Transit Time


Unlike electron flow in a conductor, electrons in a vacuum tube do not move at the speed of light. Their velocity is determined by the potential difference between the plate and the cathode. The amount of time the electrons take to travel from the cathode to the plate is called TRANSIT TIME . As a result of this time difference, the appearance of a signal at the end of a tube is not followed instantaneously by a change in current flow in the tube.

Under normal conditions, the effect of this small time lag between the input signal and a change in tube current is un-noticed. However, at frequencies such as those used in radar equipment, this is not the case. Transit time at these fre-quencies have a very marked effect on tube operation. It is a major factor that limits the use of a given tube at higher frequencies.


In your study of triodes so far, you have seen that the output of a triode circuit is developed across the tube. The output is caused by the voltage dropped across RL due to current flow from tube conduction. In all the demonstrations of gain, we assumed that RL was held constant and current through the tube was varied. In this manner we achieved a voltage gain.

If the resistance of RL is changed by the designer, the gain of a triode circuit can be either increased or decreased. This is fairly easy to understand. Assume that a circuit is composed of a triode with a plate-load resistor of 100 kohms. If a +2 volt signal causes 2 additional milliamperes to conduct through the tube, the voltage drop across RL (the output) will be.

amplification factor equation for tubes

Thus, the gain of the circuit is 100. If the plate-load resistor is reduced to 50 kohms and the input is kept at +2 volts, the gain will be reduced to:

amplification factor equation for tubes

As you can see, voltage gain depends on both the tube charact-eristics and the external circuit design.

The voltage gain is a measure of circuit efficiency, not tube efficiency.

The actual characteristics of a tube are measured by two factors: mu(µ) or AMPLIFICATION FACTOR; and TRANSCONDUCTANCE or gm. The amplification factor (represented as µ) of a tube is equal to the ratio of a change in plate voltage to the change in grid voltage required to cause the same change in plate current. This is expressed mathematically as

amplification factor equation for tubes

While this may sound complicated, it really isn't. Look at the illustration below. Here you see in view A a triode with a +1 volt input signal. At this grid voltage, current through the tube is at 1 milliampere. If the input voltage is raised to +3 volts, current through the tube increases to 2 milliamperes. The change in Eg (¨Eg) is then 2 volts.

This is shown in view B. Suppose that the grid voltage is returned to +1 volt, and the plate voltage is increased until the ammeter in view C reads 2 milliamperes of plate current. At this point plate voltage is measured. Plate voltage had to be increased by 100 volts (350-250) to get the same change in plate current (1 mA). The change in plate voltages (¨Ep) is then 100 volts. The amplification

amplification factor equation for tubes

Obtaining gain and transconductance

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 two different types of tubes, you can get an idea of their efficiency. For example, assume you have two different tubes, one with a mu of 50, and the other with a mu of 100. If you place each tube in a circuit whose input varies by 2 volts, you can expect the following changes in plate voltage.

Tube 1:

plate voltage equation for a tube

Tube 2:

plate voltage equation for a tube

Thus, you can expect twice the change in plate voltage from tube 2 as from tube 1 for the same input voltage. Therefore, tube 2 will have twice the gain of tube 1.

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