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Plate Resistance

Author: N.H. Crowhurst

Results obtained by the dynamic measurements method

When the tests are made this way, the output, or gain of the tube, goes up indefinitely, until we have difficulty in finding supply voltages high enough to maintain the same plate voltage.

A curve almost exactly like this can be made using only resistors without any tube, (A larger alternating voltage is needed, because there is no amplification, and the proper fixed resistance has to be chosen to get the curve the same shape.) From this we see that the tube is somewhat like a generator that produces an a-e voltage and has an internal resistance, the value of which is that used in the equivalent circuit that produces the same curve.

The number by which the a-c grid voltage has to be multiplied to give the input voltage in the equivalent circuit, is called the amplification factor (or magnification factor) of the tube. The value of the fixed resistor in the equivalent circuit, is called the plate resistance or a-c resistance of the tube.

The same curve can be obtained with a resistance setup

Because voltage in a series circuit is proportional to the total resistance in the circuit, we can deduce that the output voltage from the tube is given by μRc/(Rp + Rc). The bigger the coupling resistor between the supply voltage and the plate of the tube, the less the plate current must fluctuate to produce a given plate voltage fluctuation. If, in imagination, we could make the resistance and the supply voltage infinitely large, only the plate voltage would vary. The gain of the tube in this imaginary circuit is its amplification factor.

Relationship of tube circuit parameters

If there is no coupling resistor present (this was the case when we first examined the circuit), the voltage remains constant, and the current fluctuates. The plate current fluctuation for each volt of grid fluctuation is the transconductance of the tube. The relationship between these two conditions (which have no practical use, but help no end in figuring out the operation of practical tube circuits) is determined only by the parameter that we have called plate resistance.

Last Update: 2010-11-03