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R. F. Amplification Without Distortion or Reradiation
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lower will be the opposition which the condenser offers to the flow of current through it. It is possible, then, to have a frequency so high that much of the current will be passed by the condenser C. When the frequency is such that the opposition offered by the condenser is practically the same as the opposition offered by the inductance, then the circuit is said to be in resonance. Such is the case, for example, when the parallel circuit of Fig. i, made up of the condenser and inductance, is properly tuned to a particular wave.
A very small capacity may have a very harmful effect at high frequencies. In the case of radio-frequency amplification, the little condensers in the vacuum tubes themselves cause much mischief. Small condensers are formed by the grid and filament, and by the plate and grid, so that if we were to represent these little condensers on the outside of a vacuum tube, we would have a picture much like that shown in Fig. 4. These little condensers are capable of causing oscillations at high frequencies, for if a pressure exists between the points AB, it will not only send a current through the little condensers represented by the plate and filament, but it will also send a current from B to C through the little condensers represented by the plate and grid, and by the grid and filament, respectively. The current flowing through the condenser between E and G causes the proper kind of voltage to be impressed on the grid of a tube to produce an oscillating current in the plate circuit, if the plate circuit contains a small amount of inductance, as shown. Usually the grid circuit also is tuned by an inductance, as shown by
FIG. 5
A recently developed set of the Clapp-Eastham Company, having one stage of R. F., detector, and two stages of A. F. amplification
the dotted line. This aggravates the tendency for the tube to oscillate. Therefore one may construct a radio-frequency amplifier with apparently no condensers, and yet the amplifier may oscillate and give no results whatever.
Tuning the transformers of a radio-frequency amplifier has the effect of building up parallel circuits of capacitance and inductance eager to oscillate, but in the receiver illustrated in Fig. 5, this tendency is curbed as shown in Fig. 6.
The tuning element consists of a series antenna condenser Ci in series with a variometer V. The first tube is used as a radio-frequency amplifier, and it is coupled to the detector tube by means of a tuned primary radio-frequency transformer which is prevented from oscillating by means of the control condenser C2. This little condenser is a variable of about .0002 mfd. capacity. The primary of the radiofrequency transformer is tuned by means of
FIG. 6
Wiring diagram of the set shown in Fig. 5. The oscillation condenser is shown as C2. Note that the B battery feeds the plate of the R. F. tube through a choke-coil