“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, 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.
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.)
• 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.
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.
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.
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.
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, 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.