MOSFET devices of the VMOS design (Figure 7-47) are becoming a popular choice for power amplifiers, particularly those designed to switch large currents on and off’,Examples include line drivers for digital switching circuits, switching-mode voltage regulators (discussed in Chapter 17), and class-D amplifiers, which we will discus presently. One advantage of a MOSFET switch is the fact that turn-off time is not delayed by minority-carrier storage, as it is in a bipolar switch that has been driven ‘lee ply into saturation. Recall tha i current in field-effect transistors is due to tne i’Jowof majority carriers only. Also, MOSFETs are not susceptible to thermal like bipolar transistors. Finally, a MOSFET has a very large input impedance Which simplifies the design of driver circuits.
Figure 16-38 shows a simple, class-A audio amplifier that uses a VN64GA VMOS transistor to drive an output transformer. Notice that the driver for the output stage is a JlOXJFET transistor connected ill a common-source configuration. Also notice the feedback path between the transformer secondary and the input to the JFET driver. This negative feedback is incorporated to reduce distortion, as discussed earlier. The amplifier will reportedly deliver between 3 and 4 W to an 8-0 speaker with less than 2% distortion up to 15 kHz.
MOSFET power amplifiers are also constructed for class-B and class-All operation. Like their bipolar counterparts, these amplifiers can be designed without the
A two-stage, class-A audio amplifier with 1I VMOS output transistor (Courtesy of Incorporated}
need for an output transformer, since power MOSFETs arc nvailublc in complementary pairs (N-channel and Pvchanncl). Figure 16-]<) shows a typical push-pull MOSFET amplifier using complementary output devices
A class-D amplifier is one whose output is switched on and off, that is, one whose output isin it” linear range for essentially zero time during each cycle of an input sine wave. As we progress through the letters designating the various classes of operation, A, B, C, and 0, we see that linear operation occurs for shorter and shorter” intervals of time, and in class D we reach the limiting case where no linear operation occurs at all. The only time that a class-D output device is in its linear region is during that short interval required to switch from saturation to cutoff, or vice versa. In other words, the output device is a digital power switch, an application ideally suited for VMOS transistors. . A fundamental component of a class-D amplifier is a pulse-width modulator, which produces a train of pulses having widths that arc proportional to the level of the ampliflcr’s input signal. When the signal level is small, a series of narrow pulses is generated, and when the input level is large, it series of wide pulses is generated. (See Figure 17-39, which shows some typical outputs of a pulse-width modulator used in a voltage-regulator application.) As the input signal increasesand decreases, the pulse widths increase and decrease in direct proportion. Figure 16-40 shows how a pulse-width modulator can be constructed using a sawtooth gel/em/or and a voltage comparator. A sawtooth wuvctonn is one that rises linearly and then quickly switches buck to its low level. to begin another linear rise, as illustrated in the figure. When the sawtooth voltage is greater than Vit” the output of the comparator is low, and when the sawtooth falls below l'”” the comparator switches to its high output. Notice that the comparator must switch high each time the sawtooth makes its vertical descent. These time points mark the beginning ofeach new pulse. The comparator output remains high until the sawtooth rises back to the value of Vi/!, at which lime the comparator output switches low. Thus the width of the high pulse is directly proportional to the length of time it takes the sawtooth to rise to Vi/!, which is directly proportional to the level of Vi/!. As shown in the figure. the result is a series of pulses whose widths arc proportional to the level of Vi/!. Notice that the peak-to-peak voltage of the sawtooth must exceed the largest peak-to-peak input voltage for successful operation. Also, the frequency
of the sawtooth should be at least ten times as great as the highest frequency component of
The pulse-width modulator drive’ the output stage of the class-D amplifier, causing it to switch on and off as the pulses switch between high and low, Figure 16-41 shows a popular switching circuit. called a totem pole, used to drive heavy
loads. The MOSFET version of the totem pole is shown in the figure, because MOSFETs are generally used in class-D amplifiers. Bipolar versions of the totem pole are also widely used in digital logic circuits, particularly in the integratedcircuit family called TTL (transistor-transistor logic). The totem pole shown in the figure inverts, in the sense that the output is low when the input is high, and vice versa. When the input in Figure 16-41 is high, 01 and 03 are on, so the output is low (RL is effectively connected to ground through 03), Since 01 is on, the gate of 02 is low and O2 is held off. Thus, when the input is high, 02 is like an open switch and OJ is like a closed switch. When the input is low, 01 is off and a high voltage ( oo) is applied through R to ‘he gale of O2, turning it on. Thus, RL is connected through 02 to the high level V[)n. Since 03 is also off, n low input makes 03 an open switch and 02 a closed switch. The advantage of this arrangement is that the
Like the class-C amplifier, 3 class-D amplifier must have a filter to extract-or recover, the signal from the pulsed waveform. Howev r, in t, is case the signal may have many frequency components, so a tank circuit. which resonates at a single frequency, cannot he used. Instead, a low-pass filter having a cutoff frequency near the highest signal ‘requency is used. The low-pass filter suppresses the highfrequency components of the pulse train and, in effect, recove.rs the average value of the pulse train. Sit ce the average value of the pulses depends on the. pulse widths, the output of the filter is a waveform that increases and decreases as the pulse widths increase and decrease, that is, a waveform that duplicates the input signal,Vin Figure 16-42 shows a block diagram of a complete class-D amplifier, including negative feedback to reduce distortion.
The principal advantage of a class-D amplifier is that it may have a very high efficiency, approaching 100%. Like that of a class-C amplifier, the high efficiency is due to the fact that the output device spends very little time in its active region, so power dissipation is minimal. The principal disadvantages are the need for a very good low-pass filter and the fact that high-speed switching of heavy currents generates noise through electromagnetic coupling, called ‘electromagnetic interference
A class-D audio amplifier is to be driven by a signal that varies between :5 V The output is a MOSFET totem pole with VDD ;; 30 V.
1. What minimum frequency should the sawtooth waveform have?
2. What minimum peak-to-peak voltage should the sawtooth waveform have, assuming that the input is connected directly to the voltage comparator?
3. Assume that the amplifier is 100% efficient and is operated at the frequency found in (1). What average current is delivered 10 an 8-0 load when the pulsewidth modulator produces pulses that are high fof2.5 p.s?
1. The nominal audio-frequency range is 20 Hz to 20 kHz, and the sawtooth waveform should have a frequency equal to at least 10 times the highest input frequency, i.e., 10(20 kHz) = 200 kHz.
2. The peak-to-peak sawtooth voltage should at least equal the maximum peakto- peak input voltage, which is 10 V.
3. The period of the 200-kHz sawtooth is
Therefore, the pulse train is high for (2.5 p.s)/(5 p.s) = one-half the period. The totem-pole output therefore switches between 0 V and 30 V with pulse widths equal to one-half the period. Thus, the average voltage is
Related Electronics Assignments
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“Class” is in session. This discussion of electronic amplifier circuits offers an overview of the characteristics that define commonly used class designations. The class designations described are A, B, AB, C, D, E, F, G, and H.
Amplifiers and Their Applications
Amplifiers, which are fundamental elements of circuit design, take a small signal and make it larger. They drive everything from ear buds to antennas. Placed ahead of analog-to-digital converters (ADCs), they reshape signals from sources as diverse as strain gauges to ultrasound probes. By properly selecting feedback passives, they can be configured into high-pass, low-pass, bandpass, and band-elimination filters. Feed them with multiple signals, and they produce harmonics of every component of those inputs—good for some applications, a headache in others.
Background of the Class Nomenclature
Historically, amplifier class designations were related to the biasing of amplifier devices—that is, over how many degrees of each input-signal swing they conducted. It worked for classes A, B, and C. For class AB, it made a certain kind of sense. Today, it’s not as clear cut. A cynic might say that class designators can only tell us when a new class was invented.
In the beginning, classification revealed something about linearity versus efficiency. Class A amplifiers can be made very linear, but with limited efficiency. In theory, a class A amp can achieve 50% efficiency with inductive output coupling or 25% with capacitive coupling. Class B amplifiers are subject to “crossover” distortion, but efficiency runs theoretically as high as 78.5%.
Class C amplifiers offer high efficiency (up to 90%), but the high-Q tank circuits needed for their operation have narrow bands of resonance. Moreover, tank circuits for low (e.g., audio) frequencies are impractical to build, which ultimately limits them to radio frequencies. Class D’s origins emanated from Harris Semiconductor. The company introduced the first drivers for class D audio amplifiers in 1995, claiming efficiencies greater than 90%. (That unit of Harris is now part of Intersil.)
Amplifier Classes (Short Summary)
• Class A: Single-ended; the amplifier device is biased about the center of the input signal swing.
• Class B: Push-pull; each device conducts over half the input signal swing.
• Class AB: Push-pull; each device conducts over slightly more than half the input signal swing to simplify crossover.
• Class C: Used in radio-frequency applications, the output device drives a resonant “tank” circuit consisting of an inductor and one or two capacitors. It conducts for only a short portion of each input cycle.
• Class D: It’s found primarily in audio applications—either in vehicles, where it achieves high output levels, or in personal audio devices, where its efficiency contributes to long battery life. In a class D amplifier, power field-effect transistors (FETs) are driven to produce an output square-wave that switches between a high and low level at a frequency outside the range of human hearing. Instead of modulating the amplitude, internal circuitry modulates the duty cycle of the square-wave at a rate corresponding to the level of the input signal when the output is filtered down to audio band.
Classes E and F are subsets of Class C. Classes G and H are like class AB amplifiers, but with multiple power rails.
Classes A, B, and AB (Detailed Descriptions)
In class A, biasing a single active device (generally a transistor) allows it to operate in its linear conduction region during the entire input cycle. “Biasing” refers to the limiting of an input signal to a certain voltage or current range. “Linear” conduction means that changes in the amplified output of the circuit are exactly proportional to changes in the input.
Two active devices exist in class B amplifiers. The input waveform is split. One active device conducts during half of an input cycle, the other during the other half. The two halves are reassembled at the amplifier’s output. At times, class B amplifiers called “push-pull,” because the outputs of the active devices have a 180° phase relationship.
Class AB amplifiers resemble class Bs, except their active devices are biased so both conduct during an overlapping portion of each input cycle. This sacrifices a certain amount of potential gain for better linearity (i.e., there’s a smoother transition at the crossover point of the output signal). Class AB sacrifices some of that efficiency for lower distortion.
Class B and Bridge-Tied Load Amplifiers
A push-pull amplifier can be built using amplifier ICs, rather than discretes, as in the traditional class B amp. A bridge-amplifier configuration effectively doubles the voltage swing at the load. It’s possible to build a bridge-amplifier where one stage drives one side of the speaker, while a second unity-gain inverting amplifier drives the other side. However, a better configuration would enable both amplifiers to see the same input signal.
Classes G and H
Class G and H amplifiers, variations on the standard class AB, feature additional supply rails. These rails kick in when output-signal peaks would otherwise exceed the maximum voltage available from the class AB amplifier’s single voltage rail.
Class G amps employ several power rails at discrete voltage steps and switch between them as needed. Instead of providing multiple rails, class H amps track the input signal and modulate the voltage on the supply rails.
The Doherty Amplifier
Class G and H amplifiers often find their way into audio applications. However, a related but previously almost forgotten alternative called the Doherty amplifier has been revived for cell-phone applications.
Named after William Doherty, an early Bell Labs researcher, the Doherty amplifier comprises a class B “carrier” stage in parallel with a class C “peaking” stage. In the input, half the input signal drives one device in the Class B; half the other. On the output, the signals are summed. Somewhat like a class G or H amp, the class B amp sustains the output most of the time, but the class C amp cuts in on high signal peaks. The benefit of the Doherty is increased efficiency, relative to a pure class B.
Class C amplifiers feature a single active device that’s biased to conduct during only a small portion of each input-waveform cycle. Energy is driven into a high-Q, L-C “tank” circuit that continues to “ring” at its resonant frequency during the times the active device isn’t conducting.
An analogy would be the continuous tapping of a big bell with a small hammer at a rate equal to the resonant frequency of the bell. “Q” is “quality factor.” Strictly speaking, it’s the ratio of an inductor’s reactance at a given frequency to its dc resistance. More generally, though, it reflects how sharply an L-C resonant circuit is tuned (where L implies the presence of an inductance and C implies one or more capacitors).
Classes E and F
Classes E and F, much like class C, feature RF amplifier topologies that use LC tank circuits. Where class C amplifiers are widely used below 100 MHz, class E amps tend to fall into the VHF and microwave frequency ranges. The difference between class E and class C amps is the active device becoming a switch, rather than operating in the linear portion of its transfer characteristic.
Class F amplifiers resemble class E amplifiers, but use a more complex load network. In part, this network improves the impedance match between the load and the switch. Moreover, it’s designed to eliminate the input signal’s even harmonics so the switching signal is more nearly a square-wave. It improves efficiency because the switch runs at saturation or cutoff for a longer period.
Class D is most often used in audio applications. Like classes E and F, class D’s active devices—power FETs—are driven as switches, rather than in linear mode. A square-wave with a frequency that’s significantly higher than the highest frequency component of the input waveform drives the class D amp’s two devices between saturation and cutoff.
The square-wave’s pulse width or pulse density is variable, and the input signal controls one or the other. At the amplifier output, a low-pass filter attenuates the switching frequency and its harmonics, leaving only the amplified version of the input waveform.
With the FETs operating in either cutoff or saturation, losses come primarily from the transistors’ forward-voltage drops. Class D amps can achieve efficiencies as high as 90%, with distortion levels approaching class AB. A drawback of Class D concerns the challenging task of suppressing radiated and conducted interference from the switching circuitry.
- “A Sound Decision On Audio-Speaker Design Starts With The Right Amplifier”
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