amplifiers

Amplifiers are classified into classes according to their

construction and operating characteristics.

The main operating characteristics of an ideal amplifier are

are linearity, signal gain, efficiency, and power output.

One method used to distinguish the electrical characteristics of different types of amplifiers is by “class”, and as such amplifiers are

classified according to their circuit configuration and method of operation.

Then ____ is the term used to differentiate between the different amplifier types.

Amplifier Classes

Amplifier Classes represent

the amount of the output signal which varies within the amplifier circuit over one cycle of operation when excited by a sinusoidal input signal.

The classification of amplifiers ranges from entirely

linear operation (whose output is proportional to its input)

Amplifier classes are mainly lumped into

two basic groups.

The first are the

classically controlled conduction angle amplifiers forming the morecommon amplifier classes of A, B, AB and C,

The second set of amplifiers are the newer so-called

switching

Class A Amplifiers are

the most common type of amplifier as they use just one output switching transistor

This single output transistor is biased around the

Q-point within the middle of its load line and so is never driven into its cutoff or saturation regions

One of its main disadvantages is

the output transistor of a class-A never turns “OFF”

Class “A” amplifiers are considered the

best class of amplifier, to their excellent linearity, high gain and low signal distortion levels when designed correctly. Class A amplifier uses the common Emitter Configuration.

Class B amplifiers

uses two complimentary transistors bipolar of FET for each half of the waveform with its output stage configured in a “push-pull” type arrangement, so that each transistor device amplifies only half of the output waveform.

In the class B amplifier, there is no DC base bias current as its quiescent current is zero, so

its efficiency is much higher than that of the class A amplifier and is a good current amplifier. However, the price paid for the improvement in the efficiency is in the linearity of the switching device.

Then we can see that each transistor device of the class B amplifier only conducts through one half or 180 degrees of the output waveform in strict time alternation, but

as the output stage has devices for both halves of the signal waveform the two halves are combined together to produce the full linear output waveform.

This push-pull design of amplifier is obviously more efficient than Class A, at about 50%, but the problem with the class B amplifier design is that it can

create distortion at the zero-crossing point of the waveform due to the transistors dead band of input base voltages from -0.7V to +0.7.

To overcome this zero-crossing distortion (also known as Crossover Distortion)

class AB amplifiers were developed.

The AB classification of amplifier is currently one of the most common used types of

audio power amplifier design.

The class AB amplifier is a variation of a class B amplifier as described above, except that both devices are allowed to conduct at the same time around the waveform’s crossover point

eliminating the crossover distortion problems of the previous class B amplifier.

In other words, the conduction angle of a class AB amplifier is

somewhere between 180 o and 360 o depending upon the chosen bias point as shown.

The advantage of this small bias voltage, provided by series diodes of resistors,

is that the crossover distortion created by the class B amplifier characteristics is overcome,

The Class C Amplifier design has the greatest efficiency but

the poorest linearity of the classes of amplifiers mentioned here.

In other words, the conduction angle for the transistor is

significantly less than 180 degrees and is generally around the 90 degrees area.

While this form of transistor biasing gives a much-improved efficiency of around 80% to the amplifier,

it introduces a very heavy distortion of

the output signal.