When the input is at its maximum negative value, conduction through the tube decreases to .5 milliampere.

What is the peak-to-peak voltage of the output signal?
What is the phase relationship between the input and output signals?

FACTORS AFFECTING TRIODE OPERATION

The triode circuit you have just studied is a fairly simple affair.

In actual application, triode circuits are a bit more complex. There are two reasons for this. The first has to do with the triodes ability to amplify and perform other functions. Triodes come in many different types. Each of these types has different internal characteristics and different capabilities. Because of this, each triode circuit must be designed to accommodate the triodes special characteristics. The second reason for the increase in complexity has to do with DISTORTION. Distortion occurs in a tube circuit any time the output waveform is not a faithful reproduction of the input waveform.

Polarity inversion and voltage gain of the output waveform are not included in this definition of distortion. Some circuits are designed to distort the output. The reason and methods for this deliberate distortion will be covered in a later NEETS module. For the most part, however, we desire that circuits eliminate or reduce distortion.

Because the grid is close to the cathode, small changes in grid voltage have large effects on the conduction of triodes. If a large enough input signal is placed on the grid, a triode may be driven into either plate-current cutoff or plate-current saturation. When this occurs, the tube is said to be OVERDRIVEN. Overdriving is considered to be a form of DISTORTION.

Look at time zero (0) in the waveforms of figure 1-20. The input signal (E_in) is at zero volts. Grid voltage equals the bias voltage (-6 volts), and one milliampere of current is flowing through the tube (quiescent state). Plate voltage (E_p) is 200 volts.

Figure 1-20. - Overdriven triode.

On the negative half of the input signal, the grid voltage is made more negative. This reduces plate current which, in turn, reduces the voltage drop across R_L. The voltage between cathode and the plate is thereby increased. You can see these relationships by following time "a" through the three waveforms.

Now, let's assume that this particular triode cuts plate current flow off when the grid reaches -24 volts. This point is reached at time b when E_in is -18 and the bias is -6 (-18 and -6 = -24). Plate current remains cut off for as long as the grid is at -24 volts or greater.

With zero current flowing in the plate circuit, there is no voltage drop across R_L. The entire plate-supply voltage, E _bb (300 volts), appears as plate voltage between the cathode and the plate. This is shown at time b in the output signal waveform. Between time b and time c, the grid voltage is greater than -24 volts. The plate current remains cutoff, and the plate voltage remains at +300. The output waveform between time b and time c cannot follow the input because the plate voltage cannot increase above +300 volts. The output waveform is "flattopped." This condition is known as AMPLITUDE DISTORTION.

When the grid voltage becomes less negative than -24 volts, after time c, the tube starts conducting, and the circuit again produces an output.

Between time c and time d, the circuit continues to operate without distortion. At time e, however, the output waveform is again distorted and remains distorted until time f. Let's see what happened.

Remember that every cathode is able to emit just so many electrons. When that maximum number is being emitted, the tube is said to be at SATURATION or PLATE SATURATION. Saturation is reached in a triode when the voltages on the grid and plate combine to draw all the electrons from the space charge.

Now, as our grid becomes less negative (between time c and time d), and actually becomes positive (between time d and time e), the plate current increases, the voltage across R_L increases, and the plate voltage decreases.

Apparently when the grid voltage reached +12 volts at time e, the plate current reached saturation. Maximum plate current (at saturation) results in maximum voltage across R_L and minimum plate voltage. Any grid voltage higher than +12 volts cannot cause further changes in the output. Therefore, between time e and time f, the plate voltage remains at +100 volts and the waveform is distorted. This is also AMPLITUDE DISTORTION.

This has been an explanation of one cycle of an input signal that overdrives the tube. You should notice that, using the same circuit, a 50-volt peak-to-peak input signal caused a vastly different output from that caused by the 6-volt peak-to-peak input signal. The 6-volt peak-to-peak signal did not overdrive the tube. When the input signal was increased to 50-volts peak-to-peak, the tube was forced into cutoff when the grid was driven to -24 volts, and into saturation when the grid was driven to +12 volts (the grid voltage plus the signal voltage.) During these periods, the tube could not respond to the input signal. In other words, the output was distorted. A method commonly used to partially overcome distortion is to vary the bias voltage on the grid. The point at which the tube goes into cutoff or saturation can then be controlled.

For this reason tube biasing is of great importance in most tube circuits.

Q.20 The waveforms shown below are the input and output of an overdriven triode.