SEMICONDUCTOR BASICS CHAPTER 1
According to the classical Bohr model, Bohr model, the atom is viewed as having a planetary-type structure with electrons orbiting at various distances around the central nucleus. The nucleus of an atom consists of protons and neutrons. The protons have a positive charge and the neutrons are uncharged. The number of protons is the atomic number of the atom. Electrons have a negative charge and orbit around the nucleus at distances that depend on their energy level. An atom has discrete bands of energy called shells in which the electrons orbit. Atomic structure allows a certain maximum number of electrons in each shell. These shells are designated 1, 2, 3, and so on. In their natural state, all atoms are neutral because they have an equal number of protons and electrons. The outermost shell or band of an atom is called the valence band , and electrons that orbit in this band are called valence electrons. electrons. These electrons have the highest energy of all those in the atom. If a valence electron acquires enough energy from an outside source such as heat, it can jump out of the valence band and break away from its atom. Semiconductor Semiconductor atoms have four valence electrons. Silicon is the most widely used semiconductive material. Materials that are conductors have a large number of free electrons and conduct current very well. Insulating materials have very few free electrons and do not conduct current at all under normal circumstances. Semiconductive materials fall in between conductors conductors and insulators in their ability to conduct current. Semiconductor atoms bond together in a symmetrical pattern to form a solid material called a crystal . The bonds that hold a crystal together are called covalent bonds. bonds. Within the crystal structure, the valence electrons that manage to escape from their parent atom are called conduction electrons or free or free electrons. electrons. They have more energy than the electrons in the valence band and are free to drift throughout the material. When an electron breaks away to become free, it leaves a hole in the valence band creating what is called an electron-hole pair . These electron-hole pairs are thermally produced because the electron has acquired enough energy from external heat to break away from its atom. A free electron will eventually lose energy and fall back into a hole. This is called recombination. recombination . But, electron-hole pairs are continuously being thermally thermally generated so there are always free electrons in the material. When a voltage is applied across the semiconductor, the thermally produced free electrons move in a net direction and form the current. This is one type of current in an intrinsic (pure) semiconductor. Another type of current is the hole current. This occurs as valence electrons move from hole to hole creating, in effect, a movement of holes in the opposite direction. An n-type semiconductive material is created by adding impurity atoms that have five valence electrons. These impurities are pentavalent are pentavalent atoms. A p A p-type -type semiconductor is created by adding impurity atoms with only three valence electrons. These impurities are trivalent atoms. trivalent atoms. The process of adding pentavalent or trivalent impurities to a semiconductor is called doping . The majority carriers in an n-type semiconductor are free electrons acquired by the doping process, and the minority carriers are holes produced by thermally generated electron-hole pairs. The majority carriers in a p-type p-type semiconductor are holes acquired by the doping process, and the minority carriers are free electrons produced by thermally generated electronhole pairs. A pn junction is formed when part of a material is doped n-type and part of it is doped p-type. p-type. A depletion region forms starting at the junction that is devoid of any majority carriers. The depletion region is formed by ionization.
CHAPTER 2 DIODE APPLICATIONS
The single diode in a half-wave rectifier is forward-biased and conducts for 180º for 180º of the input cycle. The output frequency of a half-wave rectifier equals the input frequency. PIV (peak inverse voltage) is the maximum voltage appearing across the diode in reverse bias. bias . Each diode in a full-wave rectifier is forward-biased and conducts for 180º of the input cycle. The output frequency of a full-wave rectifier is twice the input frequency. The two basic types of full-wave rectifier are center-tapped and bridge. The peak output voltage of a center-tapped full-wave rectifier is approximately approximately one-half of the total peak secondary voltage less one diode drop. The PIV for each diode in a center-tapped full-wave rectifier is twice the peak output voltage plus one diode drop. The peak output voltage of a bridge rectifier equals the total peak secondary voltage less two diode drops. The PIV for each diode in a bridge rectifier is approximately approximately half that required for an equivalent center-tapped center-tapped configuration and is equal to the peak output voltage plus one diode drop. A capacitor-input filter provides a dc output approximately approximately equal to the peak of its rectified input voltage. Ripple voltage is caused by the charging and discharging discharging of the filter capacitor. The smaller the ripple voltage, the better the filter. Regulation of output voltage over a range of input voltages is called input or input or line regulation. Regulation of output voltage over a range of load currents is called load regulation. Diode limiters cut off voltage above or below specified levels. Limiters are also called clippers. clippers. Diode clampers add a dc level to an ac voltage. A dc power supply typically consists of an input transformer, a diode rectifier, a filter, and a regulator.
CHAPTER 3
SPECIAL-PURPOSE DIODE
The zener diode operates in reverse breakdown. breakdown. There are two breakdown mechanisms in a zener diode: avalanche breakdown and zener breakdown. breakdown. When V Z < 5 V, zener breakdown is predominant. When V Z > 5 V, avalanche breakdown is predominant. A zener diode maintains a nearly constant voltage across its terminals over a specied range of zener currents. Zener diodes are used as voltage regulators and limiters. Zener diodes are available in many voltage ratings ranging from 1.8 V to 200 V. A varactor diode acts as a variable capacitor under reverse-bias conditions. The capacitance of a varactor varies inversely with reverse-bias voltage. The current regulator diode keeps its forward curre nt at a constant specified value. The Schottky diode has a metal-to-semiconductor metal-to-semiconductor junction. It is used in fast-switching applications. The tunnel diode is used in oscillator circuits circuits.. Aka ESAKI diode An LED emits light when forward-biased. LEDs are available for either infrared or visible light. The photodiode exhibits an increase in reverse current with light intensity. The pin diode has a p region, an n region, and an intrinsic ( i ) region and displays displays a variable variable resistanc resistance e characteristic when forward-biased and a constant capacitance when reverse-biased. A laser diode is similar to an LED except that it emits coherent (single wavelength) light when the forward current exceeds a threshold value.
CHAPTER 4 BJT‘S
The BJT (bipolar junction transistor) is constructed with three regions: base, collector, and emitter. The BJT has two pn two pn junctions, the base-emitter junction and the base-collector junction. Current in a BJT consists of both free electrons and holes, thus the term bipolar . The base region is very thin and lightly doped compared to the collector and emitter regions. The two types of bipolar junction transistor are the npn and the pnp the pnp.. To operate as an amplifier, the base-emitter junction must be forward-biased and the base-collector junction must be reverse-biased. reverse-biased. This is called forward-reverse bias. bias . The three currents in the transistor are the base current (I ( I B), emitter current (I (I E), and collector current (I (I C). I B is very small compared to I C and I E.
The dc current gain of a transistor is the ratio of I C to I B and is designated designated several hundred.
. Values typically range from less than 20 to
DC
is usually referred to as hFE on transistor data sheets. The ratio of I of I C to I B is called DC. Values typically range from 0.95 to 0.99. When a transistor is forward-reverse biased, the voltage gain depends on the internal emitter resistance and the external collector resistance. A transistor can be operated as an electronic switch in cutoff and saturation. In cutoff, both pn both pn junctions are reverse-biased and there is essentially no collector current. The transistor ideally behaves like an open switch between collector and emitter. In saturation, both pn junctions are forward-biased and the collector current is maximum. The transistor ideally behaves like a closed switch between collector and emitter.
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There is a variation in DC over temperature and also from one transistor to another of the same type. There are many types of transistor packages using plastic, metal, or ceramic. It is best to check a transistor in-circuit before removing it.
Common faults are open junctions, low board.
, excessive leakage currents, and external opens and shorts on the circuit
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CHAPTER 5 TRANSISTOR BIAS CIRCUITS
The purpose of biasing a circuit is to establish a proper stable dc operating point point (Q-point). The Q-point of a circuit is defined by specific values for I for I C and V CE CE. These values are called the coordinates of the Q-point. A dc load line passes through the Q-point on a transistor's collector curves intersecting the vertical axis at approximately I C(sat) C(sat) and the horizontal axis at V CE(off) CE(off). The linear (active) operating region of a t ransistor lies along the load line below saturation and above cutoff.
The dc input resistance at the base of a BJT is approximately DCR E. Voltage-divider Voltage-divider bias provides good Q-point stability with a single-polarity single-polarity supply voltage. It is the most common bias circuit. The base bias circuit arrangement has poor stability because its Q-point varies widely with DC. Emitter bias generally provides good Q-point stability but requires both positive and negative supply voltages.
Collector-feedback Collector-feedback bias provides good stability using negative feedback from collector to base.
CHAPTER 6 BJT AMPLIFIER
A small-signal amplifier uses only a small portion of its load line under signal conditions. r parameters are easily identifiable and applicable with a transistor's circuit operation. h parameters are important to technicians and technologists because manufacturers' data sheets specify transistors using h parameters. A common common-em -emitt itter er amplif amplifier ier has good voltag voltage, e, curren current, t, and power power gains, gains, but a relati relativel vely y low input input resistance. A common-collector amplifier has high input resistance and good current gain, but its voltage gain is approximately 1. The common-base amplifier has a good voltage gain, but it has a very low input resistance and its current gain is approximately 1. A darlington pair provides beta multiplication for increased input resistance. The total gain of a multistage amplifier is the product of the individual gains (sum of dB gains). Single-stag Single-stage e amplifiers amplifiers can be connected connected in sequence sequence with various coupling methods to form multistage multistage amplifiers.
CHAPTER 7 FET’S Field-effect transistors are unipolar devices (one-charge carrier). The three FET terminals are source, drain, and gate. The JFET operates with a reverse-biased reverse-biased pn junction (gate-to-source). The high input resistance of a JFET is due to the reverse-biased gate-source gate-source junction. Reverse bias of a JFET produces a depletion region within the channel, thus increasing channel resistance. For an n-channel JFET, V GS can vary from zero negatively to cutoff, V GS(off) a p-channel -channel JFET, V GS GS(off). For a p GS can vary from zero positively to V GS(off) . GS(off) I DSS DSS is the constant drain current when V GS GS = 0. This is true for both JFETs and D-MOSFETs. A FET is called a square-law device because of the relationship of I of I D to the square of a term containing V GS GS. Unlike JFETs and D-MOSFETs, the E-MOSFET cannot operate with V GS = 0 V. GS Midpoint bias for a JFET is I D = IDSS/2, obtained by setting V GS V GS(off) GS GS(off)/3.4. The Q-point in a JFET with voltage-divider bias is more stable than in a self-biased JFET. MOSFETs differ from JFETs in that the gate of a MOSFET is insulated from the channel by an SiO 2 layer, whereas the gate and channel in a JFET are separated by a pn junction. A depletion MOSFET (D-MOSFET) can operate with a zero, positive, or negative gate-to-source voltage. The D-MOSFET has a physical channel between the drain and source. For an n-channel D-MOSFET, negative values of V GS GS produce the depletion mode and positive values produce the enhancement mode. The enhancement MOSFET (E-MOSFET) has no physical channel. A channel is induced in an E-MOSFET by the application of a V GS GS greater than the threshold value, V GS GS(th). Midpoint bias for a D-MOSFET is I D = I DSS , obtained by setting V = 0. DSS GS GS An E-MOSFET has no I DSS DSS parameter. An n-channel E-MOSFET has a positive V GS(th) A p-channel -channel E-MOSFET has a negative V GS(th) GS(th) . A p GS(th). LD MOSFET, VMOSFET, and TMOSFET are E-MOSFET technologies developed for higher power dissipation than a conventional conventional E-MOSFET.
CHAPTER 8 FET AMPLIFIER The drain of a FET is analogous analogous to the collector of a BJT, the source of a FET is analogous to the emitter of a BJT, and the gate of a FET is analogous to the base of a BJT. The transconductance, gm, of a FET relates the output current, I d , to the input voltage, Vgs. The voltage gain of a common-s common-source ource amplifier amplifier is determin determined ed largely by the transconduct transconductance ance,, g m, and the drain resistance, Rd . The internal drain-to-source resistance, r ′ ′ds s, of a FET influences (reduces) the gain if it is not sufficiently greater than R d so d that it can be neglected. An unbypassed resistance between source and ground ( R S) reduces the voltage gain of a FET amplifier. A load resistance connected to the drain of a common-source amplifier reduces the voltage gain. There is a 180º phase inversion between gate and drain voltages. The input resistance at the gate of a FET is extremely high. The voltage gain of a common-drain common-drain amplifier (source-follower) is always slightly less than 1. There is no phase inversion between gate and source in a source-follower. The input resistance of a common-gate common-gate amplifier is the reciprocal of g of g m. The total voltage gain of a multistage amplifier is the product of the individual voltage gains (sum of dB gains). Generally, higher voltage gains can be achieved with BJT amplifiers than with FET amplifiers.
CHAPTER 9 POWER AMPLIFIER A class A power amplifier operates entirely in the linear region of the transistor's characteristic curves. The transistor conducts during the full 360º of the input cycle. The Q-point must be centered on the load line for maximum class A output signal swing. The maximum efficiency of a class A power amplifier is 25 percent. A class B amplifier operates in the linear region for half of the input cycle (180º), and it is in cutoff for the other half. The Q-point is at cutoff for class B operation. operation. Class B amplifiers are normally operated in a push-pull configuration in order to produce an output that is a replica of the input. The maximum efficiency of a class B amplifier is 79 percent. A class AB amplifier is biased slightly above cutoff and operates operates in the linear region for slightly more than 180º of the input cycle. Class AB eliminates crossover distortion found in pure class B. A class C amplifier operates in the linear region for only a small part of the input cycle. The class C amplifier is biased below cutoff. Class C amplifiers are normally operated as tuned amplifiers to produce a sinusoidal output. The maximum efficiency efficiency of a class C amplifier is higher than that of either class A or class B amplifiers. Under conditions conditions of low power dissipation and high output power, the efficiency can approach 100 percent.
CHAPTER 10 AMPLIFIER FREQUENCY RESPONSE The coupling and bypass capacitors of an amplifier affect the low-frequency response. The internal transistor capacitances affect the high-frequency high-frequency response. Critical frequencies are values of frequency at which the RC circuits reduce the voltage gain to 70.7% of its midrange value. Each RC circuit RC circuit causes the gain to drop at a rate of 20 dB/decade. For the low-frequency low-frequency RC circuits, RC circuits, the highest critical highest critical frequency is the dominant critical frequency. For the high-frequency high-frequency RC circuits, RC circuits, the lowest critical lowest critical frequency is the dominant critical frequency. A decade of frequency change is a ten-times change (increase or decrease). An octave of frequency change is a two-times change (increase or decrease). The bandwidth of an amplifier is the range of frequencies between the lower critical frequency and the upper critical frequency. The gain-bandwidth gain-bandwidth product is a transistor parameter that is constant and equal to the unity-gain frequency.
CHAPTER 11 THYRISTORS AND OTHER DEVICES Thyristors are devices constructed with four semiconductor layers ( pnpn). pnpn). Thyristors include 4-layer diodes, SCRs, diacs, triacs, SCSs, and PUTs. The 4-layer diode is a thyristor that conducts when the voltage across its terminals exceeds the breakover potential. The silicon-controlled rectifier (SCR) can be triggered on by a pulse at the gate and turned off by reducing the anode current below the specified holding value. The diac can conduct current in either direction and is turned on when a breakover voltage is exceeded. It turns off when the current drops below the holding value. The triac, like the diac, is a bidirectional device. It can be turned on by a pulse at the gate and conducts in a direction depending on the voltage polarity across the two anode terminals. The silicon-controlled switch (SCS) has two gate terminals and can be turned on by a pulse at the cathode gate and turned off by a pulse at the anode gate. The intrinsic standoff ratio of a unijunction transistor (UJT) determines the voltage at which the device will trigger on. The programmable unijunction transistor (PUT) can be externally programmed to turn on at a desired anode-to-gate voltage level. The insulated-gate bipolar bipolar transistor (IGBT) combines the input characteristics of a MOSFET with the output characteristics of a BJT. The IGBT has three terminals: emitter, gate, and collector. IGBTs are used in high-voltage switching applications. Latch-up can occur in an IGBT when the maximum collector current is exceeded. In a phototransistor, base current is generated by light input. Light acts as the trigger source in light-activated SCRs (LASCRs). Optical coupling devices provide electrical isolation between a source and an output circuit. Fiber optics provides a light path f rom a light-emitting device to a light-activated device. The three basic parts of a fiber-optic cable are the core, the cladding, and the jacket.
Light rays must bounce off the core boundary at an angle (angle of incidence) greater than the critical angle in order to be reflected. Light rays that strike the core boundary at an angle less than the critical angle are refracted into the cladding, resulting in attenuation attenuation of the light. The angle of reflection is equal equal to the angle angle of incidence. Three types of fiber optic cable cable are multimode step step index, single-mode step index, and multimode graded index.
CHAPTER 12 THE OPERATIONAL AMPLIFIER •
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The basic op-amp has three terminals not including power and ground: inverting (–) input, noninverting (+) input, and output. A differential amplifier forms the input stage of an op-amp. Most op-amps require both a positive and a negative dc supply voltage. The ideal op-amp op-amp has infinite infinite input input impedan impedance, ce, zero output impedanc impedance, e, infinite infinite open-loo open-loop p voltage voltage gain, gain, infinite infinite bandwidth, and infinite CMRR. A practical op-amp has very high input impedance, impedance, very low output impedance, very high open-loop voltage gain, and a wide bandwidth. Three types of op-amp input operation are the single-ended mode, the differential mode, and the common mode. Common-mode occurs when equal in-phase voltages are applied to both input terminals. The common-mode common-mode rejection ratio (CMRR) is a measure of an op-amp's ability to reject common-mode inputs. Input offset voltage produces an output error voltage (with no input voltage). Input bias current also produces an output error voltage (with no input voltage). Input offset current is the difference between the two bias currents. Open-loop voltage gain is the gain of an op-amp with no external feedback connections. Closed-loop voltage gain is the gain of an op-amp with external feedback. Slew rate is the rate in volts per microsecond at which the output voltage of an op-amp can change in response to a step input. There are three basic op-amp configurations: inverting, noninverting, and voltage-follower. voltage-follower. The three basic op-amp configurations employ negative feedback. Negative feedback occurs when a portion of the output voltage is connected back to the inverting input such that it subtracts from the input voltage, thus reducing the voltage gain but increasing the stability and bandwidth. A noninverting amplifier configuration has a higher input impedance and a lower output impedance than the op-amp itself (without feedback). An inverting amplifier configuration has an input impedance approximately approximately equal to the input resistor R i i and an output impedance approximately equal to the output impedance of the op-amp itself. The voltage-foll voltage-follower ower has the highest input impedanc impedance e and the lowest lowest output output impedanc impedance e of the three amplifie amplifier r configurations. All practical op-amps have small input bias currents and input offset voltages that produce small output error voltages. The input bias current effect can be compensated compensated for with external resistors. The input offset voltage can be compensated for with an external potentiometer between the two offset null pins provided on the IC op-amp package and as recommended by the manufacturer. The closed-loop voltage gain is always less than the open-loop voltage gain. The midrange gain of an op-amp extends down to dc. The gain of an op-amp decreases as frequency increases above the critical frequency. The bandwidth of an op-amp equals the upper critical frequency. The internal RC lag RC lag circuits that are inherently part of the amplifier stages cause the gain to roll off as frequency goes up. The internal RC lag RC lag circuits also cause a phase shift between input and output signals. Negative feedback lowers the gain and increases the bandwidth. The product of gain and bandwidth is constant for a given op-amp. The gain-bandwidth product equals the frequency at which unity voltage gain occurs.
CHAPTER 13 BASIC OP-AMP CIRCUITS
In an op-amp comparator, when the input voltage exceeds a specified reference voltage, the output changes state. Hysteresis gives an op-amp noise immunity. A comparator switches to one state when the input reaches the upper trigger point (UTP) and back to the other state when the input drops below the lower trigger point (LTP). The difference between the UTP and the LTP is the hysteresis voltage. Bounding limits the output amplitude of a comparator. The output voltage of a summing amplifier is proportional to the sum of the input voltages.
An averaging amplifier is a summing amplifier with a closed-loop gain equal to the reciprocal of the number of inputs. In a scaling adder, a different weight can be assigned to each input, thus making the input contribute more or contribute less to the output. Integration is a mathematical process for determining the area under a curve. Integration of a step produces a ramp with a slope proportional to the amplitude.
Differentiation is a mathematical process for determining the rate of change of a function. Differentiation of a ramp produces a step with an amplitude proportional to the slope.
CHAPTER 14 SPECIAL-PURPOSE SPECIAL-PURPOSE OP-AMP CIRCUITS
A basic instrumentation amplifier is formed by three op-amps and seven resistors, including the gainsetting resistor RG. An instrumentation amplifier has high input impedance, high CMRR, low output offset, and low output impedance. The voltage gain of a basic instrumentation amplifier is set by a single external resistor. An instrumentation amplifier is useful in applications where small signals are embedded in large commonmode noise. A basic isolation amplifier has three e lectrically isolated parts: input, output, and power. Most isolation amplifiers use transformer coupling for isolation. Isolation Isolation amplifiers amplifiers are used to interface interface sensitive sensitive equipment equipment with high-volta high-voltage ge environment environments s and to provide protection from electrical shock in certain medical applications. The operational transconductance amplifier (OTA) is a voltage-to-current amplifier. The output current of an OTA is the input voltage times the transconductance. In an OTA, transconductance varies with bias current; therefore, the gain of an OTA can be varied with a bias voltage or a variable resistor. The operation of log and antilog amplifiers is based on the nonlinear (logarithmic) characteristics of a pn junction. A log amplifier has a BJT in the feedback loop. An antilog amplifier has a BJT in series with the input. Logarithmic amplifiers are used for analog multiplication and division.
CHAPTER 15 ACTIVE FILTERS
The bandwidth in a low-pass filter equals the critical frequency because the response extends to 0 Hz. The bandwidth in a high-pass filter extends above the critical frequency and is limited only by the inherent frequency limitation of the active circuit. A band-pass filter passes all frequencies within a band between a lower and an upper critical frequency and rejects all others outside this band. The bandwidth of a band-pass filter is the difference between the upper critical frequency and the lower critical frequency. A band-stop filter rejects all frequencies within a specified band and passes all those outside this band. Filters with the Butterworth response characteristic have a very flat response in the passband, exhibit a roll-off of –20 dB/decade/pole, and are used when all the frequencies in the passband must have the same gain. Filters with the Chebyshev characteristic have ripples or overshoot in the passband and exhibit a faster roll-off per pole than filters with the Butterworth characteristic. Filters with the Bessel characteristic are used for filtering pulse waveforms. Their linear phase characteristic results in minimal waveshape distortion. The roll-off rate per pole is slower than for the Butterworth. In filter terminology, a single RC circuit is called a pole. Each pole in a Butterworth filter causes the output to roll off at a rate of –20 dB/decade. The quality factor Q of a band-pass filter determines the filter's selectivity. The higher the Q, the narrower the bandwidth and the better the selectivity. The damping factor determines the filter response characteristic (Butterworth, Chebyshev, or Bessel).
CHAPTER 16 OSCILLATORS
Sinusoidal oscillators operate with positive feedback. The two conditions for positive feedback are the phase shift around the feedback loop must be 0º and the voltage gain around the feedback loop must equal 1. For initial start-up, the voltage gain around the feedback loop must be greater than 1. Sinusoidal RC oscillators include the Wien-bridge, phase-shift, and twin-T. Sinusoidal LC oscillators include the Colpitts, Clapp, Hartley, Armstrong, and crystal-controlled.
The feedback signal in a Colpitts oscillator is derived from a capacitive voltage divider in the LC circuit. The Clapp oscillator is a variation of the Colpitts with a capacitor added in series with the inductor. The feedback signal in a Hartley oscillator is derived from an inductive voltage divider in the LC circuit. The feedback signal in an Armstrong oscillator is derived by transformer coupling. Crystal oscillators are the most stable type. A relaxation oscillator uses an RC timing circuit and a device that changes states to generate a periodic waveform. The frequency in a voltage-controlled oscillator (VCO) can be varied with a dc control voltage. The 555 timer timer is an integra integrated ted circuit circuit that that can be used used as an osc oscill illato ator, r, in additi addition on to many many other other applications.
CHAPTER 17 COMMUNICATIONS COMMUNICATIONS CIRCUITS
In amplitude modulation (AM), the amplitude of a higher-frequency carrier signal is varied by a lowerfrequency modulating signal (usually an audio signal). A basic superheterodyne AM receiver consists of an RF amplifier (not always), a mixer, a local oscillator, an IF (intermediate frequency) amplifier, an AM detector, and audio and power amplifiers. The IF in a standard AM receiver is 455 kHz. The AGC (automatic gain control) in a receiver tends to keep the signal strength constant within the receiver to compensate for variations in the received signal. In frequency modulation (FM), the frequency of a carrier signal is varied by a modulating signal. A superheterodyne FM receiver is basically the same as an AM receiver except that it requires a limiter to keep the IF amplitude constant, a different kind of detector or discriminator, and a de-emphasis network. The IF is 10.7 MHz. A four-quadrant linear multiplier can handle any combination of voltage polarities on its inputs. Amplitude modulation is basically a multiplication process. The multiplication of sinusoidal signals produces sum and difference frequencies. The output spectrum of a balanced modulator includes upper-side and lower-side frequencies, but no carrier frequency. The output spectrum of a standard amplitude modulator includes upper-side and lower-side frequencies and the carrier frequency. A linear multiplier is used as the mixer in receiver systems. A mixer converts the RF signal down to the IF signal. The radio frequency varies over the AM or FM band. The intermediate frequency is constant. One type of AM demodulator consists of a multiplier followed by a low-pass filter. The audio and power amplifiers boost the output of the detector or discriminator and drive the speaker. A voltage-controlled oscillator (VCO) produces an output frequency that can be varied by a control voltage. Its operation is based on a variable reactance. A VCO is a basic frequency modulator when the modulating signal is applied to the control voltage input. A phase-locked loop (PLL) is a feedback circuit consisting of a phase detector, a low-pass filter, a VCO, and sometimes an amplifier. The purpose of a PLL is to lock onto and track incoming frequencies. A linear multiplier can be used as a phase detector. A modem is a modulator/demodulator.
DTE stands for digital terminal equipment. DCE stands for digital communications equipment.
CHAPTER 18 VOLTAGE REGULATORS
Voltage regulators keep a constant dc output voltage when the input or load varies within limits. A basic voltage regulator consists of a reference voltage source, an error detector, a sampling element, and a control device. Protection circuitry is also found in most regulators. Two basic categories of voltage regulators are linear and switching. Two basic types of linear regulators are series and shunt. In a series linear regulator, the control element is a transistor in series with the load. In a shunt linear regulator, the control element is a transistor in parallel with the load. Three configurations for switching regulators are step-down, step-up, and inverting. Switching regulators are more efficient than linear regulators and are particularly useful in low-voltage, high-current applications. Three-terminal linear IC regulators are available for either fixed output or variable output voltages of positive or negative polarities. An external pass transistor increases the curr ent capability of a regulator. The 7800 series are three-terminal IC regulators with fixed positive output voltage. The 7900 series are three-terminal IC regulators with fixed negative output voltage.
The LM317 is a three-terminal IC regulator with a positive variable output voltage. The LM337 is a three-terminal IC regulator with a negative variable output voltage. The 78S40 is a switching voltage regulator.
CHAPTER 19 PROGRAMMABLE ANALOG ARRAYS • • •
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An FPAA consists of two t wo or more configurable analog blocks. Most FPAAs use switched-capacitor switched-capacitor circuits. A switch capacitor circuit is based on charge flow and can emulate a resistor with a value determined by the capacitance value and the frequency at which it is switched. The AN221E04 FPAA has four configurable analog blocks (CABs) arranged in a 2X2 matrix. An FPAA can be programmed or configured statically while installed on a development or evaluation board or dynamically dynamically while operating in a system. Development Development software provides for entering a circuit design using a computer, simulating the design to make sure that it operates as expected, and downloading the design to an FPAA device. The setup for statically programming an FPAA requires a computer running the development software, an FPAA device installed on a development or evaluation board, and a standard between the computer port and the board. Typical development software provides configurable analog modules that can be dragged and dropped into a design window on the computer screen and then connected. Configurable analog modules (CAMs) include standard analog functions such as filters, gain amplifiers, rectifiers, comparators, summing amplifiers, integrators, differentiators, and so on with programmable parameters.
