Lesson 6. Electronic devices: amplifiers, diodes, oscillators

TEXT 6 A. Amplifiers;

Grammar Exercises (Word Order in a Simple Sentence. Negative Simple Sentence. Word Order in Interrogative Sentences);

TEXT 6 B. Diodes;

TEXT 6 C. Requirements for Oscillators.

Pre-Text Exercises

Pre-Text Exercises

Active Vocabulary

I. Practise reading the words written below. You’ll come across them in text 6 a.

amplifier …………... pick-up cartridge ….. conventionally …….. pre-amplifier ……… power amplifier …… to drive ……………. circuit ……………... configuration ……… alternation voltage … to conduct …………. base-emitter ……….. junction …………… bias ………………... potential …………... substantially ………. output ……………... linear ……………… curve ……………… resistor ……………. collector …………... headroom …………. excursion ………….. gain ……………….. leakage current ……. ambient …………… feedback loop …….. input ………………. to reduce …………..

усилитель звукосниматель, датчик обычно предварительный усилитель усилитель мощности запускать цепь устройство; форма переменное напряжение проводить излучатель узел, соединение смещение потенциал существенно, значительно мощность линейный кривая сопротивление, реостат коллектор, сборник габаритная высота сдвиг, отклонение; полюс усиление текущий ток окружающий контур обратной связи ввод, подача понижать, ослаблять

II. Put the words from the box into the correct columns below. Translate them into Russian.

conventionally, to drive, amplification, to cause, re-quirement, incapable, to conduct, condition, simply, positive, resistor, accurately, voltage, linear, ambient,

manufacture, to compensate, leakage

Noun (what/who?)	Adjective (what kind of?)	Verb (what to do?)	Adverb (in what manner?)
… … …	… … …	… to compensate …	approximately … …

III. Read text 6 a. Try to understand its content. Text 6 a. Amplifiers

A system diagram of a record player amplifier, where the small electric signal produced by the player’s pick-up cartridge must be amplified to a sufficiently large extent to a speaker, looks quite simple. The symbols for the cartridge, amplifier and speaker are those conventionally used. In practice the amplifier is quite complicated, and can be broken down into two main sections: the preamplifier, which deals with the amplification of the small signal from the cartridge; and the power amplifier which deals with the high-power amplification necessary to drive the speaker.

Although a transistor amplifier circuit has simple configuration and it is satisfactory for demonstrating ‘transistor action’, it is incapable of amplifying an audio signal.

Consider the effect of amplifying the alternating voltage from the pick-up cartridge to terminal A. Since the transistor conducts only when the base-emitter junction is forward-biased. only the parts of the signal that are positive relative to the emitter will cause the transistor to conduct.

As well as amplifying the input signal, the transistor is rectifying it. Once, in a while this happens in an amplifier under fault conditions, and it sounds terrible. It is also apparent that a proportion of the positive part of the signal is lost as well, since the transistor will not conduct, even with the base-emitter junction forward-biased, until the potential exceeds 0.7 V for a silicon transistor. This is a substantially higher voltage than a magnetic pick-up cartridge producer, so in practice the output of the circuit would be zero.

A solution to the problem is simply to connect a suitable positive potential to the base of the transistor, ensuring that it is always forward-biased so that it operates somewhere about the mid-point of the linear region of the characteristic curve. This is best done with a potential divider network involving two resistors. This does at least produce an output, if the two biased resistors R1 and R2 are exactly the right values; the resistors should be chosen so that the collector is at half the line voltage with no signal applied. This gives approximately equal ‘headroom’ for the signal in its positive and negative excursions.

Because of the tiny differences in manufacture, it is not possible to specify the gain of a transistor very accurately, a gain variation of +-50 per cent being quite common. Using this circuit would mean careful measurements and a new pair of resistors for each individual transistor used – not an ideal requirement for mass production. The leakage current of the transistor, and thus its operating point, will change the temperature. What is needed is a circuit that will automatically compensate for variations in gain of the transistor, and also for changes in ambient temperature.

In this circuit it is the values of the resistors (only) that determine the operating point. The gain of the transistor, leakage and temperature has practically no effect. The circuit involves a feedback loop. Feedback is a principle that is often used in electronics, and you will find it in many different contexts. In this case negative feedback is used. Negative feedback consists of a connection from the оutput of a system back to the input arranged, so that change in output reduces is causing that change.