Text 25. Transistor Radio Frequency Amplifiers

Like valve amplifiers a transistor r. f. amplifier may be of the tuned or of the band-pass variety.

On long, medium and short waves, the transistor is usually con­nected into a common emitter circuit while in the VHF and UHF bands use is sometimes made of the common base arrangement.

Transistor amplifiers differ from valve amplifiers in inter­stage coupling. Operation of a transistor amplifier is affected by the output resistance of the transistor, which is much low­er than the output resistance of an amplifier valve. This is why transformer and tapped- coil coupling is used extensively in r. f. transistor amplifiers.

T he stability of the operation of a transistor amplifier largely de­pends on the position of the quiescent operating point. To stabilize this point the circuit employs negative direct-current feedback provid­ed by R₃ (Fig. 9) connected in the emitter circuit. Such an arrange­ment is similar to the current feedback arrangement in valve circuits.

To eliminate a. c. feedback R₃ is bypassed to earth by C_b. Should the operating point shift due to temperature changes it will be restored by the feedback voltage built up across R₃ and applied to the transistor base. It should be noted however that in both configurations the tuned circuit may be connected to the collector circuit directly, provided the output resistance of the transistor is sufficiently high.

Notes

frequency ['friikwansi]	частота, частотність
amplifier ['æmplifaia]	радіопідсилювач
valve [Vaelv]	клапан, електронна лампа
band	смуга частот
medium ['mkdjsm]	середній
to emit [l'mit]	виділяти, випромінювати
stage ['steid3]	стадія; етап
quiescent [kwai'esnt]	постійне навантаження
to operate f'opsreit]	управляти
Feedback	зворотний зв'язок, регенерація

Computing Technique Text 1. The Computer

A computer is a machine that can take in information, perform different operations and provide answers. A computer can perform logical and mathematical operations such as addition, subtraction, mul­tiplication, division, and some more complex mathematical operations.

Logical operations deal with selecting, comparing, matching and so on meeting different needs of the users.

All the operations of a computer are performed at high speed in some kind of language (marks or symbols).

The computer has pervaded most fields of human activity and may well be the most important innovation of our age. Born out of the technology of communication, it is capable of handling enormous amounts of information at tremendous speeds. What makes it so potent is the fact that a single mechanism can perform any informa- tion-processing task. The same mechanism can control industrial pro­cesses, guide space vehicles or help to teach children. This diversity of tasks is made possible by the simple idea of the stored program.

The trick is to control electronically the nature and sequence of arithmetical and logical processes that are themselves electronic. In other words, what determines whether an addition, multiplication or some other operation is executed, what determines the inputs of the operation and what determines the disposition of the result are not built into the machine but are part of the electronic process itself. A program is the enumeration of theses determining commands; it speci­fies the method used for the solution of a problem in detail. When the machine is in operation, both the commands and the numbers or symbols being processed are constantly being taken out of and put into a depository of information known as a memory.

The commands, numbers or symbols needed in a processing task — known collectively as words — are stored in the memory, each with a certain “address”. The address identifies the stored word and determines a definite physical location within the memory de­vice. The power and universality of programming arise from the ca­pacity to address the memory selectively, that is, to direct a word into any address and to retrieve it in a very short time, regardless of how the address was previously used.

Notes

addition O'di/n] — додавання

subtraction [sab'traekjn] — віднімання

multiplication [.тліцріі'кеї/п] — множення

division [di'vi3n]		ділення
to select [si'lekt]		відбирати
to match		узгоджувати
to pervade [pa:'veid]	розповсюджувати
sequence ['si:kwans]	послідовність
to execute ['eksikju:t]	виконувати
disposition ^disps'zi/n]	розміщення
in detail ['di:teil] d	детально
epositary [di'pszitsn]	склад
to store [sto:]	зберігати
capacity [ka'pæsiti]	обсяг
to retrieve [ri'tri:v]	відновлювати
regardless [ri'ga:dlis]	незважаючи
previously ['priivjasli]	раніше