- #1
bitrex
- 193
- 0
Looking at the equations for the gain of a common cathode tube amplifier with the cathode resistor unbypassed (though the question in general could apply to similar configurations with BJTs and FETs) I see that the gain of the amplifier is something like [tex]\frac{\mu R_{l}}{R_{p} + R_{k}(\mu +1) + R_{l}}[/tex], where Rl is the load resistance, Rp is the dynamic plate resistance, mu is the amplification factor, and Rk is the unbypassed cathode resistance. This is degenerative feedback where the feedback is proportional to the current through the device - the gain of the amplifier is reduced and the dynamic plate resistance is increased. However, in a cathode follower, when the emitter resistor is unbypassed the dynamic plate resistance is decreased by the factor [tex]\frac{R_{p}}{\mu +1}}[/tex]. This must be voltage feedback since the input impedance is increased and the output impedance is reduced. What I can't see is how the feedback changed from current feedback to voltage feedback by just taking the output from the cathode instead of the anode.