classes of amplifier

Classes of Amplifier

Electronic Engineering

© University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License.

Amplifier Compensation

The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1st year undergraduate programme.

The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

Contents Amplification Classes Class A Class B Class C Class AB Credits

In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see:Clayton G, 2000, Operational Amplifiers 4th Ed, Newnes James M, 2004, Higher Electronics, Newnes

Amplification ClassesAmplifiers are divided into classes. Each class describes how the amplifying device reacts to an applied input. The amplifying device may be:

a transistor (BJT), a field effect transistor (FET) or even a valve.

The amplifying devices can be biased to operate as either Class A, Class B or Class C. Each of these are examined below.


Class A

The circuit below shows a simple amplifier

Output

20vThe bias components have been left out of the circuit. The bias point is selected so that the output “sits” at the mid point of the supply and the input is on a relatively linear section of the input characteristic – see below:


0.6v

Vbe

Ib

Bias point

The complete range of input variation produces a corresponding change in Ib. This produces a corresponding change in Ic which is converted into an output voltage by the load resistor shown in the circuit diagram. If we were to monitor the output for various input levels we would observe the following:


0

5

10

15

20

No input Small input Maximum input

The amplifying device, in this case a transistor, conducts all the time. Ideally, with no input signal present the transistor would be biased to keep the output at 10v (half supply). This allows the output to “swing” by + and – 10v.


The efficiency of the circuit is very low, less than 50% as the transistor conducts even when the input is zero. Current flows through the load in order to produce the 10v drop. This means that energy is being dissipated in the form of heat.

Advantages:Good quality output signal as the transistor is operating on a linear portion of it’s characteristic.

Disadvantage:Poor efficiency limits its use as a power amplifier – tends to be used for pre-amplifiers only.


Class BSame circuit as before but the bias point selection is different.

0.6v

Vbe

Ib

Bias point

The bias point is selected so that the transistor is just at the point of turning on. This means that if a positive input is applied the transistor conducts but if a negative input is applied no conduction takes place. This would produce the following:


0

5

10

15

20

No input Small input Maximum input

Note: The amplifier produces 180 phase shift so this is why we see negative half cycles on the output. As the circuit stands it is of little use as an amplifier but what we can do is to combine two transistors, each of which conducts in the opposite half cycle.

The overall result of the above effects is that the Class B arrangement tends to have difficulty whenever the signal waveform changes polarity and we have to switch off one transistor and turn on the other. The result is what is called Crossover Distortion and this can have a very bad effect on small level or high speed waveforms.

0

5

10

15

20This problem is enhanced due to Non-Linearities in the transistors – i.e. that the output current and voltage don’t vary linearly with the input level – which are worst at low currents.


This places the amplifier designer/user in a dilemma. The Class A amplifier uses devices that always pass high currents, and small signals only modulate these by a modest amount, avoiding the above problems. Alas, Class A is very power inefficient. Class B is far more efficient, but can lead to signal distortions. The solution is to find a ‘half-way house’ that tries to take the good points of each and minimise the problems. The most common solution is Class AB amplification, see later.

Advantages:More efficient than Class A – quoted value is 78%

Disadvantages:Cross over distortionNormally requires a positive and negative supply.

Class C.Once again what makes an amplifier Class C is the bias point selection.

0.6v

Ib

Bias point

As the bias point is selected below the turn on point for the transistor, only a portion of the positive half cycle will be amplifier. This makes it of no use as an audio amplifier. Where it finds its use is in circuits where a short burst of energy is required every cycle. An example of this is a tuned circuit that will oscillate but does require an energy boost to keep it going.

Class AB

The bias point is selected so that the device conducts for more than the positive half cycle but not for the whole wave. We have a situation where both transistors in the push pull arrangement are conducting simultaneously. The effect of this is to reduce/eliminate the distortion whilst ensuring that the efficiency is better than Class A, but not as good as Class B.


This resource was created by the University of Wales Newport and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.

© 2009 University of Wales Newport

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