2.15 Part 15

2.15.1 Remember pronunciation of the following words

faithful, impose, distortion, blur, straight.

2.15.2 Find antonyms, translate them

primary (winding) a, weak a, like adv, great a, directly adv, output n, above adv, positive a, rise v, input n, fall v, negative a, secondary (winding) a, small a, strong a, unlike adv, indirectly adv, below adv.

VACUUM TUBE AMPLIFIER

1. One of the most important functions of the vacuum tube is its use as an amplifier of radio frequency or audio currents. The input resistor represents some part of any circuit in which a weak but varying current is flowing and the output resistor represents another circuit to which a stronger current of the same form is delivered. In some cases these are resistors but in others they are the primary and secondary windings of separate transformers. The source of the additional energy is the B-battery plate supply.

2. In amplifying any given signal or current a faithful reproduction of the wave form must be carried out, otherwise distortion will result; musical sounds from a radio will be blurred. To amplify without distortion a tube must be used that has a long straight section in its characteristic curve and it should be operated at the center of this straight portion. Such operation is shown by the graph in Figure 13. To make the tube operate at M a small battery, called a C-battery or C-bias, is inserted in the grid circuit to maintain the grid at a negative potential. For the curve and tube shown this requires —5 volts, while for other types and tubes it might well require greater or smaller potentials.

3. When no input signal potentials are imposed the grid is held at —5 volts and a steady current of 150 milliamperes flows through the plate and output circuit. If now an alternating current like a radio frequency of constant amplitude is impressed across the input terminals, the grid potential will rise and fall in the same way, and an undistorted but amplified current will flow in the plate and output circuit. If the input radio frequency is voice modulated, the amplified current will also be voice modulated without distortion. It should be noted that if the impressed grid voltage variations are too large, say —20 to +10 volts, the amplified currents will reach the curved portions of the curve above and below and distortion of the wave form will result. As long as the tube is operated on the straight portion of the curve, plate current is directly proportional to the impressed grid potential, and faithful amplification takes place.

Notes to the text

To make the tube operate at M a small battery called а С battery or C-bias is inserted in the grid circuit to maintain the grid at a negative potential. — Чтобы заставить лампу работать в точке М, на ее сетку подается потенциал от небольшой батареи С, которая называется батареей смещения и поддерживает сетку под отрицательным потенциалом.

Impressed grid voltage variations — изменения приложенного напряжения на сетке.

say — скажем.