Unit 10. Amplifiers and oscillators

1. Words to remember.

apmlifier – усилитель

amplitude – амплитуда

bias – смещение

grid – энергетическая система

inductance – индуктивность

capacitance – ёмкость

feed-back – обратное питание

shift – изменение

sustained – длительный

plate – анод (лампы)

2. Read the text. Amplifiers and oscillators

One of the most important applications of electron tubes and transistors is to amplify voltage, power, or current. In an electron tube the plate collects the current in much the same manner as the collector electrode of a transistor. Electrons travel from cathode to plate in a tube, and current carriers travel from emitter to collector in a transistor. In a triode electron tube the electrons pass through the control- grid region and in a transistor the current passes through the base.

The bias voltage between control grid and cathode controls plate current in an electron tube; the bias current between base and emitter controls collector current in a transistor. Any transistor or electron tube connected in such a manner as to use its amplification capability may correctly be referred to as an amplifier.

It is hardly possible to attach too much importance to the study of amplifiers since there exists scarcely a unit of electronic equipment that does not utilize the principles of amplification.

THE AMPLIFIER

In electronics an amplifier may be defined as ‘any device that will receive at its input a signal of given amplitude of voltage and deliver at its output a signal of greater amplitude’. The devices most common to all electronic circuits in which amplification is one of the requirements are the electron tube and the transistor.

The manner in which amplifier devices are connected into the associated circuits governs the amount of amplification, for example, the three basic circuits for transistors indicate how different amounts of amplification can be obtained. The common-base circuit produces a current transfer of less than unity and has an output resistance that is very high with respect to the resistance to the input resistance. The common-emitter circuit produces a typical current gain of 50 and has an output resistance that is medium with respect to the input resistance. The common- collector also produces a very high current gain but has an output resistance much lower than the input resistance. Thus it is seen that many different conditions can be established and different amounts of amplification obtained by circuit design.

Of the three basic circuits, the common- emitter has the advantages of high current gain, high voltage gain, and medium input and output resistance.

OSCILLATORS

Although oscillators perform definite functions in both receiver and transmitter circuits, in all cases they can be considered as devices used for the purpose of generating frequencies. The frequencies generated are determined by the value of circuit components. The elements of an oscillator circuit may consist of an inductance- capacitance (LC) network, or a resistance- capacitance (RC) network. Tube and transistor oscillators operate essentially the same. Oscillators are similar to the amplifiers previously discussed except that a portion of the output power is returned to the input network in phase with the starting power (regenerative or positive feedback) to sustain oscillations. DC bias voltage requirements for oscillators are similar to those for amplifiers. An oscillator functions on the ability of an electron tube or transistor to amplify.

By applying a portion of the amplified output back to the input circuit as regenerative feedback, input circuit losses can be overcome and the circuit will oscillate. Therefore, any amplifier supplying its own input is called an oscillator. The most important consideration is that the feedback signal is applied in phase with the signal at the input. Since there is a 180 phase shift between the input and output of electron-tube and common-emitter amplifiers the feedback circuit must provide an additional 180 phase shift.

For sustained oscillations in a transistor oscillator, the power gain of the amplifier network must be at least equal to or greater than unity. When the amplifier power gain becomes less than unity, oscillations become smaller with time, or ‘damped’, until they cease to exist. In practical oscillatory circuits, power gains greater than unity are required because the power output is divided between the load and the feedback network. The feedback power must be equal to the input power plus the losses in the feedback network to sustain oscillations.

The increasing use of electrical energy will require electric power systems with transmission capacities greater than existing systems. The need for much transmission capacities in the future will stem primarily from the incentive to make optimum use of available resources.

One of the most efficient methods to increase transmission capacity is to increase the line voltage, rather than the line current which is responsible for transmission loss and conductor heat. The increase in voltage will increase dielectric stresses; but even today, in the era of new and sophisticated insulating materials, air appears to be the most attractive insulating medium to use in connection with the transmission of these tremendously high power quantities. Air has traditionally been the least expensive and most convenient medium for insulation and cooling, and its potential limits are yet unexplored. Ultra High Voltage (UHV) research for overhead transmission lines is directed toward cities greater than existing systems.

It is important to keep in mind that the limits of UHV ac transmission are not determined by insulation alone but rather by complex social and economic problems, which must be weighted against alternative solutions (UHV dc, more circuits at lower voltages, underground transmission) capable of transmitting the same amount of power.