INTRODUCTION TO ELECTRON TUBES

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Figure 1-28.—Obtaining gain and transconductance.As you can see, mu is a measure of the ability of a tube to amplify. By comparing the mu of twodifferent types of tubes, you can get an idea of their efficiency. For example, assume you have twodifferent tubes, one with a mu of 50, and the other with a mu of 100. If you place each tube in a circuitwhose input varies by 2 volts, you can expect the following changes in plate voltage.Tube 1:1-40
Tube 2:Thus, you can expect twice the change in plate voltage from tube 2 as from tube 1 for the same inputvoltage. Therefore, tube 2 will have twice the gain of tube 1.TransconductanceTransconductance is a measure of the change in plate current to a change in grid voltage, with platevoltage held constant. The unit for conductance is the mho (siemens), pronounced "moe."Transconductance is normally expressed in either micromhos or millimhos. Mathematically,transconductance is expressed by the formula:Examine figure 1-28, views A and B, again. In view A, the input voltage is +1 volt. At +1 volt E g ,the plate current is equal to 1 milliampere, with a plate voltage of 250 volts. In view B, the input voltage(E g ) is raised to +3 volts. g, as before, is equal to 2 volts. This increase in grid voltage causes platecurrent to increase to 2 milliamperes. The change in plate current (I p ) is then equal to 1 milliampere.Thus, transconductance (g m ) is equal to:Remember that the voltage gain of a circuit is measured by the ratio of the change in plate voltage tothe change in grid voltage. Because plate voltage is developed across a resistor, the more current varieswith a given input signal, the greater will be the output (E = I × R). If you have two tubes, one with a gmof 500 mhos and the other with a gm of 500 Assume that the circuit in which you wish to use one of these tubes has a load resistor of 100 kohms andthat E g will be 2 volts. The voltage gain of these two circuits will be:1-41
Page 1 and 2: CHAPTER 1INTRODUCTION TO ELECTRON T
Page 3 and 4: Figure 1-2.—Edison's experimental
Page 5 and 6: The path of the second series circu
Page 7 and 8: The reference point in diode and ot
Page 9 and 10: DIRECTLY HEATED.—The filament tha
Page 11 and 12: Figure 1-9.—Diode construction.Th
Page 13 and 14: The silver spot you will sometimes
Page 15 and 16: Figure 1-13.—E p - I p characteri
Page 17 and 18: You should note that there is very
Page 19 and 20: applications in other NEETS modules
Page 21 and 22: was a variation in plate current of
Page 23 and 24: Most amplifier circuits are designe
Page 25 and 26: ·acts to reduce or eliminate grid
Page 27 and 28: cathode, through the grid, to the p
Page 29 and 30: Now, let's assume that this particu
Page 31 and 32: Figure 1-22.—Cathode bias.The onl
Page 33 and 34: The capacitor, then, can be said to
Page 35 and 36: ight-hand plate of C c . This is th
Page 37 and 38: Figure 1-27.—Classes of amplifier
Page 39: Thus, the gain of the circuit is 10
Page 43 and 44: As you can see, tube 2 is 10 times
Page 45 and 46: Figure 1-31.—Basic tetrode circui
Page 47 and 48: Figure 1-32.—Noise in a tetrode c
Page 49 and 50: The CATHODE is the electron-emittin
Page 51 and 52: A TRIODE is basically a diode with
Page 53 and 54: of a triode to the input. This effe
Page 55 and 56: A15. A plate load resistor R LA16.

INTRODUCTION TO ELECTRON TUBES

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