In the circuits above, we see the reference points move from the top tube's input being ground referenced to the bottom tube's input being so referenced. We also see the top tube's cathode serving as a reference as well as the bottom tube's plate. Now, how do we design around these reference points?
    The circuit below shows the output stage configured as an amplifier. Each tube sees the same cathode-to-grid signal and each provides gain. Referencing solely from ground, the signal going into the top triode must be much larger than that going to the bottom triode, as the top triode cathode's voltage swing must be subtracted from it. Bootstrapping is the technique used in this circuit to create the huge voltage swing. The capacitor that bridges the top triode's cathode to the driver tube's choke provides the bootstrapping, the positive feedback that allows such gain. 

     The bottom triode's input receives only the normal gain from the driver tube, about 7 in this example. This means the additional gain must come from an input stage with a gain of at least 6 to drive the 2A3 to full output with an input voltage of 1 volt. The obvious temptation is to use a bypass capacitor across the cathode resistors, as this would greatly increase the gain of the driver stage, possibly eliminating the need for an additional gain stage. Unfortunately, the bypass capacitor would also lower the output impedance of the driver, which would undermine the bootstrapping technique.  Without the bypass capacitor, the effective rp is equal to rp plus the cathode resistor value against the sum of the mu plus 1:
       rp' = rp + (mu + 1)Rk
In this example, the effective rp becomes 330k. (Ultimately, the pentode may be a better choice than the triode for the driver stage.)

< PREVIOUS   Copyright © 2001 GlassWare   All Rights Reserved