Characteristics
In triode datasheets, characteristics linking the anode current(Ia) to anode voltage (Va) and grid voltage (Vg) are usually given. From here, a circuit designer can choose the operating point of the particular triode.
In the example characteristic shown on the image, if an anode voltage Va of 200 V and a grid voltage bias of -1 volt are selected, a plate (anode) current of 2.25 mA will be present (using the yellow curve on the graph). Changing the grid voltage will change the plate current; by suitable choice of a plate load resistor, amplification is obtained.
In the class A triode amplifier, an anode resistor would be connected between the anode and the positive voltage source. For example, with Ra=10000 Ohms, voltage drop on it will be
VRa=Ia×Ra=22.5 V,
if anode current of Ia=2.25 mA is chosen.
Now, if the input voltage amplitude (at the grid) changes from -1.5 V to -0.5 V (difference of 1 V), anode current will change from 1.2 to 3.3 mA (see image). This will result in anode resistor voltage drop changes from 12 to 33 V (difference of 21 V).
Since grid voltage changes from -1.5 V to -0.5 V, and anode resistor voltage drop from 12 to 33 V, amplification of signal resulted. Amplification factor is 21 - output voltage amplitude divided by input voltage amplitude.
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