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 transistors.
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
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
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 the 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 gets.
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
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 the input.
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 frequency.
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
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 theater systems.
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
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 (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 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 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
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.