There are 4 different amplifier classes.

Class A is used in applications where the output is an amplified  replica of the input signal. Class A consists of both small-signal and  large signal amplifiers if operated in the linear region at all times.  Class A also consists of Power Amplifiers which are a large signal type  amplifier that's objective is to distribute power to a load. These  amplifiers are rated for more than 1W and needs a form of heat  dissipation when used in an application. Class A
Amplifiers achieve maximum signal when q-point is center of the ac load line.  

Class B is biased at cutoff, operating in the linear region for 180o of the input cycle and is in cutoff for 180o  . This class of amplifiers are more efficient than class A in terms of  output power to input power ratio. The class B amplifier however only  conducts for the positive have of a sinusoidal waveform. An operation  referred to as a push-pull consists of two Class B amplifiers working  together to achieve conduction on the negative portion of the cycle. 

Class AB is similar to Class B in the sense that it is more efficient  than a class A amplifier. A class AB amplifier in the push-pull stage  can be used to overcome crossover distortion which occurs when the dc  base voltage in a class B amplifier reaches zero. When this happens  neither transistor is conducting so the biasing is adjusted to overcome  the VBE of the transistors. 

 Class C is a nonlinear amplifier, meaning that the output is not a  replica of the output. The transistor is biased below cutoff with a  negative VBB supply putting the Q-point outside the load  line. The transistor is turned on for a short interval when the base  voltage exceeds the barrier potential of the base-emitter junction while  the AC source peak voltage is slightly greater than

.

• Explain what is meant by 'TFT' that you always find in the specs for laptop displays.

TFT stands for Thin Film Transistor and is used to enhance the operation of the LCD displays."The  thin film transistor (TFT) is a type of semiconductor device used in  display technology to enhance efficiency, compactness, and cost of the  product. In conjunction with its semiconductor properties, the TFT LCD  is an active matrix display, controlling pixels individually and  actively rather than passively, furthering the benefits of this  semiconductor device."(Orient Display, 2022).

• Discuss an application of Darlington Pair transistors.

A Darlington Pair transistor consists of two bipolar junction  transistors combined to yield a high current gain. The first transistor  amplifies then the second further amplifies. can consist of either two  PNP transistors or two NPN transistors connected back-to-back sharing a  common collector. 

References:

How does TFT LCD Display Work | Orient Display 

 Links to an external site.

Darlington Transistor: What is it? (Darlington Pair) | Electrical4U 

 Links to an external site.

October 11, 2020 by Electrical4U