Tube DIY Asylum

RE: Output Impedance of CF w/ CCS Sink

69.19.14.42

In Reply to: RE: Output Impedance of CF w/ CCS Sink posted by . on June 30, 2008 at 09:11:25

An even closer approximation is Zo ~= (Ra + Za)/ mu
where
Ra = tube anode resistance (Rp)
Za = anode circuit impedance (power supply impedance, anode resistor, ...)
which closely resembles 1/gm for small Za.
A better approximation still would take into account the cathode resistor or CCS being in parallel with the load.
Note that this does not result in a Thevenin equivalent circuit with an ideal voltage source in series with a resistor(R=Zo). This is only the case when sourcing current into the load. The CF circuit can only sink current through the CCS or cathode resistor. The Thevinin equivalent for sinking current is the cathode resistor in the case of a resistor loaded CF and an intersting function of load impedance and attempted voltage swing for the CCS loaded CF.
Cheers,
Michael

Edits: 07/01/08

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