Cathode follower question

[rongon]

Hi again...

Here are some squiggles I drew:



I was thinking that if a CCS was a good thing as the load for a cathode follower, then a choke should be too.

I know the choke pictured above doesn't have much inductance (22H). Is that still a problem considering the very low output Z of a CF?

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I know a choke isn't as good a load as a CCS, and will have bandwidth limitations a CCS might not, but my question is meant more for my learning/understanding than for building the best possible circuit.

[Miles Prower]

Originally Posted by rongon

I was thinking that if a CCS was a good thing as the load for a cathode follower, then a choke should be too.

I know the choke pictured above doesn't have much inductance (22H). Is that still a problem considering the very low output Z of a CF?

For any cathode follower, the larger the tail load, the better. 22H corresponds to: Xl= 2K77 at f= 20Hz. You'd ideally be best off with an r(k)= 276R, which would mean a g(m)= 3.62mA/V, or better. This isn't so difficult to pull off, and so the choke load will work just fine. (Assuming it's a decent choke designed for the purpose and not something like a PS ripple choke.)

I know a choke isn't as good a load as a CCS, and will have bandwidth limitations a CCS might not, but my question is meant more for my learning/understanding than for building the best possible circuit.

Asking is the first step to learning.

[Allen Wright]

Your theory is correct, a choke can replace a CCS - but only under certain conditions. But neither circuit is much good as drawn, as both will very easily overload with any sort of signal level. The cathode of a CF should sit well above gnd, ideally around half the B+ rail to allow maximum voltage swing before any clipping, up or down. This means the grid of the CF needs to be elevated to the same voltage, and the cathode impedance adjusted to suit. Note that a choke will not work with near half the B+ across it...something will smoke.

Regards, Allen

[SY]

Thanks for sitting in for me, Allen!

[Wavebourn]

Originally Posted by Allen Wright

Your theory is correct, a choke can replace a CCS - but only under certain conditions. But neither circuit is much good as drawn, as both will very easily overload with any sort of signal level. The cathode of a CF should sit well above gnd, ideally around half the B+ rail to allow maximum voltage swing before any clipping, up or down. This means the grid of the CF needs to be elevated to the same voltage, and the cathode impedance adjusted to suit. Note that a choke will not work with near half the B+ across it...something will smoke.

A choke does not need to. It may sit at zero (ideally) DC, and swing way below zero.

In case of CCS load it should sit at (B+ - Vs)/2
where Vs is a saturation voltage on the max current.
In case of resistor load it should sit at (B+ - Vs)*3/4

[Jeb-D.]

A choke does not need to. It may sit at zero (ideally) DC, and swing way below zero.

I'm glad somebody is thinking outside of the sand box.

[Allen Wright]

Originally Posted by Wavebourn

A choke does not need to. It may sit at zero (ideally) DC, and swing way below zero.

In case of CCS load it should sit at (B+ - Vs)/2
where Vs is a saturation voltage on the max current.
In case of resistor load it should sit at (B+ - Vs)*3/4

1/ This may be all correct, but it's really splitting hairs. The OP had some basic questions, he doesn't need or want a learned maths lession.

2/ The schema the OP drew shows a choke in a zero biased triode CF. Assuming the choke has enough DCR to act as a suitable cathode bias R, it will still have VERY limited output swing, and make massive 2nd harmonic distortion before clipping.

Regards, Allen

[Wavebourn]

Originally Posted by Allen Wright

1/ This may be all correct, but it's really splitting hairs. The OP had some basic questions, he doesn't need or want a learned maths lession.

2/ The schema the OP drew shows a choke in a zero biased triode CF. Assuming the choke has enough DCR to act as a suitable cathode bias R, it will still have VERY limited output swing, and make massive 2nd harmonic distortion before clipping.

Unfortunately without some math your "very" and "massive" are undefined.
Triode CF is not ZERO biased. It is biased up to the current needed for a maximum of a negative voltage swing on a given output load. Second harmonic distortions will be way below what you may get from an identical common cathode stage with choke loaded anode.

Edit: you may search for Altec 1570 schemo.

[Wavebourn]

Originally Posted by Jeb-D.

I'm glad somebody is thinking outside of the sand box.

Thank you. I'm not so old to forget physics' basics I learned 40 years ago.

[rongon]

A little controversy!

2/ The schema the OP drew shows a choke in a zero biased triode CF. Assuming the choke has enough DCR to act as a suitable cathode bias R, it will still have VERY limited output swing, and make massive 2nd harmonic distortion before clipping.

With a DCR of about 1k ohms, that simple resistance with 8mA through it should raise the cathode to +8V, right? That should give us the necessary grid bias, right? Not enough of a load, but then you add...

...The inductance of the choke, which will resist changes in current, creating an AC load that causes a signal voltage swing across the resistance... I think.

Being a buffer with a grid bias of +8V, wouldn't a stage like this CF be used to match a previous high-Z gain stage (amplifying a 500mV signal to let's say 2Vrms) to a subsequent stage requiring a lower Z? So an 8V grid bias would be plenty in that application, right?

In case of CCS load it should sit at (B+ - Vs)/2
where Vs is a saturation voltage on the max current.
In case of resistor load it should sit at (B+ - Vs)*3/4

That little bit of math I can do. Only thing is, what defines Vs? In the CF with choke in the cathode, the bias is +8V, is Vs = 8V?

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