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AUDIO AMPLIFIERS
The final amplifier is the power source of every audio installation. Its job is to
convert a small alternating voltage into a powerful signal for driving loudspeakers
with as little distortion as possible.
Delivering a large amount of power is not a simple task for an amplifier. Voltage
amplification and current amplification are both necessary in order to provide
sufficient power to speakers connected to the amplifier. To produce power
amplification in a final amplifier, various concepts have been developed for using
transistors or FET’s to generate high-quality output signals and to improve the
efficiency of the output stage.
Final amplifiers are normally classified according to the configuration of the output
stage. This determines their efficiency and quality since the output stage is where
the actual power amplification takes place.
Class A amplifiers
A class A amplifier has very low efficiency but it has almost no crossover distortion.
Class A final amplifier is the simplest configuration and also one of the best
configurations for high quality audio reproduction and can be implemented using a
standard emitter follower. The quiescent current through the transistor is equal to
the peak AC output current, which means that the transistor is biased in the middle
of its working range and simply conducts more or less current when driven by an
alternating voltage.
The efficiency of a class A amplifier is very low: 25% at maximum output amplitude
and even less at low signal levels. The efficiency can be improved by using a
symmetrical design with 2 transistors, but even so the highest effic. is 50%.
This is the most linear of the classes, meaning the output signal is a truer
representation of what was imputed. Here are the characteristics of the class:
1.
The output device (transistor) conducts electricity for the entire cycle of input
signal. In other words, they reproduce the entire waveform in its entirety.
2.
These amps run hot, as the transistors in the power amp are on and running
at full power all the time.
3.
There is no condition where the transistor(s) is/are turned off. That doesn't
mean that the amplifier is never or can never be turned off; it means the
transistors doing the work inside the amplifier have a constant flow of
electricity through them. This constant signal is called "bias".
4.
Class A is the most inefficient of all power amplifier designs, averaging only
around 20.
Because of these factors, Class A amplifiers are very inefficient: for every watt of
output power, they usually waste at least 4-5 watts as heat. They are usually very
large, heavy and because of the 4-5 watts of heat energy released per watt of
output, they run very hot, needing lots of ventilation (not at all ideal for a car, and
rarely acceptable in a home). All this is due to the amplifier constantly operating at
full power. The upside is that these amps are the most enjoyed of all amplifiers.
These amps dig out musical detail, since the transistor reproduces the entire audio
waveform without ever cutting off. As a result the sound is cleaner and more linear;
that is, it contains much lower levels of distortion.
They are the most accurate of all amps available, but at significant cost to
manufacture, because of tight tolerances, and the additional components for cooling
and heat regulation.
Power
Load Voltage (at Speaker)
Load Current (through Speaker)
Supply Voltage
Supply Current
20W (continuous)
12.65 Volts RMS (17.9 Volts Peak)
1.58 Amps RMS (2.23 Amps Peak)
+/- 20 Volts (constant)
+/- 2.25 Amps (peak)
Table 1 - Class-A Amplifier Requirements (Approx.) - 8Ω Load
Class-A Benefits
Since Class-A amps are inefficient, generate lots of heat, and require a far more
complex power supply than conventional Class-AB amplifiers, there have to be some
compelling reasons to use this arrangement. The first is circuit simplicity. In the light
of the above discussion, the circuit is not simple, but for the audio signal it can be far
less complex than for a conventional power amp.
The benefit of this is that the signal is subjected to comparatively little amplification,
resulting in an open loop (i.e. without feedback) gain which is generally fairly low probably less than 250 (48dB), and possibly as low as 50 or so (34dB). This means
that very little overall feedback is used, so stability and phase should be excellent
over the audio frequencies. A well designed Class-A amplifier should not require any
frequency compensation (or very little), so the open loop gain will remain reasonably
constant over the audio range. This results in superior transient response, and
dramatically reduced "Transient Inter modulation Distortion" (or TID, aka Dynamic
Inter modulation Distortion), which is thought by many designers to be caused by
phase and time delays between the input and feedback signals. It may be possible
that this is the cause, although the existence of TID is virtually zero in any
competently designed amp.
The simple fact is that the more amplifying devices that are introduced into the
chain, the more phase shift must be introduced. No amplifying device is capable of
responding instantaneously to a change of input - all have some inherent delay
(which usually includes different turn-on and turn-off times). With fewer devices in
the audio circuit, there must be less delay between a change in the input causing a
change in the output. The simplified topology used for most Class-A amps can also
be used with Class-AB - often with very good results indeed.
Figure 3 - Crossover Distortion
Figure 3 shows the crossover distortion of a Class-B type amplifier. This is
exaggerated for clarity, and the "soft" crossover distortion shown is actually more
typical of a push-pull valve (vacuum tube) or lateral MOSFET amplifier (without
proper bias adjustment). As can be seen, when the signal is reduced, the ratio of
distortion to signal will become much worse, resulting in an increase in distortion as
power is reduced. Indeed, this is exactly what happens in many amplifiers, but it
generally is "swamped" by so much feedback that it seems to disappear. It can be
seen from the diagram that for this crossover distortion to appear, the amplifier's
gain must fall as the signal level approaches 0 Volts.
The point that distortion "seems to disappear" is the operative term here - it does
not go away at all, and worse, as the crossover point is reached, the open loop gain
of the amplifier is reduced, meaning that there is not as much feedback as at higher
signal levels. This will be apparent to readers with an electronics background - note
that near the crossover point, the amplitude of the signal is much lower than it
should be (this is what causes the problem in the first place!). Since the amplitude is
reduced, it is obvious that the amplifier's gain must be lower at this level than at
higher levels.
Class B amplifiers
The class B configuration employs 2 transistors, each of which conducts for exactly
half of the signal cycle. In the quiescent state, no current at all flows through the
transistors. The efficiency of the A class B output stage is around 78% but the
primarily disadvantage of this configuration is the transfer distortion that occurs each
time the load must be transferred from one transistor to the other. This leads to
crossover distortions, which is a quite audible degradation of the signal waveform. To
solve this, class A and class B were combined to produce class AB, this is a class B
configuration in which a small quiescent current flows, causing the output stage to
work in class A at low power levels.
In this amp, the positive and negative halves of the signal are dealt with by different
parts of the circuit. The output devices continually switch on and off. Class B
operation has the following characteristics:
1.
The input signal has to be a lot larger in order to drive the transistor
appropriately.
2.
This is almost the opposite of Class A operation
3.
There have to be at least two output devices with this type of amp. This
output stage employs two output devices so that each side amplifies each half
of the waveform.
4.
Each output device is on for exactly one half of a complete signal cycle.
These amps run cooler than Class A amps, but the sound quality is not as pure, as
there is a lot of "crossover" distortion, as one output device turns off and the other
turns on over each signal cycle.
This type of amplifier design, or topology, gives us the term "push-pull," as this
describes the tandem of output devices that deliver the audio signal to your
speakers: one device pushes the signal, the other pulls the signal. They can be less
expensive, because one can use two cheap output devices instead of one highquality one in the design.
As I mentioned before, the input signal has to be lot larger, meaning that from the
amplifier input, it needs to be "stepped up" in a gain stage, so that the signal will
allow the output transistors to operate more efficiently within their designed
specifications. This means more circuitry in the path of your signal, degrading sound
even before it gets to the output stage.
Class AB
this is the compromise of the bunch. Class AB operation has some of the best
advantages of both Class A and Class B built-in. Its main benefits are sound quality
comparable to that of Class A and efficiency similar to that of Class B. Most modern
amp designs employ this topology.
Its main characteristics are:
1.
In fact, many Class AB amps operate in Class A at lower output levels, again
giving the best of both worlds
2.
The output bias is set so that current flows in a specific output device for
more than a half the signal cycle but less than the entire cycle.
3.
There is enough current flowing through each device to keep it operating so
they respond instantly to input voltage demands.
4.
In the push-pull output stage, there is some overlap as each output device
assists the other during the short transition, or crossover period from the
positive to the negative half of the signal.
There are many implementations of the Class AB design. A benefit is that the
inherent non-linearity of Class B designs is almost totally eliminated, while
avoiding the heat-generating and wasteful inefficiencies of the Class A design.
And as stated before, at some output levels, Class AB amps operate in Class
A. It is this combination of good efficiency (around 50) with excellent linearity
that makes class AB the most popular audio amplifier design.
There are quite a few excellent Class AB amps available. This is the design I
recommended for most general-use applications in home and car. Usually,
parts choice rivals that of Class A amps, and dollar for dollar these are some
of the best values in stereo amplification. There can be some variation in
design principle, but generally these are well-designed amps since their
function is very well-understood by audio designers.
Class C
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. The usual application for class-C amplifiers is
in RF transmitters operating at a single fixed carrier frequency, where the distortion is controlled
by a tuned load on the amplifier. The input signal is used to switch the active device causing
pulses of current to flow through a tuned circuit forming part of the load.
The class-C amplifier has two modes of operation: tuned and untuned. The diagram 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., an inductive-capacitive filter plus a load resistor) is used, two things
happen. The first is that the output's bias level is clamped with the average output voltage equal
to the supply voltage. This is why tuned operation is sometimes called a clamper. This allows the
waveform to be restored to its proper shape despite the amplifier having only a one-polarity
supply. This is directly related to the second phenomenon: the waveform on the center frequency
becomes less distorted. The residual distortion 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.
Class D
These amplifiers are erroneously called "digital" amplifiers by the press and many
audio "experts." Here's the skinny on Class D:
1.
While some Class D amps do run in true digital mode, using coherent binary
data, most do not.
2.
They are better termed "switching" amplifiers, because here the output
devices are rapidly switched on and off at least twice for each cycle.
3.
Depending on their switching frequency, they may be "switched on" or "off"
millions of times a second.
4.
Class D operation is theoretically 100% efficient, but in practice, they are
closer to 80-90% efficiency.
5.
This efficiency gain is at the cost of high-fidelity.
Think of Class D amps as being similar to a switch able power supply, but with audio
signals controlling, or modulating, the switching action. To do this, you use a
technology called Pulse Width Modulation (or PWM, a technology found in many CD
players).
According to experts, audio signals can be used to modulate a PWM system to create
a high power audio amplifier at fairly low voltages using very small components.
Class D audio uses a fixed, high frequency signal having pulses that vary in width
based on input signal amplitude. So, for example, a deep bass note creates a large
pulse in the carrier signal. This can be translated into a musical signal by the on/off
nature of the output devices.
Class D amplifiers are generally used for non-high-fidelity, or subwoofer applications.
There is a fifth (and, nominally, a sixth) class of amplifier, but they are rarely seen in
practice in the consumer market. One is the Class G and the other Class H. These
are similar in design to Class AB topologies, but both feature two power supplies that
switch on or off, depending on the musical signal imputed. 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 amps are
designed to use the same topology as Class G, but it provides just enough voltage
for optimum operation of the output devices. Again, its an attempt to increase
efficiency, but at the expense of fidelity ultimately.
In summary:
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Class G and H amplifiers add complexity to the signal and degrade it because
of the need for switching depending on the input signal
Class D amplifiers are models of efficiency, but with a loss of detail and
fidelity
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Class B amplifiers generally introduce some crossover distortion, but move
away from Class D, G, and H's extreme non-linearity.
Class AB amplifiers may introduce some crossover distortion, but they get
closer to the ideal of Class A for most of its operating regime.
They are indeed the best compromise of performance versus cost.
Class A amplifiers introduce no crossover distortion and are the most
desirable amps to own, but they are expensive, run hot, and have to be very
well-built.
Conclusion
The quest for high-fidelity, coupled with efficiency has driven amplifier design for
decades. Electrical properties of the available electronics and the never-bending
laws of electrical behavior have created a multitude of solutions for those trying
to design high-powered, great-sounding, and efficient amps. Look for the best
balance of performance for the buck and let your ears be your guide and you'll
choose the best products, regardless of price and class.