As the top pentode is clearly acting as a Cathode Follower, it will experience a decrease in current conduction because the positive pulse will force its cathode to be even more positive than its grid, all of which in turn will serve to decrease the voltage at the amplifier's output.
  So far the output stage has an output impedance equal to 1/2 Gm, where Gm is the transconductance of the output tubes. The output impedance can be further decreased by wrapping a feedback loop around the amplifier. Here is where the second triode's grid comes into play. This grid is at ground level and it represents a high input impedance. These features serve to make adding a feedback loop easy: no additional capacitors, no critically valued, low resistance feedback resistor. Instead, all that is needed is a two resistor voltage divider. While we must be mindful of the Miller effect capacitance from the second triode's grid to its plate, we enjoy greater latitude in the choice of resistor values than previously. Notice that when the positive going pulse is forced on to the amplifier's output, the feedback will send a correcting signal to both output tubes. Let's follow the path.

  The pulse cause the second triode to increase its current conduction, which develops a greater voltage across its plate resistor, which then forces the top output tube's current conduction to decrease, thereby pushing the output negative. Now the pulse which caused the second triode to increase its conduction also causes an increase in the common cathode resistor, which will cause the first triode to see its cathode moving positive while its grid remain fixed at 0 volts. The first triode conducts less and thus its plate resistor sees less current flowing through it, which results in a positive pulse being fed into the bottom output tube's grid. The bottom output tube then conducts more current, which will work to pull the output voltage down. 
  Of course, the gain of the triodes will greatly magnify the errant pulse at the output and this gain is what feeds the feedback. If the open-loop gain of the amplifier is 10 and we feed all of this signal back into the amplifier, the output impedance will decrease by ten fold, as should the distortion and noise. Furthermore the feedback will extend the bandwidth of the amplifier. Magic?


pg. 10

NEXT >   Copyright © 2000 GlassWare   All Rights Reserved