CLASSES OF AMPLIFIER for study purposes and for S.M.I student,

2. S.M.I university

3. AMPLIFIER???
An amplifier is an electronic device that
increase the power, voltage or current of a
signal. Amplifiers are used in wireless
communication and broadcasting and in audio
equipments of all kinds.

4. Classes of Amplifier:
There are four common classes of amplifier in the high-fidelity
reproduction of audio:
 Class A
 Class B
 Class AB
 Class D

5. “Class A”
Here are the characteristics of the class:
 The output device (transistor) conducts electricity for the entire cycle of input
signal. In other words, they reproduce the entire waveform in its entirety.
 These amps run hot, as the transistors in the power amp are on and running at
full power all the time.
 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".
 Class A is the most inefficient of all power amplifier designs, averaging only
around 20.
 They usually waste at least 4-5 watts as heat
.

6.  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.
Figure: of class A “amplifier”

7. “Class B”
In this amp, the positive and negative halves of the signal are dealt with by different
parts of circuit. The output devices continually switch on and off. Class B operation
have the following characteristics:
The input signal has to be a lot larger in order to drive the transistor
appropriately.
This is almost the opposite of Class A operation.
There have to be at least two output devices wit
h this type of amp. This output stage employs two output devices so that each side
amplifies each half of the waveform. [li Either both output devices are never
allowed to be on at the same time, or the bias (remember, that trickle of
electricity?) for each device is set so that current flow in one output device is zero
when not presented with an input signal.
Each output device is on for exactly one half of a complete signal cycle.

8. 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.
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.
Figure: of Class B Amplifier

9. “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:
In fact, many Class AB amps operate in Class A at lower output levels, again
giving the best of both worlds
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.
There is enough current flowing through each device to keep it operating so they
respond instantly to input voltage demands.
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.

10. 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.
Figure: of class AB Amplifier

11. “Class D”
These amplifiers are erroneously called "digital" amplifiers by the press and
many audio "experts." Here's the skinny on Class D:
While some Class D amps do run in true digital mode, using coherent binary
data, most do not.
They are better termed "switching" amplifiers, because here the output devices
are rapidly switched on and off at least twice for each cycle.
Depending on their switching frequency, they may be "switched on" or "off"
millions of times a second.
Class D operation is theoretically 100% efficient, but in practice, they are
closer to 80-90% efficiency.
This efficiency gain is at the cost of high-fidelity.

12. Think of Class D amps as being similar to a switchable 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).
Class D amplifiers are generally used for non-high-fidelity, or subwoofer
applications.
Figure: of class D Amplifier

13. “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.