Classes of Amplifiers
Most mobile amplifiers use complementary transistor pairs to drive the speakers. In this
configuration there is a transistor (or group of transistors) which conducts current from the positive
power supply voltage for the positive half of the audio waveform and a different transistor (or group
of transistors) which conducts current from the negative power supply voltage for the negative half
of the waveform. There are some amplifiers which use the same transistor(s) to drive both the
positive and the negative halves of the waveform.
NOTE:Amplifiers in classes A, B, and AB operate their output transistors in a 'linear' mode. Class
'D' amplifiers operate their outputs in 'switch' mode.
Imagine that you are the amplifier's output device(s) and you must support a 10 pound iron weight
(the speaker load). The most difficult method (linear mode) would be to hold the weight straight out
in front of you. This would very roughly simulate the linear mode architecture. Your muscles would
start to ache in a short amount of time. Think of this pain as the power dissipation in output
In this example, you can support the weight in one of two positions. In the first position, you can
hold the iron weight directly over your head with your elbows locked so that your're not really using
very much effort to support the weight. In the second position, you would let the weight hang down
by your side. This would also use very little effort from your muscles. If you held it directly over
your head half of the time and by your side for the other half of the time, it's position would
'average' out to be the same as if you held it out straight in front of you like in the previous (linear
mode) example. This would roughly simulate the switch mode which we will discuss later in this
page. You can see that with this method (switch mode), there would also be little pain (power
dissipation) involved in supporting the weight.
Amplifiers do not actually increase the strength of an electronic signal. What happens instead, the
signal is copied and enlarged. There are different schemes
for amplifying the signal. There are different classes of amplifiers. These classes are A, AB, and C.
There have been some special classes such as G, created by Hatachi. Class H created by
Soundcraftsman. Class D for the so-called digital amps and Class T for Tripath's digital amplifiers.
Input and output variables
Electronic amplifiers use two variables: current and voltage. Either can be used as input, and either
as output leading to four types of amplifiers. In idealized form they are represented by each of the
four types of dependent source used in linear analysis, as shown in the figure, namely:
A comparison of output signals for the different amplifier classes of operation.
Class A amplifiers use one or more transistors that conduct during both the positive and negative
cycles of the signal. This Class of amplifier has the lowest distortion but it is very inefficient and
generates a lot of heat. A Class A amplifier requires that the amplifier generate the full current no
matter what the output is. If you were simply listening to FM or watching a movie, the amplifier
would be consuming as much power as if you had it turned up to full volume.
Class B amplifiers use one transistor to conduct the positive portion of the waveform and another
transistor to conduct the negative portion of the waveform. 99% of all audio amplifiers today are
Class B. Class B amplifier can be built today so
that its distortions are well below what the human ear can detect and nearly to the point where it is
Class C amplifiers conduct less than 50% of the input signal and the distortion at the
output is high, but high efficiencies (up to 90%) are possible. Some applications (for example,
megaphones) can tolerate the distortion. 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 Class C amp. has two modes of operation: tuned, and untuned. The diagram below shows a
waveform from a simple class C circuit without the tuned load. This is called untuned operation, and
the analysis of the waveforms shows the massive distortion that appears in the signal. When the
proper load (e.g., a pure inductive-capacitive filter) is used, two things happen. The first is that
output's bias level is clamped, so that the output variation is centered at one-half of the supply
voltage. This is why tuned operation is sometimes called a clamper. This action of elevating bias
level allows the waveform to be restored to its proper shape, allowing a complete waveform to be
re-established despite having only a one-polarity supply. This is directly related to the second
phenomenon: the waveform on the center frequency becomes much less distorted. The distortion
that is present is dependent upon the bandwidth of the tuned load, with the center frequency seeing
very little distortion, but greater attenuation the farther from the tuned frequency that the signal
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 extracted by the tuned load
(e.g., a high-quality bell will ring at a particular frequency when it is hit periodically with a hammer).
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.
In the Class D amplifier the input signal is converted to a sequence of higher voltage output pulses.
The averaged-over-time power value of these pulses are directly proportional to the instantaneous
amplitude of the input signal. The frequency of the output pulses is typically ten or more times the
highest frequency in the input signal to be amplfied. The output pulses contain inaccurate spectral
components (that is, the pulse frequency and its harmonics) which must be
removed by a low-pass passive filter. The resulting filtered signal is then an amplified replica of
These amplifiers use pulse width modulation, pulse density modulation (sometimes referred to as
pulse frequency modulation) or more advanced form of modulation such as Delta-sigma modulation
(for example, in the Analog Devices AD1990 Class-D audio power amplifier). Output stages such
as those used in pulse generators are examples of class D amplifiers. The term Class D is usually
applied to devices intended to reproduce signals with a bandwidth well below the switching
Class D amplifiers can be controlled by either analog or digital circuits. The digital control
introduces additional distortion called quantization error caused by its conversion of the input signal
to a digital value.
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 completly on or completely off, rather than operated in linear mode.
A MOSFET operates with the lowest resistance when fully-on and thus has the lowest power
dissipation when in that condition, except when fully off. (When operated in a linear mode the
MOSFET has variable amounts of resistance that vary linearly with the input voltage and the
resistance is something other than the minimum possible, therefore more electrical energy is
dissipated as heat.) Compared to class A/B operation, class D's lower losses permit the use of a
smaller heat sink for the MOSFETS while also reducing the amount of AC power supply power
required. Thus, Class D amplifiers do not need as large or as heavy power supply transformers or
heatsinks, so they are smaller and more compact in size than an equivalent Class AB amplifier.
Class D amplifiers have been 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 nearly all are used in applications where quality is not a
factor, such as modestly-priced bookshelf audio systems and "DVD-receivers" in mid-price home
Class T amps are a more refined switching amplifier developed by Tripath. It uses signal
processing to eliminate the switching distortion of Class D. nOrh is currently working with parts
from Tripath to determine the sonic merits using the Tripath parts. Our current view is that
advantage to using Class T and Class D amps is not to achieve better sound than can currently be
achieved with standard A or A/B amplifiers. Rather it is an attempt to create a lower priced
amplifier that offers good performance.
The Class E 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
large L (inductance) to the supply voltage. The supply voltage is connected to ground via a large
capacitor to prevent any RF-signals leaking into the supply. The class-E amplifier adds a C
between the transistor and ground and uses a defined L1 to connect to the supply voltage.
Class E Amplifier
The following description ignores DC, which can be added afterwards easily. The above mentioned
C and L are in effect a parallel LC-circuit to ground. When the transistor is on, it pushes through
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 L0. 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 C0 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, L0, C and C0) 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.