Radio Broadcast (Nov 1923-Apr 1924)

How the C Battery Prevents Distortion 299 + RMP. B FIG. I Two-stage audio amplifier. If the transformers are of reliable make and matched for the tubes, any distortion must occur in the tube circuit Therefore, we will focus attention upon the vacuum tube and see what part it plays in the amplifier. Also what other apparatus, if any, must be added to the amplifier so that the vacuum tube will operate with maximum efficiency, eliminate distortion, and still afford maximum amplification. What is the three-element vacuum tube as used in our present-day amplifiers? It is an instrument consisting of an evacuated bulb, containing a filament that can be made incandescent so as to emit electrons; a grid, and another metallic object called the plate. Further, it is a voltage-operated device, which, when an alternating potential is impressed upon the grid, will produce the same alternations in the current flowing in the plate circuit but of greater amplitude. Now the question might arise in the mind of the reader: How can distortion occur in a vacuum tube if the alternations impressed upon the grid are reproduced in the plate circuit? This can best be explained by means of a plate-current, gridvoltage characteristic curve (Fig. 2). Before going into the explanation of the curve, a few remarks concerning the plate current might help the uninitiated. Experiments have determined that when a filament is contained in an evacuated chamber and made incandescent, it will emit electrons at a certain rate per second, the rate depending upon the degree of incandescence. Also, that when another metallic object in the form of a plate is placed into that chamber and given a positive charge in respect to the filament (the positive terminal of the B battery is always connected to the plate), the electrons will be attracted to the plate, and a stream of electrons will flow from the filament to the plate. Now if we place a meter that will indicate the flow and quantity of any current in the plate circuit, we will find that when there is a flow of electrons within the tube (from filament to plate), there is a certain value of current flowing in the plate circuit. So we can say that the electronic flow is equal to a current flow within the tube. And we find that by varying the brilliancy of the filament (thus varying the number of electrons that are emitted from the filament per second), we can vary the plate current; also, that we can vary the plate current by varying the value of the voltage applied to the plate, for the greater the positive charge on the plate, the greater its attracting power, and a greater number of electrons are attracted to the plate. The plate current corresponds to the number of electrons reaching the plate per second. If we maintain the filament current constant and the plate voltage is also maintained at a constant value, there will be a definite number of electrons flowing across inside the tube, and the plate current will be constant. This value of the plate current is usually expressed as the amount of current the tube draws. When the plate voltage is held constant and the filament current varied from minimum to maximum, there will be a variation in the plate current from minimum to maximum. The maximum indicates that the positive charge on the plate has attracted all the electrons possible, and any increase in filament current in order to increase the number of electrons emitted will be of no use. When this stage is reached, we have reached the saturation point of the tube, i.e. for that plate voltage. Now let us go back to the characteristic curve of the tube. We are to illustrate how distortion may occur within the vacuum tube, and we shall also show how it can be eliminated. The curve illustrates the operation of a three fig. 2 Showing the relationship between the current and the grid voltage when the plate voltage and filament current are held constant QRID VOLTAGE