Negative Transconductance Oscillator

A negative transconductance oscillator consists of a pentode with the screen and suppressor grids coupled together. The screen is more positive than the plate. The cathode supplies electrons to both the screen and the plate. If the screen voltage rises, this increase is transferred over to the suppressor. It becomes more positive than before. A large quantity of electrons passes to the plate and a small quantity of electrons passes to the screen. Thus, the screen current is reduced. A negative resistance characteristic is produced in the screen circuit at the terminals.

A tuned circuit is connected to the terminals and in this way oscillations are produced.

In what way is the screen current reduced?

What does negative transconductance oscillator consist of?

 

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What are the names of the Oscillators?

Many types of oscillator circuits are used nowadays. Their tube types, tank circuits, and feedbacks are different.

Examine the diagrams given below. To answer this question read the following texts.

1. The Electron-coupled Oscillator uses a multi-grid vacuum tube with a cathode and two grids operating in common. In this device the plate circuit load is coupled to the oscillator through the electron stream. The diagram shows a tetrode variant using a series-fed oscillator.

2. The Dynatron Oscillator uses the negative resistance characteristic of a tetrode tube to eliminate the resistance of its tank circuit in order to keep oscillations.

3. The Blocking Oscillator uses a plate transformer to provide the feedback voltage.

4. A Multivibrator is a form of oscillator using two RC-controlled triodes.

5. The Tuned Plate - Tuned Grid oscillator has parallel or resonant circuits in both plate and grid circuits. The necessary feedback is supplied by the plate-to-grid interelectrode capacitance.

Name the five oscillators in Fig. 20.

Fig. 20

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Solar Energy

The Sun radiates to our planet great amounts of energy. This energy input fuels all our biological processes and is the original source for hydropower, wind power, and fossil fuel resources. Environmentally and economically, die Sun is our greatest energy source. Why then is the direct use of solar energy so limited for industrial, domestic, commercial, and transportation purposes?

First, solar energy arrives in small quantities - only about one kW per square meter; second, it does not arrive at all during night hours. Third, it is very difficult to transform it into useful energy forms except low-grade heat.

Why is the direct use of solar energy limited nowadays?

 

                                                                                              

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Superconductivity

In the electric generating field, as well as in the electric-transmission and storage-technology areas, considerable attention is focused on the use of superconductivity conductors. Cooling an electric conductor to temperatures close to absolute zero (-273°C) results in the conductors’ losing their resistance to electric current. Accordingly, their ohmic losses also drop greatly. Current densities of tens of millions amperes per sq. cm can be tolerated in such conductors.

Cryogenic technology being now mastered, one can foresee the possibility of supercooling not only generators but also underground transmission lines and storage coils.

Are these statements true or false?

a) Conductors can never lose their resistance to the flow of electric current.

b) Generators and underground transmission lines can be supercooled.

 

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Metals Used as Conductors

The use of electricity depends upon a means of conducting it from its source to the point where it is to be used. Copper has been used as a conductor since the beginning of the industry and no proper substitute has been found. Only one metal, silver, is more efficient, but it has too high cost to be extensively used.

Aluminium, because of its lightness, is used in common practice for transmission lines where long spans are necessary. It has, compared to other metals, a conducting capacity of about 60 per cent of that of copper.