Transconductance is a measure of the change in plate current to a change in grid voltage, with plate voltage held constant. The unit for conductance is the mho (siemens), pronounced "moe." Transconductance is normally expressed in either micromhos or millimhos. Mathematically, transconductance is expressed by the formula:
Examine the illustration below, views A and B, again. In view A, the input voltage is +1 volt. At +1 volt Eg, the plate current is equal to 1 milliampere, with a plate voltage of 250 volts. In view B, the input voltage (Eg) is raised to +3 volts. š( g, as before, is equal to 2 volts. This increase in grid voltage causes plate current to increase to 2 milliamperes. The change in plate current (šIp) is then equal to 1 milliampere. Thus, transconductance (gm) is equal to:
Obtaining gain and transconductance.
Remember that the voltage gain of a circuit is measured by the ratio of the change in plate voltage to the change in grid voltage. Because plate voltage is developed across a resistor, the more current varies with a given input signal, the greater will be the output (E = I × R). If you have two tubes, one with a gm of 500 mhos and the other with a gm of 500 PKRV \RX FDQ HVWLPDWH WKH UHODWLYH JDLQ RI WKHVH WZR WXEHV Assume that the circuit in which you wish to use one of these tubes has a load resistor of 100 kohms and that šEg will be 2 volts. The voltage gain of these two circuits will be:
As you can see, tube 2 is 10 times the amplifier that tube 1 is.
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