Lighthouse Electric Tutorials

Part 4.

Tube Characteristics and Operation

As we mentioned earlier, a vacuum tube with a control grid has amplifying ability. A small a-c voltage applied to the grid causeslarge variations in a-c plate current. Since the grid is maintained at negative potential (in most cases) over the complete cycle of operation, no electrons are attracted to it and there is practically no grid current present. A resistor, called theload resistoris connected in series with the plate circuit. Plate current responding to the input signal develops a large voltage drop across the plate resistor. Since the output voltage is greater than the applied grid voltage, the tube operates as an amplifier. A triode is a power amplifier as well as a voltage amplifier. Plate resistance is also listed in tube manufacturer's data.Triode Characteristic CurvesIn the families of curves shown in Fig.4-1a-b-c, each curve is obtained by applying different voltages to the grid and plate and noting the corresponding plate current. TheEp-Ipcurves are derrived by keeping the grid voltageEgconstant and plotting the plate voltageEpagainst the plate currentIp. TheEg-Ipcurves are derrived by keeping the plate voltageEpconstant and plotting the grid voltageEgagainst the plate currentIp. The third family of curves are obtained by holding the plate currentIpconstant, and plotting the grid voltageEgagainst the plate voltageEp. Most commonly used curves are theEp-Ipfamily and are published in manufacturer's tube manuals.

Fig.4-1a

Fig.4-1b

Fig.4-1c

Tube ParametersThree important parameters is obtainable from either curve family. These parameters or ratios, are theamplification factor, thetransconductanceand the internal a-c (dynamic)plate resistance. The amplification factormuis defined as the ratio of plate-voltage change to grid-voltage, at constant plate current.

Grid-platetransconductance, or mutual conductance,Gmis the ratio of plate-current change to grid-voltage change producing it, at constant plate voltage. The unit of transconductance is the mho, and is expressed by the formula:

Plate Resistance.The triode has a d-c and an a-c plate resistance. The dc plate resistance is simly theEp/Ipratio at a chosen point on a curve and determines how much power is consumed by the tube as heat -plate dissipation. Dynamic plate resistance is different in value from the d-c plate resistance. Internal a-c plate resistanceRpis given by the ratio of plate-voltage change to plate-current change producing it, at constant grid voltage. The plate resistance is stated in Ohm's:

Taking all the above into account, the following relationship exists between the three tube coefficients:

Load LinesOne can apply characteristic curves to find the output current and voltage values forany operating condition.The strait line drawn across theEp-Ipcurves is called theload line. It is a plot of corresponding plate-current and plate-voltage values in the triode circuit. This method is shown in Fig.4-2

Fig.4-2

One end of the load line is located at zero plate current and zero voltage drop across the load resistor. The other end islocated at the point where the plate current is not limited by the tube, the tube is replaced by a short circuit. Current flow at this point is limited only by the load resistance. The slope of the load line is determined by the value of the load resistor. The path of operation for the amplifier circuit must always fall along the load line. Let's examine one example shown in Fig.4-2. The grid bias value is -8volt. With no signal applied to the grid, the static values Ip and Ep are located at the intersection of the -8 Volt curve and the load line. This point is called theoperating(P) orquescentpoint, corresponds to Ip = 4.2 mA an Ep=132 Volts. Let's assume we apply 4 Volts a-c signal voltage to the grid. Since the grid is biased at -8 Volts, that 4 volt signal will cause the grid voltage to swing between -4 and -12 Volts. The two curves,Eg=-4 andEg-12, cross the load line, which coresponds toEp-min=102 Volts andEp-max168 Volts,Ip-max=5.1 mA andIp-min=3.3 mA. The portion of the load line between those two points is called theoperating range.

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