Editing Triode (section)

== Characteristics ==
[[Image:TriodeECC83Characteristic1.png|right|upright=1.66|ECC83 triode operating characteristic|thumb]]
In triode datasheets, characteristics linking the anode current (I<sub>a</sub>) to anode voltage (V<sub>a</sub>) and grid voltage (V<sub>g</sub>) are usually given. From here, a circuit designer can choose the [[biasing|operating point]] of the particular triode. Then the output voltage and amplification of the triode can be evaluated graphically by drawing a [[load line (electronics)|load line]] on the graph.

In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltage V<sub>a</sub> of 200&nbsp;V and a grid [[voltage bias]] of &minus;1&nbsp;V. This implies a quiescent plate (anode) current of 2.2&nbsp;mA (using the yellow curve on the graph). In a [[class-A amplifier|class-A]] triode amplifier, one might place an anode resistor (connected between the anode and the positive power supply). If we choose  R<sub>a</sub>&nbsp;=&nbsp;10000&nbsp;Ω, the voltage drop on it would be V<sub>+</sub> &minus; V<sub>a</sub> = I<sub>a</sub> × R<sub>a</sub> = 22&nbsp;V for the chosen anode current of I<sub>a</sub> = 2.2&nbsp;mA. Thus we require a power supply voltage V<sub>+</sub> = 222&nbsp;V in order to obtain V<sub>a</sub>&nbsp;=&nbsp;200&nbsp;V on the anode.

Now suppose we impress on the &minus;1&nbsp;V bias voltage a signal of 1&nbsp;V peak-peak, so that the grid voltage varies between &minus;0.5&nbsp;V and &minus;1.5&nbsp;V. When V<sub>g</sub>&nbsp;=&nbsp;&minus;0.5&nbsp;V, the anode current will increase to 3.1&nbsp;mA, lowering the anode voltage to V<sub>a</sub> =  V<sub>+</sub>&nbsp;&minus;&nbsp;10&nbsp;kΩ&nbsp;×&nbsp;3.1&nbsp;mA = 191&nbsp;V (orange curve). When V<sub>g</sub>&nbsp;=&nbsp;&minus;1.5&nbsp;V, the anode current will decrease to 1.4&nbsp;mA, raising the anode voltage to V<sub>a</sub> = V<sub>+</sub>&nbsp;&minus;&nbsp;10&nbsp;kΩ&nbsp;×&nbsp;1.4&nbsp;mA = 208&nbsp;V (green curve). Therefore a 1&nbsp;V peak-peak signal on the input (grid) causes an output voltage change of about 17&nbsp;V. 

Thus voltage amplification of the signal is obtained. The ratio of these two changes, the [[Gain (electronics)#Voltage gain|voltage amplification factor]] (or ''mu'') is 17 in this case. It is also possible to use triodes as [[Valve amplifier#Output_impedance|cathode follower]]s in which there is no voltage amplification but a huge reduction in dynamic [[Electrical impedance|impedance]]; in other words, the ''current'' is [[Gain (electronics)#Current gain|greatly amplified]] (as it also is in the [[Amplifier#Common terminal|common-cathode configuration]] described above). Amplifying either the voltage or current results in power amplification, the general purpose of an amplifying tube (after all, either the current or voltage alone could be increased by decreasing the other just using a transformer, a passive device).