverse direction than in the forward direction. Tunnel diodes exhibit negative resistance, which can be used in highfrequency oscillators. PIN diodes use a forward-biased control current to change its resistance in RF and microwave communication circuits. Derivations (5-3) +VS Series current: (5-8) RS VR(out) VS 2 VZ IS 5 _______ R RZ VR(out) < ___ R VR(in) RZ S S (5-9) Load voltage: Maximum series resistance: RS(max) +VS + VL – VZ (5-5) RS VR(in) VZ (5-4) Output ripple: +VS(min) ( ) VS(min) RS(max) 5 _____ 2 1 RL(min) Vz Load current: IL +VS (5-10) + VL RL VL IL 5 ___ RL – Maximum series resistance: RS(max) +VS(min) VZ (5-6) RL(min) VZ VL 5 VZ IL(max) Zener current: IS VS(min) 2 VZ RS(max) 5 __________ I +VS L(max) IZ IL IZ 5 IS 2 IL (5-13) LED current: RS +VS (5-7) Change in load voltage: VD +VS RZ 182 + VL – VS 2 VD IS 5 ________ R S DVL 5 IZRZ Chapter 5 Figure 6-32 Transistor curve tracer test. Copyright © Tektronix, Printed with permission. All rights reserved. Summary SEC. 6-1 THE UNBIASED TRANSISTOR smaller, typically less than 5 percent of the emitter current. Saturation and cutoff are used in digital circuits. A transistor has three doped regions: an emitter, a base, and a collector. A pn junction exists between the base and the emitter; this part of the transistor is called the emitter diode. Another pn junction exists between the base and the collector; this part of the transistor is called the collector diode. SEC. 6-3 TRANSISTOR CURRENTS SEC. 6-5 THE BASE CURVE SEC. 6-2 THE BIASED TRANSISTOR For normal operation, you forward-bias the emitter diode and reverse-bias the collector diode. Under these conditions, the emitter sends free electrons into the base. Most of these free electrons pass through the base to the collector. Because of this, the collector current approximately equals the emitter current. The base current is much BJT Fundamentals The ratio of the collector current to the base current is called the current gain, symbolized as dc or hFE. For low-power transistors, this is typically 100 to 300. The emitter current is the largest of the three currents, the collector current is almost as large, and the base current is much smaller. SEC. 6-4 THE CE CONNECTION The emitter is grounded or common in a CE circuit. The base-emitter part of a transistor acts approximately like an ordinary diode. The base-collector part acts like a current source that is equal to dc times the base current. The transistor has an active region, a saturation region, a cutoff region, and a breakdown region. The active region is used in linear amplifiers. The graph of base current versus base-emitter voltage looks like the graph of an ordinary diode. Because of this, we can use any of the three diode approximations to calculate the base current. Most of the time, the ideal and the second approximation are all that is necessary. SEC. 6-6 COLLECTOR CURVES The four distinct operating regions of a transistor are the active region, the saturation region, the cutoff region, and the breakdown region. When it is used as an amplifier, the transistor operates in the active region. When it is used in digital circuits, the transistor usually operates in the saturation and cutoff regions. The breakdown region is avoided because the risk of transistor destruction is too high. 231 SEC. 6-7 TRANSISTOR APPROXIMATIONS SEC. 6-10 VARIATIONS IN CURRENT GAIN SEC. 6-13 RECOGNIZING SATURATION Exact answers are a waste of time in most electronics work. Almost everybody uses approximations because the answers are adequate for most applications. The ideal transistor is useful for basic troubleshooting. The third approximation is needed for precise design. The second approximation is a good compromise for both troubleshooting and design. The current gain of a transistor is an unpredictable quantity. Because of manufacturing tolerances, the current gain of a transistor may vary over as much as a 3;1 range when you change from one transistor to another of the same type. Changes in the temperature and the collector current produce additional variations in the dc gain. SEC. 6-8 READING DATA SHEETS SEC. 6-11 THE LOAD LINE Transistors have maximum ratings on their voltages, currents, and powers. Small-signal transistors can dissipate 1 W or less. Power transistors can dissipate more than 1 W. Temperature can change the value of the transistor characteristics. Maximum power decreases with a temperature increase. Also, current gain varies greatly with temperature. The dc load line contains all the possible dc operating points of a transistor circuit. The upper end of the load line is called saturation, and the lower end is called cutoff. The key step in finding the saturation current is to visualize a short between the collector and the emitter. The key step to finding the cutoff voltage is to visualize an open between the collector and emitter. The idea is to assume that the npn transistor is operating in the active region. If this leads to a contradiction (such as negative collector-emitter voltage or collector current greater than saturation current), then you know that the transistor is operating in the saturation region. Another way to recognize saturation is by comparing the base resistance to the collector resistance. If the ratio is in the vicinity of 10 ;1, the transistor is probably saturated. SEC. 6-9 SURFACE-MOUNT TRANSISTORS Surface-mount transistors (SMTs) are found in a variety of packages. A simple three-terminal gull-wing package is common. Some SMTs are packaged in styles that can dissipate more than 1 W of power. Other surface-mount devices may contain (house) multiple transistors. SEC. 6-12 THE OPERATING POINT SEC. 6-14 THE TRANSISTOR SWITCH Base bias tends to use the transistor as a switch. The switching action is between cutoff and saturation. This type of operation is useful in digital circits. Another name for switching circuits is two-state circuits. SEC. 6-15 TROUBLESHOOTING The operating point of the transistor is on the dc load line. The exact location of this point is determined by the collector current and the collector-emitter voltage. With base bias, the Q point moves whenever any of the circuit values change. You can use a DMM or ohmmeter to test a transistor. This is best done with the transistor disconnected from the circuit. When the transistor is in the circuit with the power on, you can measure its voltages, which are clues to possible troubles. Definitions (6-2) (6-3) DC alpha: DC beta (current gain): IC IC IC dc 5 __ I IC dc 5 __ I IB B E IE Derivations (6-1) Emitter current: (6-4) IC IB Collector current: IC IE 5 IC 1 IB IB IC 5 dcIB bdc IE 232 Chapter 6
