Description of Amplifier Classes
Description of Amplifier Classes
Linear Amplifiers - Classes A, AB, B, C, F1
Switching Mode Amplifiers - Classes D, E, F2, F3, S
Amplifier classes: Audio power amplifiers are classified primarily by the design of the
output stage. Classification is based on the amount of time the output devices operate
during each cycle of signal swing. Also defined in terms of output bias current, (the
amount of current flowing in the output devices with no signal).
Class A operation is where both devices conduct continuously for the entire cycle of
signal swing, or the bias current flows in the output devices at all times. The key
ingredient of class A operation is that both devices are always on. There is no
condition where one or the other is turned off. Because of this, class A amplifiers are
single-ended designs with only one type polarity output devices. Class A is the most
inefficient of all power amplifier designs, averaging only around 20%. Because of this,
class A amplifiers are large, heavy and run very hot. All this is due to the amplifier
constantly operating at full power. The positive effect of all this is that class A designs
are inherently the most linear, with the least amount of distortion.
Class B operation is the opposite of class A. Both output devices are never allowed to
be on at the same time, or the bias is set so that current flow in a specific output
device is zero when not stimulated with an input signal, i.e., the current in a specific
output flows for one half cycle. Thus each output device is on for exactly one half of a
complete sinusoidal signal cycle. Due to this operation, class B designs show high
efficiency but poor linearity around the crossover region. This is due to the time it
takes to turn one device off and the other device on, which translates into extreme
crossover distortion. Thus restricting class B designs to power consumption critical
applications, e.g., battery operated equipment, such as 2-way radio and other
communications audio.
Class AB operation allows both devices to be on at the same time (like in class A), but
just barely. The output bias is set so that current flows in a specific output device
appreciably more than a half cycle but less than the entire cycle. That is, only a small
amount of current is allowed to flow through both devices, unlike the complete load
current of class A designs, but enough to keep each device operating so they respond
instantly to input voltage demands. Thus the inherent non-linearity of class B designs
is eliminated, without the gross inefficiencies of the class A design. It is this
combination of good efficiency (around 50%) with excellent linearity that makes class
AB the most popular audio amplifier design.
Class AB plus B design involves two pairs of output devices: one pair operates class
AB while the other (slave) pair operates class B.
Class D operation is switching, hence the term switching power amplifier. Here the
output devices are rapidly switched on and off at least twice for each cycle. Since the
output devices are either completely on or completely off they do not theoretically
dissipate any power. Consequently class D operation is theoretically 100% efficient,
but this requires zero on-impedance switches with infinitely fast switching times -- a
product we're still waiting for; meanwhile designs do exist with true efficiencies
approaching 90%.
Class G operation involves changing the power supply voltage from a lower level to a
higher level when larger output swings are required. There have been several ways to
do this. The simplest involves a single class AB output stage that is connected to two
power supply rails by a diode, or a transistor switch. The design is such that for most
musical program material, the output stage is connected to the lower supply voltage,
and automatically switches to the higher rails for large signal peaks. Another approach
uses two class AB output stages, each connected to a different power supply voltage,
with the magnitude of the input signal determining the signal path. Using two power
supplies improves efficiency enough to allow significantly more power for a given
size and weight. Class G is becoming common for pro audio designs. Class H
operation takes the class G design one step further and actually modulates the higher
power supply voltage by the input signal. This allows the power supply to track the
audio input and provide just enough voltage for optimum operation of the output
devices. The efficiency of class H is comparable to class G designs.
With the Proliferation of Different Amp Types, Which is the One for You? In the past
we had essentially two types of amplifiers to choose from: Class "AB" and class "A".
Today we have AB, A, D, G, H, & T, in addition to some that do not have a class name.
New technology brought down the size and price while improving performance and
efficiency. We'll review the various topologies of the modern amplifier, spending extra
time on the aspect of efficiency (as the quest for smaller, more efficient designs have
spawned the class D, G, H, & T designs). We'll also try to dispel some of the
misconceptions and folklore that seem to surround amp design.
Class A: A small signal modulates a larger current. This larger current is present when
the small signal is not present. Efficiency up to about 26 percent. Excellent quality
Class B: Uses a push-pull arrangement where one amplification device operates on
the positive side of the waveform and another operates on the negative side.
Efficiency up to 75%. Sounds bad because of distortion caused by switching from one
device to another.
Class C: A small signal turns a larger signal on or off. There is no in-between state.
Efficiency up to 90 percent. Not usable for audio. Audio requires accurate
reproduction of all levels—not just no power and full power levels.
Class D: A variation of Class C. Class D is a way of modulating a Class C amplifier
to allow it to carry audio information. Sounds very good with latest technology.
Efficiency up to 90 percent. Produces Electromagnetic Interference (EMI)
Class AB: A variation of Class B. Always has a small current flowing (class A
operating region) and this eliminates the switching distortion inherent in Class B.
Efficiency up to 65 percent. Sounds excellent—if well-designed
Class H: A variation of Class AB. Changes the power supply voltage to the amplifier
depending on the signal level. Improved dynamic efficiency. Requires complex power
supply. Single tone efficiency up to 65 percent Sounds excellent—if well-designed Class-I Class-T
Amplifier classes
Amplifier circuits are classified as A, B, AB and C for analog designs, and class D
and E for switching designs. For the analog classes, each class defines what
proportion of the input signal cycle (called the angle of flow) is used to actually
switch on the amplifying device:
Class A
100% of the input signal is used (conduction angle a = 360° or 2π)
Class AB
more than 50% but less than 100% is used. (181° to 359°, π < a < 2π)
Class AB1 applies to tube or transistor amplifiers in class AB where the grid or base
is more negatively biased than it is in class A.
Class AB2 applies to tube or transistor amplifiers in class AB where the grid or base
is often more negatively biased than in AB1, and the input signal is often larger. When
the drive is high enough to make the grid or the base more positive, the grid or base
current will increase. It is possible depending on the level of the signal input for the
amplifier to move from class AB1 to AB2.
Class B
50% of the input signal is used (a = 180° or π)
Class C
less than 50% is used (0° to 179°, a < π)
This can be most easily understood using the diagrams in each section below. For the
sake of illustration, a bipolar junction transistor is shown as the amplifying device, but
in practice this could be a MOSFET or vacuum tube device. In an analog amplifier,
the signal is applied to the input terminal of the device (base, gate or grid), and this
causes a proportional output drive current to flow out of the output terminal. The
output drive current is obtained from the power supply. The voltage signal shown is
thus a larger version of the input, but has been changed in sign (inverted) by the
amplification. Other arrangements of amplifying device are possible, but that given
(common emitter, common source or common cathode) is the easiest to understand
and employ in practice. If the amplifying element is linear, then the output will be
faithful copy of the input, only larger and inverted. In practice, transistors are not
linear, and the output will only approximate the input. Non-linearity is the origin of
distortion within an amplifier. Which class of amplifier (A, B, AB or C) depends on
how the amplifying device is biased — in the diagrams the bias circuits are omitted
for clarity.
Any real amplifier is an imperfect realization of an ideal amplifier. One important
limitation of a real amplifier is that the output it can generate is ultimately limited by
the power available from the power supply. An amplifier can saturate and clip the
output if the input signal becomes too large for the amplifier to reproduce.
Class A
Class A amplifiers amplify over the whole of the input cycle such that the output
signal is an exact scaled-up replica of the input with no clipping. Class A amplifiers
are the usual means of implementing small-signal amplifiers. They are not very
efficient — a theoretical maximum of 50% is obtainable, but for small signals, this
waste of power is still extremely small, and can be easily tolerated. Only when we
need to create output powers with appreciable levels of voltage and current does Class
A become problematic. In a Class A circuit, the amplifying element is biased such that
the device is always conducting to some extent, and is operated over the most linear
portion of its characteristic curve (known as its transfer characteristic or
transconductance curve). Because the device is always conducting, even if there is no
input at all, power is wasted. This is the reason for its inefficiency.
Class A Amplifier
If high output powers are needed from a Class A circuit, the power wastage will
become significant. For every watt delivered to the load, the amplifier itself will, at
best, waste another watt. For large powers this will call for a large power supply and
large heat sink to carry away the waste heat. Class A designs have largely been
superseded for audio power amplifiers, though some audiophiles believe that Class A
gives the best sound quality, due to it being operated in as linear a manner as possible.
In addition, some aficionados prefer thermionic valve (or "tube") designs over
transistors, for a number of reasons: Tubes are more commonly used in class A
designs, which have an asymmetrical transfer function. This means that distortion of a
sine wave creates both odd- and even-numbered harmonics. They claim that this
sounds more "musical" than the purely odd harmonics produced by a symmetrical
push-pull amplifier.[1][2] Though good amplifier design can avoid inducing any
harmonic patterns in a sound reproduction system, the differences in harmonic content
are essential to the sound of intentional electric guitar distortion. Another is that
valves use many more electrons at once than a transistor, and so statistical effects lead
to a "smoother" approximation of the true waveform — see shot noise for more on
this. Field-effect transistors have similar characteristics to valves, so these are found
more often in high quality amplifiers than bipolar transistors. Historically, valve
amplifiers often used a Class A power amplifier simply because valves are large and
expensive; Many Class A designs uses only a single device. Transistors are much
cheaper, and so more elaborate designs that give greater efficiency but use more parts
are still cost effective. A classic application for a pair of class A devices is the
long-tailed pair, which is exceptionally linear, and forms the basis of many more
complex circuits, including many audio amplifiers and almost all op-amps. Most
Class A amplifiers are not used for op-amps; they are used as medium-power,
low-efficiency, and high-cost(!) audio amplifiers. The power consumption is unrelated
to the output power: at idle (no input), the power consumption is essentially the same
as at high output volume. The result: a considerable amount of power is radiated by
the heat sinks. Some owners of Class A amplifiers claim that in the winter they heat
their houses with the power wasted by the amplifiers. In summer, the situation would
be worse. Air conditioners could remove the extra heat but it would have to go
somewhere, typically outside. The air conditioners would use even more power than
the power amplifiers. Of course, Class A amplifiers are used only by the extremely
wealthy. This simplifies some problems, because only the costliest solutions will be
Class B and AB
Demonstration of Theoretical efficiency of Class B amplifierClass B amplifiers only
amplify half of the input wave cycle. As such they create a large amount of distortion,
but their efficiency is greatly improved and is much better than Class A. Class B has a
maximum theoretical efficiency of 78.5%. This is because the amplifying element is
switched off altogether half of the time, and so cannot dissipate power. A single Class
B element is rarely found in practice, though it can be used in RF power amplifiers
where the distortion is unimportant. However Class C is more commonly used for
Class B Amplifier
A practical circuit using Class B elements is the complementary pair or "push-pull"
arrangement. Here, complementary devices are used to each amplify the opposite
halves of the input signal, which is then recombined at the output. This arrangement
gives excellent efficiency, but can suffer from the drawback that there is a small glitch
at the "joins" between the two halves of the signal. This is called crossover distortion.
A solution to this is to bias the devices just on, rather than off altogether when they
are not in use. This is called Class AB operation. Each device is operated in a
non-linear region which is only linear over half the waveform, but still conducts a
small amount on the other half. Such a circuit behaves as a class A amplifier in the
region where both devices are in the linear region, however the circuit cannot strictly
be called class A if the signal passes outside this region, since beyond that point only
one device will remain in its linear region and the transients typical of class B
operation will occur. The result is that when the two halves are combined, the
crossover is greatly minimised or eliminated altogether.
However, it is important to note that while the efficiency of Class AB is greater than
Class A, it is less than Class B. (To say that its efficiency is greater than Class A is a
trival statement, because the efficiency of a Class A circuit is vanishingly small.)
Class B Push-Pull Amplifier
Class B or AB push-pull circuits are the most common form of design found in audio
power amplifiers. Class AB is widely considered a good compromise for audio
amplifiers, since much of the time the music is quiet enough that the signal stays in
the "class A" region, where it is reproduced with good fidelity, and by definition if
passing out of this region, is large enough that the distortion products typical of class
B are relatively small. Class B and AB amplifiers are sometimes used for RF linear
amplifiers as well. Class B amplifiers are also favored in battery-operated devices,
such as transistor radios.
Negative feedback
Feedback feeds the difference of the input and part of the output back to the input in a
way that cancels out part of the input. The main effect is to reduce the overall gain of
the system. However the unwanted signals introduced by the amplifier are also fed
back. Since they are not part of the original input, they are added to the input in
opposite phase, subtracting them from the input.
Careful design of each stage of an open loop (non-feedback) amplifier can achieve
about 1% distortion. With negative feedback, 0.001% is typical. Noise, even crossover
distortion can be practically eliminated. Feedback was originally invented so that
replacing a burnt-out vacuum tube would not change an amplifier's performance
(manufacturing realities require that tubes and transistors with the same part number
will have close but not identical gain). Negative feedback also compensates for
changing temperatures, and degrading or non-linear components. While amplifying
devices can be treated as linear over some portion of their characteristic curve, they
are inherently non-linear; their physics dictates that they operate using a square law.
The result of non-linearity is distortion.
The application dictates how much distortion a design can tolerate. For hi-fi audio
applications, instrumentation amplifiers and the like, distortion must be minimal,
often better than 1%.
While feedback seems like a universal fix for all the problems of an amplifier, many
believe that negative feedback is a bad thing. Since it uses a loop, it takes a finite time
to react to an input signal, and for this short period the amplifier is "out of control." A
musical transient whose timing is of the same order as this period will be grossly
distorted, even though the amplifier will show incredibly good distortion performance
on steady-state signals. This, essentially, is the rationale for the existence of "transient
intermodulation distortion" in amplifiers which was exhaustively discussed and
debated from the late 1970s through much of the 1980s [3]. Proponents of feedback
refute this, saying that the feedback "delay" is of such a short order that it represents a
frequency vastly outside the bandwidth of the system, and such effects are not only
inaudible, but not even present, as the amplifier will not respond to such high
frequency signals.
The argument has caused controversy for many years, and has led to all sorts of
interesting designs — such as feedforward amplifiers (e.g. digital signals on many
cell-site base-station transmitters are precompensated for the radio amplifier's
distortion). The fact remains that the majority of modern amplifiers use considerable
amounts of feedback, though many of the high-end audiophile designs seek to
minimise this.
Whatever the merits of these arguments about its effect on waveform distortion,
feedback also affects the output impedance of the amplifier and therefore its damping
factor. Roughly speaking, the damping factor is a measure of the ability of the
amplifier to control the speaker. All other things being equal, the greater the amount
of feedback, the lower its output impedance and the higher its damping factor. This
has an effect on the low frequency performance of many speaker systems where low
damping factors lead to irregular bass response.
The concept of feedback is used in operational amplifiers to precisely define gain,
bandwidth and other parameters.
A practical circuit
For the purposes of illustration, this practical amplifier circuit is described. It could be
the basis for a moderate-power audio amplifier. It features a typical (though
substantially simplified) design as found in modern amplifiers, with a class AB
push-pull output stage, and uses some overall negative feedback. Bipolar transistors
are shown, but this design would also be realisable with FETs or valves.
A practical amplifier circuit
The input signal is coupled through capacitor C1 to the base of transistor Q1. The
capacitor allows the AC signal to pass, but blocks the DC bias voltage established by
resistors R1 and R2 so that any preceding circuit is not affected by it. Q1 and Q2 form
a differential amplifier (an amplifier that multiplies the difference between two inputs
by some constant), in an arrangement known as a long-tailed pair. This arrangement is
used to conveniently allow the use of negative feedback, which is fed from the output
to Q2 via R7 and R8. The negative feedback into the difference amplifier allows the
amplifier to compare the input to the actual output. The amplified signal from Q1 is
directly fed to the second stage, Q3, which provides further amplification of the signal,
and the DC bias for the output stages, Q4 and Q5. R6 provides the load for Q3 (A
better design would probably use some form of active load here, such as a
constant-current sink). So far, all of the amplifier is operating in Class A. The output
pair are arranged in Class AB push-pull, also called a complementary pair. They
provide the majority of the current amplification and directly drive the load,
connected via DC-blocking capacitor C2. The diodes D1 and D2 provide a small
amount of constant voltage bias for the output pair, just biasing them into the
conducting state so that crossover distortion is minimised. This design is simple, but a
good basis for a practical design because it automatically stabilises its operating point,
since feedback internally operates from DC up through the audio range and beyond.
Further circuit elements would probably be found in a real design that would roll off
the frequency response above the needed range to prevent the possibility of unwanted
oscillation. Also, the use of fixed diode bias as shown here can cause problems if the
diodes are not both electrically and thermally matched to the output transistors — if
the output transistors turn on too much, they can easily overheat and destroy
themselves, as the full current from the power supply is not limited at this stage. A
common solution to help stabilise the output devices is to include some emitter
resistors, typically an ohm or so. Calculating the values of the circuit's resistors and
capacitors is done based on the components employed and the intended use of the
For the basics of radio frequency amplifers using valves, see Valved RF amplifiers.
Class C
Class C amplifiers conduct less than 50% of the input signal and the distortion at the
output is high, but efficiencies of up to 90% can be reached. Some applications can
tolerate the distortion, such as megaphones. A much more common application for
Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by
using tuned loads on the amplifier stage. The input signal is used to roughly switch
the amplifying device on and off, which causes pulses of current to flow through a
tuned circuit. The tuned circuit will only resonate at particular frequencies, and so the
unwanted frequencies are dramatically suppressed, and the wanted full signal (sine
wave) will be abstracted by the tuned load. Provided the transmitter is not required to
operate over a very wide band of frequencies, this arrangement works extremely well.
Other residual harmonics can be removed using a filter.
Class C Amplifier
Class D
Main article: Switching amplifier
A class D amplifier is a power amplifier where all power devices are operated in
on/off mode. Output stages such as those used in pulse generators are examples of
class D amplifiers. Mostly though, the term applies to devices intended to reproduce
signals with a bandwidth well below the switching frequency. These amplifiers use
pulse width modulation, pulse density modulation (sometimes referred to as pulse
frequency modulation) or more advanced form of modulation such as Sigma delta
modulation (see for example Analog Devices AD1990 Class-D audio power
amplifier). The input signal is converted to a sequence of pulses whose averaged
value is directly proportional to the amplitude of the signal at that time. The frequency
of the pulses is typically ten or more times the highest frequency of interest in the
input signal. The output of such an amplifier contains unwanted spectral components
(i.e.. the pulse frequency and its harmonics) that must be removed by a passive filter.
The resulting filtered signal is then an amplified replica of the input.
The main advantage of a class D amplifier is power efficiency. Because the output
pulses have a fixed amplitude, the switching elements (usually MOSFETs, but valves
and bipolar transistors were once used) are switched either on or off, rather than
operated in linear mode. This means that very little power is dissipated by the
transistors except during the very short interval between the on and off states. The
wasted power is low because the instantaneous power dissipated in the transistor is
the product of voltage and current, and one or the other is almost always close to zero.
The lower losses permit the use of a smaller heat sink while the power supply
requirements are lessened too.
Class D amplifiers can be controlled by either analog or digital circuits. A digital
controller introduces additional distortion called quantisation error caused by its
conversion of the input signal to a digital value.
Class D amplifiers were widely used to control motors, and almost exclusively for
small DC motors, but they are now also used as audio amplifiers, with some extra
circuitry to allow analogue to be converted to a much higher frequency pulse width
modulated signal. The relative difficulty of achieving good audio quality means that
the vast majority appear in applications where quality is not a factor, such as
miniature audio systems and "DVD-receivers".
High quality Class D audio amplifiers are now, however, starting to appear in the
market. Tripath have called their revised Class D designs Class T. Perhaps more
famously, Bang and Olufsen's ICEPower Class D system has been used in the Alpine
PDX range and some of the PRS range of Pioneer along with other manufacturers.
These revised designs have been said to rival good traditional AB amplifiers in terms
of quality.
Before these higher quality designs existed an earlier use of Class D amplifiers and
prolific area of application is high-powered, subwoofer amplifiers in cars. Because
subwoofers are generally limited to a bandwidth of no higher than 150 Hz, the switch
speed for the amplifier does not have to be as high as for a full range amplifier. The
drawback with Class D designs being used to power subwoofers is that their output
filters (typically inductors that convert the pulse width signal back into an analogue
waveform) lower the damping factor of the amplifier. This means that the amplifier
cannot prevent the subwoofer's reactive nature from lessening the impact of low bass
sounds (as explained in the feedback part of the Class AB section). Class D amplifiers
for driving subwoofers have become so inexpensive that a true 1 kW of power output
can be had for less than 250USD (retail). Efficiencies are in the 80% to 95% range.
D does not stand for "digital"
The letter D used to designate this type of amplifier is simply the next letter after C,
and does not stand for digital. Class D and Class E amplifiers are sometimes
mistakenly described as "digital" because the output waveform superficially
resembles a pulse-train of digital symbols, but a Class D amplifier merely converts an
input waveform into a continuously pulse-width modulated (square wave) analog
signal. (A digital waveform would be pulse-code modulated.)
Class E
The class E/F amplifier is a highly efficient switching power amplifier, typically used
at such high frequencies that the switching time becomes comparable to the duty time.
As said in the class-D amplifier the transistor is connected via a serial-LC-circuit to
the load, and connected via a large L (inductivity) to the supply voltage. The supply
voltage is connected to ground via a large capacitor to prevent any RF-signals to leak
into the supply. The class-E amplifier adds a C between the transistor and ground and
uses a defined L (RFC in the figure) to connect to the supply voltage.
Class E Amplifier
The following description ignores DC, which can be added afterwards easily. The
above mentioned C (Cp in the figure) and L are in effect a parallel LC-circuit to
ground. When the transistor is on, it pushes through the serial LC-circuit into the load
and some current begins to flow to the parallel LC-circuit to ground. Then the serial
LC-circuit swings back and compensates the current into the parallel LC-circuit. At
this point the current through the transistor is zero and it is switched off. Both
LC-circuits are now filled with energy in the C and the Ls. The whole circuit performs
a damped oscillation. The damping by the load has been adjusted so that some time
later the energy from the Ls is gone into the load, but the energy in both Cs peaks at
the original value, to in turn restore the original voltage, so that the voltage across the
transistor is zero again and it can be switched on.
With load, frequency, and duty cycle (0.5) as given parameters and the constraint that
the voltage is not only restored, but peaks at the original voltage, the four parameters
(L,L,C,C) are determined. The class F-amplifier takes the finite on resistance into
account and tries to make the current touch the bottom at zero. This means the voltage
and the current at the transistor are symmetric with respect to time. The Fourier
Transform allows an elegant formulation to generate the complicated LC-networks. It
says that the first harmonic is passed into the load, all even harmonics are shorted and
all higher odd harmonics are open.
Class F and the even harmonics
In push-pull amplifiers and in CMOS the even harmonics of both transistors just
cancel. Experiment tells that a square wave can be generated by those amplifiers and
math tells that square wave do consist of odd harmonics only. In a class D amplifier
the output filter blocks all harmonics, that means the harmonics see an open load. So
even small currents in the harmonics suffice to generate a voltage square wave. The
current is in phase with the voltage applied to filter, but the voltage across the
transistors is out of phase. Therefore there is a minimal overlap between current
through the transistors and voltage across the transistors. The sharper the edges the
lower the overlap. While class D sees the transistors and the load as to separate
modules the class F admits imperfections like the parasitics of the transistor and tries
to optimize the global system to have a high impedance at the harmonics. Of course
there has to be a finite voltage across the transistor to push the current across the on
state resistance. Because the combined current through both transistors is mostly in
the first harmonic it looks like a sine. That means that in the middle of the square the
maximum of current has to flow, so it may make sense to have a dip in the square or
in other words to allow some over swing of the voltage square wave. A class F load
network by definition has to transmit below a cut off frequency and to reflect above.
Any frequency lying below the cut off and having its second harmonic above the cut
off can be amplified, that is an octave bandwidth. On the other hand a LC series
circuit with a large L and a tunable C may be simpler to implement. By reducing the
duty cycle below 0.5, the output amplitude can be modulated. The voltage square
waveform will degrade, but any overheating is compensated by the lower overall
power flowing. Any load mismatch behind the filter can only act on the first harmonic
current waveform, clearly only a purely resistive load makes sense, then the lower the
resistance the higher the current. Class F can be driven by sine or by a square wave,
for a sine the input can be tuned by an L to increase gain. If class F is implemented
with a single transistor the filter is complicated to short the even harmonics.
All previous designs use sharp edges to minimize the overlap. Class E uses a
significant amount of second harmonic voltage. The second harmonic can be used to
reduce the overlap with edges with finite sharpness. For this to work energy on the
second harmonic has to flow from the load into the transistor, and no source for this is
visible in the circuit diagram. In reality the impedance is mostly reactive and the only
reason for it is that class E is a class F amplifier with a very simplified load network
and thus has to deal with imperfections. Note how in many amateur simulations of
class E amplifiers sharp current edges are assumed nullifying the very motivation for
class E and measurements near the transit frequency of the transistors show very
symmetric curves, which look much similar to class F simulations.
Stuff belonging to class D:
The main concept used in this amplifier is to model the active switching device, such
as a transistor or MOSFET, as a linear combination of two parts: (1) a (theoretical)
"perfect" switching element, and (2) a complex network of parasitic elements attached
to it (capacitors, inductors and resistors). After the decomposition, it becomes trivial
to eliminate the losses of each part:
(1) The "perfect" switching element should be switched ON during zero-voltage
crossing, and should be switched OFF during zero-current crossing. Thus the
switching element either conducts current, or has some non-zero voltage on it, but
never both at the same time. Because the dissipated power is equal to current x
voltage, it becomes zero. This can be arranged by adjusting the phase (capacitor) and
DC bias (resistor) of the signal going into the transistor input.
(2) The imaginary part of the impedance of the parasitic elements can be tuned, one
by one, by matching them to another passive element with the complex conjugate
impedance, thus leaving only the real part of the complex impedance.
In theory, the only remaining loss is the real part of the impedance of the parasitic
elements in the system, which cannot be avoided. This class of amplifier is unique to
radio frequency ranges, where the amplifier analysis is usually done in the frequency
domain and not in the voltage/current domain. This class is further divided to
subclasses depending on which harmonics of the signal are taken into account during
zero-voltage switching (ZVS) and zero-current switching (ZCS), with names such as
Class E/F2,odd; Class F^-1; and so on. It is still an active area of research and new
variants appear from time to time, usually with the letters "E" and "F" somewhere in
class name.
The figure above shows a schematic of a class-E/F amplifier that uses this principle to
achieve high efficiency.
The switch is periodically opened and closed at the frequency of operation. Usually,
but not always, the switching duty ratio is 50%. The RF choke has comparatively
large inductance so that in effect it functions as a constant current source. Other
passive device values are chosen such that the following conditions are satisfied
simultaneously. (1) The voltage across the switch at the instant of closing is zero. (2)
The time derivative of voltage across the switch is at zero when the switch turns on.
Moreover, Ls and Cs forms a resonating filter at the frequency of operation.
In practical implementations a transistor is substituted for a switch, but is operated
either in saturation (on) or in cut-off (off). The theoretical efficiency of a class-E
amplifier is 100% with ideal components. However, practical circuits do exhibit a
number of weaknesses that make them less than 100% efficient. These effects include
finite switching speed, finite on-resistance and non-zero saturation voltage of the
transistor as well as lossy passive components at high frequencies. Typical efficiency
is about 60% at an operating frequency of 1-2 GHz.
This amplifier class is specially designed for the amplification of square waves, such
as those used to transmit data in purely digital form. “Square” waves or pulses have
special needs due to their frequency characteristics, since they require the faithful
reproduction of the very high frequencies present in their leading and trailing edges,
without adding artifacts such as ringing or overshoot during the amplification process.
Consideration must be made as well for the lower frequency components introduced
by the switching levels, such as the impedance of the output load, which is often in
the form of a transmission line.
The class E amplifier was invented in 1972 by Nathan O. Sokal and Alan D. Sokal,
and details were first published in 1975 [4]. Some earlier reports on this operating
class have been published in Russian.
Class G
Demonstration of Theoretical efficiency of Class G amplifier part1
Demonstration of Theoretical efficiency of Class G amplifier part2Class G amplifiers
are a more efficient version of class AB amplifiers, which use "rail switching" to
decrease power consumption and increase efficiency. The amplifier has several power
rails at different voltages, and switches between rails as the signal output approaches
each. Thus the amp increases efficiency by reducing the "wasted" power at the output
Class H
A Class H amplifier takes the idea of Class G one step further creating an infinite
number of supply rails. This is done by modulating the supply rails so that the rails
are only a few volts larger than the output signal at any given time. The output stage
operates at it's maximum effiency all time. Switched mode power supplies can be
used to create the tracking rails. Significant efficiency gains can be achieved but with
the draw back of more complicated supply design and reduced THD performance.
Other classes
Several audio amplifier manufacturers have started "inventing" new classes as a way
to differentiate themselves. These class names usually do not reflect any revolutionary
amplification technique, and are used mostly for marketing purposes. This can easily
be determined by the fact that the class name is trademarked or copyrighted. For
example, Crowns K and I-Tech Series as well as several other models utilise Crowns
patented Class-I (or BCA) technology. Lab Gruppen use a form of class D amplifier
called class TD or Tracked Class D which tracks the waveform to more accurately
amplify it without the drawbacks of traditional class D amplifiers.
"Class T" is a trademark of TriPath company, which manufactures audio amplifier
IC's. This new class "T" is a revision of the common class D amplifier, but with
changes to ensure fidelity over the full audio spectrum, unlike traditional class D
designs. It operates at a frequency of 650kHz, with a proprietary modulator.
Class Z is a trademark of Zetex semiconductor is a direct digital feedback technology.
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