Self-Biased Cathode Follower Output Point

[Ardee]

The output from a self-biased cathode follower is usually taken from the cathode. My gut feeling is that it should be taken from the junction below the bias resistor, where all three resistors meet. Can anyone please explain the advantage of taking the output from the cathode?

 

[Wavebourn]

Lower output resistance.

 

[Ardee]

OK, but surely the output resistance relies on feedback so having the bias resistor in the feedback loop will not help?

 

[SY]

If you draw the equivalent circuit, the bias resistor is in series with the signal, increasing the source resistance. That's why the textbook CFs use a bypass cap across that resistor when taking the signal from the junction of the grid bias return.

 

[DF96]

If you take output from the cathode then the bias resistor is not in the feedback loop, although it does appear in parallel with the output. Maybe you are confused by the grid resistor, which is not for feedback but merely for bias?

As far as feedback is concerned, the output voltage is applied to the cathode while the grid is grounded by whatever feeds it with signal. You can't see the feedback loop because it contains no extra components.

 

[Ardee]

Thanks SY, that does seem to make sense. My thinking was that if the output is taken from below the bias resistor, the resistor just adds to the rp, whereas if the output comes from the cathode, the bias resistor and load resistor form a potential divider for the feedback.

Thanks DF96, My basic understanding of how it works was lacking.