International projectionist (Jan-Dec 1946)

FIGLRE 2. A simple amplifier circuit using a triode tube. on the grid. This will cause the plate current to decrease. Hence, when the voltage on the grid is varied in accordance with a signal. the plate curent varies with the signal. It takes only a very small input signal to give a large variation in plate current. If the plate current is passed through a large resistor, the varying current will produce a large voltage across the resistor. Assuming that the tube is operated properly, very little distortion will result and the voltage across the plate resistor will have the same characteristics as the alternating voltage on the grid. From this discussion it can be seen that a small voltage placed on the grid will result in a large voltage being produced across the plate resistor, and it is in this manner that the tube can amplify very small signals. The invention of the triode tube made loudspeaker radio reception possible. A simple triode amplifier circuit is shown in Fig. 2. The grid is seen to connect to a "C" battery through a resistor. The "C" battery provides the negative bias for the grid. The cathode is tied to the positive side of the "C" battery, which means that the grid will be negative with respect to the cathode. The signal is applied across the resistor in the grid circuit. This resistor is known as the grid input resistor and its value usually is about one meg-ohm or more. A large value of resistance is necessary here in order to keep the input signal from being shunted to ground. If the resistor is made too large, distortion will result due to contact potential and large negative charges accumulating on the grid. The plate is connected to a larse "B" batterv throuah -p-se FIG. 3 FIG. 4 a resistor which i known as the plate load resistor. It can be seen that the positive end of the battery is nearest the plate and the negative end is connected to the cathode. This connection makes the plate positive with respect to the cathode. The plate must be positive with respect to the cathode in order for plate current to flow. The plate load resistor should be large ii much amplification is desired, the usual values being from 0.1 meg-nhnito 0.5 meg-ohms. The plate load resistor rarely exceeds 0.5 meg-ohms unless special circuits are used. The output voltage, which is the amplified signal, appears across the plate load resistor. From this point the output signal may be fed into a loudspeaker, or it may be fed into the grid of a second tube if additional amplification is required. Inter-Electrode Capacitance The grid, plate, and cathode of a triode form an electro-static system, each electrode acting as one plate of a small condenser. The capacitances are those existing between grid and plate, arid OUTPUT FIGLRE 3. (left) Interelectrode capacitances present in a triode tube. FIGURE 4. (right) Interelectrode capacitance is reduced in a tetrode tube by means of the screen grid. FIGLRE 5. A simple amplifier circuit using a tetrode tube. and cathode, plate and cathode, and are known as inter-electrode capacitances. As a general rule, the capacitance between grid and plate is of the most importance. In high-gain radio-frequency amplifier circuits, this capacitance can produce undesired coupling between the input and the output circuits. This coupling is undesirable in an amplifier because it results in unwanted oscillations, instability, and unsatisfactory performance. The inter-electrode capacitances which exist in a triode may be noted in Fig. 3. If an additional electrode is placed between the grid and plate, the grid to plate capacitance can be reduced so that its effects on tube performance are negligible. This additional electrode which looks like the control grid, is called a screen grid, and it shields the control grid from the plate. It may be considered as tending to form two condensers between the grid and the plate. These condensers will be in series, so the total capacitance between control grid and plate will be reduced con C B FIGLRE 6. A simple amplifier circuit using a pentode tube. siderably. The screen grid acting in conjunction with a by-pass condenser which is connected externally between screen grid and cathode, will reduce the grid to plate capacitance from an average of 10 micromicrofarads for a triode to about 0.01 micromicrofarads. or less, for a tetrode. The method by which the capacitance between grid and plate is reduced is illustrated in Fig. 4. The screen grid is operated with a positive voltage, usually less than the plate voltage. The screen is closer to the cathode than the plate, and has a greater effect on the plate curent than is exerted by the plate. This makes the plate current somewhat independent of plate voltage and makes possible the design of tubes having higher amplification factors than is possible with triodes. The low inter-electrode capacitance makes it possible to obtain this high amplification without plate to grid feedback and resultant instability. In Fig. 5 we see the same amplifier circuit that is shown in Fig. 2, except that a screen grid tube, called a tetrode because it contains four elements, is now used in place of a triode. In the tetrode type of vacuum tube the screen grid is placed between the control grid and plate. Although the voltage on the screen is usually less than that on the plate, it still is a fairly large positive charge. This positive voltage attracts the electrons strongly because of the magnitude of the voltage and the proximity of the screen to the cathode. The electrons, for the greater part, pass through the screen grid and move on to the plate because of the wide spaces between the wires which make the screen grid. The electrons which move on to the DECEMBER QUESTIONS AND ANSWERS 1> (Q-) Which type of cathode gives the most emission for the least amount of filament power? (A.) Oxide-coated cathode. 2. (Q.) "which types of cathodes are best suited for tubes that are designed for portable receivers? (A.) Directly-heated cathodes. JANUARY 1946 21