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In Reply to: RE: Unbypassed cathode resistor and feedback posted by danlaudionut on October 26, 2009 at 16:25:47
A cathode follower is 100 percent NFB on a tube stage. That's why vout ~ vin. Your schematic is missing something: Rk=Ra.
Since the follower portion is directly 100% NFB such that vo(+) = vin and vk = va so that |vo(+)| = |vo(-)|, then the anode is in complete control of the voltage of the cathode. So it follows that the anode is under direct influence of 100% NFB and 100% tracking from that. The anode in other words sounds like the cathode, just inverted, and from solid 100% NFB.
It's "instantaneous" NFB by virtue that there is so little lag up to a very high frequency. There is no perfect "instantaneous" response as electrons have delay. It just gets really short in delay. In this manner, no compensation need be there since the phase margin is about the full 180 degrees to a frequency the gain goes to an unimportant level as it gets sucked up by grid capacitance.
So what is the tube in between doing? It's the modulating voltage that maintains vo(+)=vo(-), or vo(+) + vpk + vo(-)(w/respect to virtual ground at B+) = B+ = constant. The current varies, the voltages vary, but there are those two equal voltages that must stay equal or the circuit isn't working. But this circuit can only do that by 100% NFB at vk (and hence vp w/ respect to B+).
Vk is DC, vk is AC, BTW. So all this is an AC analysis. I hope this helps you get it.
-Kurt
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