|
|
This is so because the amplification performed on the input signal also acts on the capacitance between the grid and the plate, amplifying it; thus making it much more of a burden. This is, in a nutshell, the dreaded Miller effect: the multiplying of the grid-to-plate capacitance by the gain of the triode.
Cathode
So far we have covered only one of the triode's three controlling elements (the grid). The cathode can also be used to control the flow of current through the triode.
In fact, the cathode is slightly more effective at controlling the current flow than the grid. But unlike the grid, the cathode presents a low-impedance input and thus requires more effort to move its voltage up or down. (When the cathode is used as the control element, i.e. as the input, the triode is being used in the grounded-grid topology.)
|
|
|
|
mu, Gm, and rp
The ratio of the plate's effectiveness over the grid's effectiveness in controlling current flow from cathode to plate defines the mu or amplification factor or µ of a triode. And the measure of any controlling element's ability to vary current conduction in response to a change in its voltage goes by the name of transconductance. Each of the triode's three elements displays its own amount of transconductance. The most commonly specified transconductance is that of the grid, which often is labeled Gm or mutual conductance and noted in micro-siemens, the siemens (S) being the unit of conductance, the inverse of resistance. The plate's transconductance equals 1/rp; the grid's, mu/rp; and the cathode's, (mu +1)/rp.
A quick review: the triode can only conduct current from its cathode to its plate (and, in some cases, to its grid); the triode offers resistance to the flow of current that results in a voltage drop across the triode, much like the voltage drop across a resistor; and both the grid and the cathode are much more effective than the plate in controlling the flow of current through the triode. Given this short explanation of the triode's workings, we can move on to how to set a triode's idle current with a cathode resistor.
Cathode Bias
In the absence of a cathode resistor, with both the grid and the cathode seeing the same voltage, the triode's rp offers the only controlling opposition to the flow of current through the triode. This configuration can result in a great deal of current, as Vp (the cathode-to-plate voltage) divided by the triode's rp, roughly equals the amount of current. For example, a triode with an rp of 2k under a Vp of 100 volts will draw 50 mA of current, which against the 100 volts equals 5 watts of heat dissipation by the triode. Doubling the B+ voltage would also double the idle current and thus quadruple the dissipation, as both the current and the voltage have doubled.
|
|
