Audio Amplification (part iii)
The Triode Behavior
After the path breaking invention of Triode by Lee De Forrest, today over 100 years later it is the same triode found in some of the world’s best high fidelity amplifiers and despite dramatic technological innovations and inventions over the century, this piece of vintage technology has no competition whatsoever.
Needless to say, this device indeed needs a deeper study and deserves attention. Today triodes are made in various types and characteristics and some of the most popular, proven and high quality triode tubes which are made today are dual which means 2 triode sections within one tube.
Following is a list of Different Triode Tubes available today and used quite often.
Understanding the Capabilities & Limitations of Triodes
In one of the paragraphs above, we came across the statement that the triodes are suitable for preamplifier for audio frequencies and not in case of radio frequencies.
We will understand why this so and what limits Triode’s capabilities.
The electrodes of a triode valve are in very close proximity to each other this leads to an inherent behaviour of the various electrode pairs to act like capacitors (we have learned that Capacitors are nothing but 2 parallel plates facing each other & separated).
In triodes the inter-electrode capacitances are quite small and are normally less than some 2.5 pF. These small capacitances have negligible effect on circuit operation in some applications, but in others they are of importance and have to be taken into account.
Since the signal is applied at the Grid, the two capacitances, 1. between Grid & Cathode (Cgk) and 2. Between Grid and Anode come into effect (Cga). However there can be more complicated combination of capacitances due to the presence of heater as well, but we will ignore them due to their practical insignificance.
On June 11 1919, John Milton Miller, got his paper published (Click here to read), highlighting the effect of these inherently present capacitances in triode tubes and subsequently known as Miller Effect or Miller Capacitance.
Take a look at the typical triode connected and configured as amplifier, with grid being fed the voltage signal to be amplified.
We know that the signal which is to be applied for amplification is not DC of course, hence the Grid- Cathode and Grid to Anode capacitors will cause the current to flow from the signal source as these capacitors provide a path, otherwise the grid is not connected in any manner to any other electrode. A remarkable thing which happens because of the amplification happening in the tube. At the anode since the voltage is amplified by the amplification factor, say A (assume 100), and also since a fall in grid voltage attracts more electrons and more flow at anode, and the inrese as well, it is clear that the signal amplified at anode gets inverted. Hence there is a difference of 100+1 (100 – (-1)) i.e., 101, ( for ex., 1v at grid gets amplified to -100V at plate, hence a difference of 101).
This leads to the source experiencing an enhanced capacitance by a factor of (Amplification +1). This effect is known as MILLER EFFECT.
If we take a small detour here and brush up the understanding of how the capacitors and inductors behave with frequency, it will make things pretty clear for the learning to come yet.
If the 12AX7 triode discussed above is employed in an audio amplifier circuit with conventional resistance-capacitance coupling to the grid, this input reactance could result in a loss in overall circuit gain at the higher audio frequencies. Such a loss may be acceptable in, say, a guitar amplifier but it would have to be taken into account in the design of an amplifier intended for high fidelity reproduction and certainly not acceptable at much higher frequencies like radio transmission applications.