The triode is somewhat similar in operation to the n-channel JFET; it is normally on, and exhibits progressively lower and lower plate/drain current as the grid/gate is pulled increasingly negative relative to the source/cathode. Cutoff voltage corresponds to the JFET's pinch-off voltage (Vp) or VGS(off); i.e., the voltage point at which output current essentially reaches zero. This similarity is limited, however. The triode's anode current is highly dependent on anode voltage as well as grid voltage, thus limiting the voltage gain. Because, in contrast, the JFET's drain current is virtually unaffected by drain voltage, it appears as a constant-current device, similar in action to a tetrode or pentode tube (high dynamic output impedance). Both the JFET and tetrode/pentode valves are thereby capable of much higher voltage gains than the triode which seldom exceeds 100. However the power gain, or the output power obtained from a certain AC input voltage is often of greater interest. When these devices are used as cathode followers (or source followers), they all have a voltage "gain" of just under 1, but with a large current gain.
The triode is somewhat similar in operation to the n-channel JFET; it is normally on, and exhibits progressively lower and lower plate/drain current as the grid/gate is pulled increasingly negative relative to the source/cathode. Cutoff voltage corresponds to the JFET's pinch-off voltage (Vp) or VGS(off); i.e., the voltage point at which output current essentially reaches zero. This similarity is limited, however. The triode's anode current is highly dependent on anode voltage as well as grid voltage, thus limiting the voltage gain. Because, in contrast, the JFET's drain current is virtually unaffected by drain voltage, it appears as a constant-current device, similar in action to a tetrode or pentode tube (high dynamic output impedance). Both the JFET and tetrode/pentode valves are thereby capable of much higher voltage gains than the triode which seldom exceeds 100. However the power gain, or the output power obtained from a certain AC input voltage is often of greater interest. When these devices are used as cathode followers (or source followers), they all have a voltage "gain" of just under 1, but with a large current gain.