Text 10 The structure of the triode transistor

The triode transistor itself takes several distinct forms. There are point-contact transistors and junction transistors.

The point-contact transistor consists of two point-contact diodes which are very close together. The current in either diode will have an important effect upon the current in the other diode. It is possible by biasing the diodes properly to obtain power amplification of electric signals, the spacing between the points being only a few mils. The contacts are applied to a carefully treated surface. One of the points is designated as the collector and biased in the reverse direction. The third contact is a large-area contact usually plated to the back of the polished or sandblasted base. Power amplification is obtained by virtue of the fact that the variation in the emitter current causes as large or larger variation in the collector current.

A device which is more stable both mechanically and electrically can be constructed by forming junctions rather than point contacts. The junction transistor consists of two p – n junctions on opposite sides of a thin slab of semiconductor crystal. The most common low-frequency transistor is probably the alloy type. The collector junction is made larger than the emitter junction to improve the collector action.

A pair of closely spaced p – n junctions may be grown in a bar of semiconductor crystal as the crystal is pulled. Transistors formed in this manner are known as grown –junction transistors. A series of n and p regions may be formed by changing the chemical composition of the melt or by changing the rate of crystal formation. The crystal containing the junctions is then cut into bars. Contacts are made to the ends and to the base region. The above mentioned processes may be used to form either n – p – n or p – n – p transistors. The p – n – p transistor is most easily made by grown-junction method.

The existence of two types of transistor, the n – p – n and p – n – p, offer an unusual amount of flexibility in circuit design.

Text 11 radio waves

Electrical energy that has escaped into free space exists in the form of electromagnetic waves. These waves, which are commonly called radio waves, travel with the velocity of light and consist of magnetic and electrostatic fields at right angles to each other and also at right angles to the direction of travel.

One half of the electrical energy contained in the wave exists in the form of electrostatic energy, while the remaining half is in the form of magnetic energy.

The essential properties of a radio wave are the frequency, intensity, direction of travel, and plane of polarization. The radio waves produced by an alternating current will vary in intensity with the frequency of the current and will therefore be alternately positive and negative.

The distance occupied by one complete cycle of such an alternating wave is equal to the velocity of the wave divided by the number of cycles that are sent out each second and is called the wave length.

The relation between wave length λ in meters and frequency f in cycles per second is therefore

.

The quantity 300 000 000 is the velocity of light in meters per second. The frequency is ordinarily expressed in kilocycles, abbreviated KC; or in megacycles, abbreviated MC. A low-frequency wave has a long wave length while a high frequency corresponds to a short wave length.

The strength of a radio wave is measured in terms of the voltage stress produced in space by the electrostatic field of the wave and is usually expressed in microvolts stress per meter.

Since the actual stress produced at any point by an alternating wave varies sinusoidally from instant to instant, it is customary to consider the intensity of such a wave to be the effective value of the stress, which is 0.707 times the maximum stress in the atmosphere during the cycle. The strength of the wave measured in terms of microvolts per meter of stress in space is exactly the same voltage that the magnetic flux of the wave induces in a conductor 1 meter long when sweeping across this conductor with the velocity of light.

Thus the strength of a wave is not only the dielectric stress produced in space by the electrostatic field, but it also represents the voltage that the magnetic field of the wave will induce in cutting across a conductor.

In fact, the voltage stress produced by the wave can be considered as resulting from the movement of the magnetic flux of the same wave.

The minimum field strength required to give satisfactory reception of a wave depends upon a number of factors, such as frequency, type of signal involved, and amount of interference present. Under some conditions radio waves having signal strengths as low as 0.1 μγ per meter are usable. Occasionally signal strengths as great as 5,000 to 30,000 μγ per meter are required to ensure entirely satisfactory reception at all times.

In most cases the weakest useful signal strength lies somewhere between these extremes.

A plane parallel to the mutually perpendicular lines of electrostatic and electromagnetic flux is called the wave front.

The wave always travels in a direction at right angles to the wave front, but whether it goes forward or backward depends upon the relative direction of the lines of electromagnetic and electrostatic flux.

If the direction of either the magnetic or electrostatic flux is reversed, the direction of travel is reversed; but reversing both sets of flux has no effect.

The direction of the electrostatic lines of flux is called the direction of polarization of the wave. If the electrostatic flux lines are vertical the wave is vertically polarized; when the electrostatic flux lines are horizontal and the electromagnetic flux lines are vertical, the wave is horizontally polarized.