Re: NFB Quiz



Alex wrote:

To continue that dropped negative feedback thread, please find below a
couple of self-test questions.

It is well known, that plate resistance Ra (of a triode) equals Ra = mu/S,
where:
mu - amplification factor of the tube (triode);
S - transconductance.

According to RDH4, and conventions set up 70 years ago, mu is µ and S is
gm.

So hence Ra = µ / gm.



Question 1: What will Ra be if the grid and the plate are AC coupled (e.g.,
via a 0.1uF capacitor)? (Assume that the grid leak resistor is very large
and can be ignored; cathode self bias resistor is bypassed by an infinitely
large capacitor.)

Ra remains the same no matter what anode load you have and Ra is
dependant
for its value in ohms on the Ia flow, and as Ia changes, so does gm,
while µ is the most
constant parameter which is mainly dependant on the electrode spacings.

Please read RDH4 more fully than you have. I promise your brain
won't liquify if you do.



Now here is a parable.
Once upon a time there lived a triode class A amplifier lover. He was always
being attacked by transistor lovers who ridiculed him that his valve
amplifiers could not provide low output impedance and hence had poor
damping. This triode fan, being a smartass, decided to shut up his critics
forever by using a class A cathode follower output stage. Well, he had to
get about 500V peak-to-peak drive for that, which he brilliantly managed by
using in the first stage that super-high-mu high voltage triode for picture
tube shunt voltage regulation and using 5kV anode supply for such a driver
stage... But this is not the point. The point is:

Question 2: What is the output resistance of the cathode follower given that
the triode has transconductance of S and amplification factor of mu?

The Rout at the cathode of a cathode follower is 1 / gm in parallel with
any connected load.
Usually Rl is very much greater than 1 / gm, and is thus ignored, and
Rout is simply 1 / gm.

Since Ra = µ / gm, then gm = µ / Ra, so Rout of the CF can be expressed
as Ra / µ in ohms.

ANd BTW, triode users will tell people who put up with transistor
shortcomings
that triodes are unique in that their internal electrostatic local NFB
makes the Ra usually a much lower value than the load connected to the
triode,
so a 300B with Ra = 800 ohms is happiest with RL = 3.kk, and a natural
DF of 4 exists.

With any power BJT, the load suitable might be 8 ohms, but the collector
resistance is many
times higher, and DF is exptremely poor unless lots of series voltage
NFB is used
by using an emitter follower connection.

Now with a 300B, µ = about 4, and gm approx = 4 / 800 = 5mA/V, so Rout
in CF = 800 / 4 = 200 ohms.

The cathode load SHOULD STILL BE 3.2k, and preferably direct transformer
coupling lest cut off
distortion occur.

Otherwise the power from the tube cannot be yielded because the V swing
I swing in the tube
must be the same as in the plate loaded case, so the transistor lover
cannot be
taken seriously if he says the 300B in CF is poor because it cannot
drive an 8 ohm load,
even if it had a 200 ohm : 8 ohm OPT.
The SAME OPT of 3.2k:8 ohms must be used for the 300B in CF mode.

However, this use in CF of a 300B to drive an OPT will produce an
astoundingly
good DF = 3,200 / 200 = 16, instead of the 4, when used just as a plate
loaded tube.

Trouble is you need about 250Vrms of grid drive from a low distortion
wide bandwidth
voltage amp, and so nobody builds amps using 300Bs connected as cathode
followers.




After you get answers for Q1 and Q2, compare the answers and make comments.

What comments would you make?


And, those who could not answer these questions in 3 minutes (not looking
into engineering databooks), feel free to unsubscribe from the r.a.t.

Hi-hi.

[Correct answers will be posted later.]

For parables, see the New Testament.

For tube electronics, don't bother with Alex just now, read RHD4
instead.

Patrick Turner.



Regards,
Alex
.

Relevant Pages

  • Re: NFB Quiz
    ... Ra remains the same no matter what anode load you have and Ra is ... for its value in ohms on the Ia flow, and as Ia changes, so does gm, ... Once upon a time there lived a triode class A amplifier lover. ... What is the output resistance of the cathode follower given ...
    (rec.audio.tubes)
  • Re: NFB Quiz
    ... Ra remains the same no matter what anode load you have and Ra is ... for its value in ohms on the Ia flow, and as Ia changes, so does gm, ... Once upon a time there lived a triode class A amplifier lover. ... What is the output resistance of the cathode follower given ...
    (rec.audio.tubes)
  • Re: Where is Grahams mixer design? (was Re: RAT Tube Mixer update?)
    ... Then use a CCS for the cathode dc supply. ... The anode load produces the output required, and the Rout = the load RL ... you need a low source resistance maybe say 500 ohms. ... But classic circuits used a triode CF to drive a cathode of a second ...
    (rec.audio.tubes)
  • NFB in triodes was Re: Pentode gm wired as a triode
    ... If ya can't see the difference between a resistor and a triode ... How come the resistance went low when we applied a higher Ea? ... The rise in anode voltage raises the force of attraction of electrons at ... If the anode voltage rises while grid voltage remains constant, ...
    (rec.audio.tubes)
  • NFB in triodes was Re: Pentode gm wired as a triode
    ... If ya can't see the difference between a resistor and a triode ... How come the resistance went low when we applied a higher Ea? ... The rise in anode voltage raises the force of attraction of electrons at ... If the anode voltage rises while grid voltage remains constant, ...
    (rec.audio.tubes)