III Answer the following questions:

1. What kind of amplifier is Class A?

2. Why did valve often use a Class A amplifier?

3. What do valves use?

IV Read and memorise the following words:

1. dissipate – розсіювання

2. mismatch – несполучність, несумістність

3. drawback – відпускати

4. bais – відхиляти, відводити

5. remain – залишатися

Class b and ab

Class B amplifiers only amplify half of the input wave cycle. As such they create a large amount of distortion, but their efficiency is greatly improved and is much better than Class A. Class B has a maximum theoretical efficiency of 78.5%. This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A single Class B element is rarely found in practice, though it can be used in RF power amplifiers where the distortion levels are less important. However Class C is more commonly used for this.

A practical circuit using Class B elements is the complementary pair or "push-pull" arrangement. Here, complementary or quasi-complementary devices are used to each amplify the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch at the "joins" between the two halves of the signal. This is called crossover distortion. A solution to this is to bias the devices to be just on, rather than completely off when they're not in use. This is called Class AB operation.

Each device is operated in a non-linear region which is only linear over half the waveform, but still conducts a small amount on the other half. Such a circuit behaves as a class A amplifier in the region where both devices are in the linear region, however the circuit cannot strictly be called class A if the signal passes outside this region, since beyond that point only one of the devices will remain in its linear region and the transients typical of class B operation will occur. The result is that when the two halves are combined, the crossover is greatly minimised or eliminated altogether.

Class AB sacrifices some efficiency over class B in favor of linearity, so will always be less efficient. It is typically much more efficient than class A.

Class B or AB push-pull circuits are the most common design type found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Class B amplifiers are also favored in battery-operated devices, such as transistor radios.

V Answer the following questions:

1. What is the efficiency of Class B amplifier?

2. What is crossover distortion called?

3. What is Class AB operation called?

4. Where Class B and AB amplifiers used in?