Lighthouse Electric Tutorials
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 the load resistor is 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 Curves In 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. The Ep-Ip curves are derrived by keeping the grid voltage Eg constant and plotting the plate voltage Ep against the plate current Ip. The Eg-Ip curves are derrived by keeping the plate voltage Ep constant and plotting the grid voltage Eg against the plate current Ip. The third family of curves are obtained by holding the plate current Ip constant, and plotting the grid voltage Eg against the plate voltage Ep. Most commonly used curves are the Ep-Ip family and are published in manufacturer's tube manuals.
Tube Parameters Three important parameters is obtainable from either curve family. These parameters or ratios, are the amplification factor, the transconductance and the internal a-c (dynamic) plate resistance. The amplification factor mu is defined as the ratio of plate-voltage change to grid-voltage, at constant plate current.
Grid-plate transconductance, or mutual conductance, Gm is 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 the Ep/Ip ratio 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 resistance Rp is 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 Lines One can apply characteristic curves to find the output current and voltage values for any operating condition. The strait line drawn across the Ep-Ip curves is called the load 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
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 the operating (P) or quescent point, 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 and Eg-12, cross the load line, which coresponds to Ep-min=102 Volts and Ep-max168 Volts, Ip-max =5.1 mA and Ip-min =3.3 mA. The portion of the load line between those two points is called the operating range.
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