III Answer the following questions:

1. How can amplifires be classified by function?

2. Where is a servo amplifier used in?

3. Where is a wideband amplifier used in?

4. What characteristics has an operational amplifier?

IV Define the tense-forms of the verbs:

1. may be classified

2. indicates

3. does not generate

4. are used

5. do not occur

6. is designed

7. are provided

V Choose the correct variant from those in brackets:

1. They are well-trained ... (in, into, to) programming, statistics, ecology problems.

2. Measurements are ... (much, more) accurate and the work is done faster.

3. Storing materials ... (for, with, to) long periods can cause problems.

4. The next important step (was, were, been) the development of robots with legs.

5. Manyfactures ... (find, found) that quality, as well as quantity and cheapness, ... (was, were) important.

VI Translate the text in writing.

Modern machinery, or what may be termed production units, consist of a large number of different parts, separate machines and pieces of apparatus, each of which is assigned a definite function. Operated together, these parts, machines and apparatus serve to carry out some specific production process. In order to design and build such production units, as well as to be able to operate and maintain them efficiently, it is necessary to have a sound knowledge of the functions of all their elements.

Every well developed machine may be considered to consist of three significantly different parts: the prime mover (motor, engine), the transmission system, and the working machine proper. The function of the first two parts, the prime mover with its system of control, and the transmission system, which may comprise a set of shafts, pulleys, belts, gears, etc., is to impart motion to and operate the working machine. Hence, since the first two parts maintain the working machine in motion, they are both embraced by the common term "drive".

Unit 13 Amplifier classes

I Read and memorise the following words:

1. switch on – включати

2. curve – вигин

3. fidelity – правильність

4. capacity – ємкість, місткість

5. biased – попереджений

6. inefficiency – неефективність

7. intentional – навмисний, з наміром

8. valve – електронна лампа

II Read and translate the text:

Amplifier circuits are classified as A, B, AB and C for analog designs, and class D and E for switching designs. For the analog classes, each class defines what proportion of the input signal cycle (called the angle of flow) is used to actually switch on the amplifying device:

Class A

Class A amplifying devices operate over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input with no clipping. Class A amplifiers are the usual means of implementing small-signal amplifiers. They are not very efficient; a theoretical maximum of 50% is obtainable with inductive output coupling and only 25% with capacitive coupling.

In a Class A circuit, the amplifying element is biased so the device is always conducting to some extent, and is operated over the most linear portion of its characteristic curve (known as its transfer characteristic or transconductance curve). Because the device is always conducting, even if there is no input at all, power is drawn for the power supply. This is the chief reason for its inefficiency.

If high output powers are needed from a Class A circuit, the power waste (and the accompanying heat) will become significant. For every watt delivered to the load, the amplifier itself will, at best, dissipate another watt. For large powers this means very large and expensive power supplies and heat sinking. Class A designs have largely been superseded for audio power amplifiers, though some audiophiles believe that Class A gives the best sound quality, due to it being operated in as linear a manner as possible which provides a small market for expensive high fidelity Class A amps. In addition, some aficionados prefer thermionic valve (or "tube") designs instead of transistors, for several claimed reasons:

tubes are more commonly used in class A designs, which have an asymmetrical transfer function. This means that distortion of a sine wave creates both odd- and even-numbered harmonics. The claim is that this sounds more "musical" than the higher level of odd harmonics produced by a symmetrical push-pull amplifier. Though good amplifier design can reduce harmonic distortion patterns to almost nothing, the increase in distortion in some amplifier designs is essential to the sound of intentional electric guitar distortion.

Another is that valves use many more electrons at once than a transistor, and so statistical effects lead to a "smoother" approximation of the true waveform — see shot noise for more on this. Junction field-effect transistors (JFETs) have similar characteristics to valves, so these are found more often in high quality amplifiers than bipolar transistors. Historically, valve amplifiers often used a Class A power amplifier simply because valves are large and expensive; many Class A designs uses only a single device.