How do I calculate or estimate the output impedance of a preamp with a
capacitor-coupled, cathode-follower output?

Grant

Grant Sellek wrote:

How do I calculate or estimate the output impedance of a preamp with a
capacitor-coupled, cathode-follower output?

Grant

Providing the coupling cap from the CF to the following amp is a large
vaue,
say 2 uF, and the following load is at least 10k or higher, the impedance
og the coupling cap
can be neglected.

The CF output Zo is approximately simply 1/gm, in parallel with the
cathode to 0V load,
and the load imposed by the following amp.
If Gm was 2 mA/V, tube Zo = 1 / 0.002 = 500 ohms,
and the other RLs attatched make little difference.

Since a CF is an example of series voltage NFB and B = 1,
then for a tube with Ra = 10k, and u = 20,
Ro = 10k / ( 1 + [20 x 1] ) = 10,000 / 21 = 476 ohms.

Standard feedback equations apply.

Patrick Turner.

Thanks Patrick for that lucid answer.

So the cathode resistor is irrelevant? e.g. in your example below,
there may be a 270R cathode resistor below the tap for the signal out.

Also, what if there were 2 120R cathode resistors in series, and the
output tap was between the 2 resistors?

Grant

Patrick Turner wrote in message ...
Grant Sellek wrote:

How do I calculate or estimate the output impedance of a preamp with a
capacitor-coupled, cathode-follower output?

Grant

Providing the coupling cap from the CF to the following amp is a large
vaue,
say 2 uF, and the following load is at least 10k or higher, the impedance
og the coupling cap
can be neglected.

The CF output Zo is approximately simply 1/gm, in parallel with the
cathode to 0V load,
and the load imposed by the following amp.
If Gm was 2 mA/V, tube Zo = 1 / 0.002 = 500 ohms,
and the other RLs attatched make little difference.

Since a CF is an example of series voltage NFB and B = 1,
then for a tube with Ra = 10k, and u = 20,
Ro = 10k / ( 1 + [20 x 1] ) = 10,000 / 21 = 476 ohms.

Standard feedback equations apply.

Patrick Turner.

Grant Sellek wrote:

Thanks Patrick for that lucid answer.

So the cathode resistor is irrelevant? e.g. in your example below,
there may be a 270R cathode resistor below the tap for the signal out.

Where did you get the 270 ohms from?



Also, what if there were 2 120R cathode resistors in series, and the
output tap was between the 2 resistors?

How are these connected?



Grant

Some cathode followers have the output from a bias R in series with RL,
but this means the Ro will be the Ro of the CF from the cathode + Rk .

The lowest Ro occurs where you have the output directly off the cathode.

Patrick Turner.



Patrick Turner wrote in message ...
Grant Sellek wrote:

How do I calculate or estimate the output impedance of a preamp with a
capacitor-coupled, cathode-follower output?

Grant

Providing the coupling cap from the CF to the following amp is a large
vaue,
say 2 uF, and the following load is at least 10k or higher, the impedance
og the coupling cap
can be neglected.

The CF output Zo is approximately simply 1/gm, in parallel with the
cathode to 0V load,
and the load imposed by the following amp.
If Gm was 2 mA/V, tube Zo = 1 / 0.002 = 500 ohms,
and the other RLs attatched make little difference.

Since a CF is an example of series voltage NFB and B = 1,
then for a tube with Ra = 10k, and u = 20,
Ro = 10k / ( 1 + [20 x 1] ) = 10,000 / 21 = 476 ohms.

Standard feedback equations apply.

Patrick Turner.

Patrick Turner wrote in message ...
Grant Sellek wrote:
So the cathode resistor is irrelevant? e.g. in your example below,
there may be a 270R cathode resistor below the tap for the signal out.

Where did you get the 270 ohms from?

OK I was not clear. Your initial reply showed Zo to be purely
dependent on the valve's properties. Although I am ignorant and hence
asked the original question, I had thought Zo would not be unaffected
by the chosen value of any cathode bias resistor on the CF valve
stage. Why? Because I thought the output tap is taken from the valve
cathode, so Zo is measured between the cathode and earth, which is in
parallel with the bias resistor.

The 270 ohms was just for example. I plucked it from a CF circuit
diagram.

Also, what if there were 2 120R cathode resistors in series, and the
output tap was between the 2 resistors?

How are these connected?

Earth to 120R to 120R to cathode. Meant to be a voltage divider for
the AC output, which will still be larger at the cathode than I want.

Some cathode followers have the output from a bias R in series with RL,
but this means the Ro will be the Ro of the CF from the cathode + Rk .

The lowest Ro occurs where you have the output directly off the cathode.

I want low Zo, and I want to divide the AC output voltage to half (or
less) of the value at the cathode. Grant


Patrick Turner wrote in message ...
Grant Sellek wrote:

How do I calculate or estimate the output impedance of a preamp with a
capacitor-coupled, cathode-follower output?

Grant

Providing the coupling cap from the CF to the following amp is a large
value,
say 2 uF, and the following load is at least 10k or higher, the impedance
of the coupling cap can be neglected.

The CF output Zo is approximately simply 1/gm, in parallel with the
cathode to 0V load, and the load imposed by the following amp.
If Gm was 2 mA/V, tube Zo = 1 / 0.002 = 500 ohms,
and the other RLs attatched make little difference.

Since a CF is an example of series voltage NFB and B = 1,
then for a tube with Ra = 10k, and u = 20,
Ro = 10k / ( 1 + [20 x 1] ) = 10,000 / 21 = 476 ohms.

Standard feedback equations apply.

Patrick Turner.