RF Cathode Follower?

[pwdrive]

Trying to get my head round this, it's regarding cathode follower output impedance (Zo), several texts quote the formula as....Zo = 1/1/RK + 1/ra + gm....where RK is the resistance of the cathode resistor (eg: the resistor that goes from cathode to deck), ra is the internal anode resistance and gm is the mutual conductance.

Suppose an operating point is chosen that requires RK to be 200 ohms to bias the grid to a chosen operating point which gives an ra of 2,800 ohms and a mutual conductance of 5ma/volt, entering the numbers results in Zo being 96.5 ohms.

Other texts say that a good approximation of Zo can be got by the following formula if RK and ra are high....Zo = 1/gm....in the above case RK and ra are not particularly high but I've put in the numbers anyways to show the difference....Zo = 200 ohms.

In the above, RK provides the grid bias, but what if I want to drive an RF transformer from the cathode circuit that has say a primary impedance of 200 ohms (secondary impedance 50 ohms) The RF transformer being wound as a transmission line transformer with a very low primary resistance which would probably be less than 1 ohm..

Should the primary be connected between the cathode and deck via a bias resistor?

Should the primary be connected directly between the cathode and deck with a separate bias supply for the grid?

Should the primary be connected across a cathode bias resistor but say via a coupling capacitor?

Anyone have any ideas?

Lawrence

[Murphyv310]

Hi.
Interesting question. I use cathode followers in many circuits and in them find them very useful.
One thing I need to ask though, do you need to use a matching transformer in the particular application? Is the resultant low impedance from the cathode suitable.
Ideally the follower would be directly coupled from its grid to the preceding stages anode. If a transformer is required then I would suggest a resistor from cathode to deck and the transformer capacitive coupled to the valves cathode, to allow the valve to follow the anode volts without excess A-K current.

[pwdrive]

Trev, I'm trying to figure out, if possible, the best way to graft the RF input port of a fully balanced mixer to a cathode follower, the mixer will be either the Minicircuits SBL-1 or the ADE-1, the mixer's RF in port is designed for 50 ohms input impedance, it's for a receiver project I'm trying to design (1.8 to 30 Mhz) The mixers VFO will be solid state but I want to use valves for the rest of the design.

From what I can gather the only way to step up that impedance for the frequencies concerned is with a wideband transmission line transformer, but apparently the maximum impedance step up ratio I'm likely to achieve if I make one of them is 1:4 (200 ohms) or possibly 1:9 (450 ohms) at a push, the grid input to the cathode follower will be parallel tuned circuit, loosely coupled to the antenna.

Any thoughts.

Lawrence.

[Murphyv310]

Hi Lawrence.
I wonder if you could use a triode/pentode like an ECF80, use the pentode as a high impedance amp, then DC couple the Pentode Anode to the Triode grid. The only downside could be DC current but you could limit the current in the triode with a suitable anode load resistor with the anode de-coupled. In that way you may be able to have your matching TX directly connected to the cathode.
I find that many circuits will work and work well against all the theory, possibly a suck it and see scenario!

[pwdrive]

Murphyv310Hi Lawrence.
I wonder if you could use a triode/pentode like an ECF80, use the pentode as a high impedance amp, then DC couple the Pentode Anode to the Triode grid. The only downside could be DC current but you could limit the current in the triode with a suitable anode load resistor with the anode de-coupled. In that way you may be able to have your matching TX directly connected to the cathode.
I find that many circuits will work and work well against all the theory, possibly a suck it and see scenario!

To make things worse Trev, I want to run the valves with 12 volts HT (12.6 volts LA battery) the nearest triode at that voltage for a cathode follower I can find is an ECC86:

http://www.r-type.org/pdfs/ecc86.pdf

Going back to that formula I posted earlier, Zo = 1/1/RK + 1/ra + gm I've plugged in some numbers for the ECC86 for an operating point with 0.25 volts -ve bias on the grid, I work it out to Zo = 55 ohms with a cathode resistor (RK) of about 80 ohms.

If that's correct then it would be a close enough match for the RF input impedance of the mixer (50) ohms

Can some kind soul check that for me?

Also in the above scenario if the RF port of the mixer was coupled to the cathode via a capacitor would that upset things within the mixer to any great extent as the transformers in the mixer are wound transmission line style.

Lawrence.

[Terry]

Can some kind soul check that for me?

Lawrence, it shouldn't be difficult to do a quick lash-up.

Measure the signal level at the cathode, then terminate in 50Ω.

If the source impedance is also 50Ω, the signal level should now be exactly half the value you measured earlier.

[Murphyv310]

Hi.
I'd be very surprised if you even get 50% from the cathode into a 50ohm load at 12v HT even with a ECC86

[pwdrive]

Yes, might have to re think the strategy, maybe a homebrew Polyakov mixer or a homebrew single balanced mixer could be incorporated into a valve receiver, the first DC receiver I built many moons ago was based on Spaargaren's design (Fig. 8):

http://www.epanorama.net/sff/Radio/Commu...rt%201.pdf

That one worked quite well as I remember, it might be adaptable to a valve receiver.

Another idea would be to use a valved single balanced valve mixer based on the CW detector used in the Marconi Atlanta shown in the schematic and in more detail in appendix 2:

http://bee.mif.pg.gda.pl/ciasteczkowypot...ual_v6.pdf

All yap at the moment though 'till I can get some bits together.

Lawrence.