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Then we find the intersection with the leftmost portion of the graph by using Ohm's law:
I = V/R.
Given a plate resistor of 2k, the intersection occurs at 100 mA. The last step is to draw a line connecting both points. This line defines the range of possible idle currents and plate voltages with this plate resistor in place.
For example, if we pick 25 mA as a suitable idle current, then the plate resistor will see 50 volts across its leads and the triode will see 150 volts minus the cathode bias voltage (about 12.5 volts).
Working Backwards
Sometimes we are presented with an existing circuit that lists the values of the plate resistor and cathode resistor, but not the operating voltages or idle current. Fortunately, we can work backwards from the resistor values to the operating points (if the B+ voltage is specified). The mathematical approach involves rewriting the previously given formula:
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and
Vp = Vb - Iq( Rk + Ra)
and
Vgk = IqRk
While the results depend on the triode's mu and rp having been accurately defined from some point close to the actual operating point, the results are usually close enough to get a good idea of what is going on in the circuit.
Inspecting the plate curves promises greater precision, but requires a good eye. The first step is to plot the plate resistor as we did before. The second step is to plot the cathode resistor line. Be sure to resist the temptation to plot its line in the same fashion as we did in the resistor-only examples, as the cathode resistor does not see the voltage marked along the x-axis! The voltage the cathode resistor sees is the potential between ground and the cathode. Consequently, we must use the gridlines to provide the voltage needed in the formula: current = voltage / resistance. But as the gridlines curve, the cathode resistor line must also slightly curve.
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