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Patrick Turner Patrick Turner is offline
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Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 18, 3:05*am, flipper wrote:
On Thu, 17 Mar 2011 05:19:15 -0700 (PDT), Patrick Turner

wrote:

snip



In RDH4, PFB is mentioned and a schematic is given where a PFB loop
between two cascaded small signal voltage triodes is boosted 12dB and
the same amount of global NFB is used. The effect of PB when the
signal is below say 10Vrms is to increase open loop sensitivty without
much increase in THD or reduction of bandwidth


PFB is PFB and has the attendant increase in distortion and reduced
bandwidth regardless of the 'voltage level'.


But RDH4 didn't make your simple generalisation or come to the same
conclusion about PFB.

It is possible to reduce distortion with bootstrapping which is mild
PFB.

Or PFB may be added to increase gain say by +12dB in a low voltage
level input stage/s without increasing THD by +12dB.
Then a higher amount of NFB can be applied and the overall THD is much
less than without PFB.

And, last but not least, Positive Current FB reduces Rout.

Sure, PCFB increases THD and reduces bandwith and increases
instability, but you don't need much of it to reduce the Rout of some
amp from say 2 ohms to say 0.2 ohms, or down to 0.0 ohms, or in fact -
0.5 ohms, or a negative figure which simply means VO increases if a
lower load is connected.
PCB is DANGEROUS because if RL becomes very low, then the amp tries
desperately and vainly to keep VO constant and it can oscillate itself
to death easily at some inconvenient frequency, and with some
resultant high expense.

The reason PCFB and PVFB isn't used is because many dumbcluck amp
designers and fuctard production managers will not be able to get a
mass made amp to conform to the behaviour of a well researched and
tested prototype. R&D and design time costs money, and what CEO can
afford to pay anything while he dreams about hs latest Cadilac?

so that global NFB may
still be applied while retaining stability, at least with resistance
loads.
RDH4 goes on to say what a marvel PFB can be. Its as if PFB offers
"free voltage gain" without using expensive hardware to get it, just
resistors are needed.


I don't know where the heck you got that idea because RDH4 doesn't say
a thing about 'free' or PFB being a 'marvel'.


RDH4 was kept brief. It was a 1,600 page book, and brevity needed to
be mantained, so much of it comes across dry as desert, and without
any distracting notions about "free", or "marvels" .

What it does say for the referenced two stage amplifier, and what you
left out, is if stage 1's distortion is *much lower* than stage 2 then
the increase in stage 1 distortion from applying local PFB is more
than offset by GNFB reducing overall distortion because that is
dominated by stage 2, which is in the global loop but not the local.
You are, in effect, 'redistributing' the distortion from stage 2 to 1.


Just in case you are confused about which section of RDH4's many pages
about FB to which I may have been referring to, I suggest you turn to
page 352 and 353 of RDH4, 4th Ed, 1955.

On page 352 there is a 3 stage feedback amp with two 6SN7 and a pair
of 6V6 output tubes in beam tetrode mode.

The amount of internal loop PVFB is +26dB.

The distortion results at the bottom of the page say...

"The intermodulation is 40% with no feedback, 8% with negative
feedback alone and 1.9% with combined positive and negative feedback
under the following conditions--output 8 watts, 4:1 ratio with
frequency 60 and 7000 c/s."

I rest my case with the jury....


Every 6dB of PFB doubles the distortion but as Cyrano Jones noted when
negotiating the price of Tribbles, "twice nothing is still nothing."
Now, the first stage isn't quite 'nothing', of course, but if it's
very low compared to the second stage then it's, to reference another
famous quote, "close enough for government work." Or amplifiers. E.g.
if you double a very small number and then halve the big one the net
result is lower overall.

But it ain't 'free' as stage 1's bandwidth is also reduced,
complicating stability.

You'd be better off with a high gain wide bandwidth section, like say
2 stages (assuming you don't introduce new problems) instead of the
one with PFB, but that costs more and just "how low can you go" before
it isn't worth dancing that hand jive?


I don't like any PFB anywhere myself.

But it is inherent with bootstrapping which is usally a very small
amount of PFB, certainly not 26dB.
Ditto with paraphase input stages as in Quad-II amps.

In amps which John Stewart was referring to which did have
bootstrapped driver RLs, the PVFB is small but it does a lot.

In Quad-II, the paraphase inverter does indeed double drive amp THD.
But overall is mainly due to the OP stage where near clipping its THD
might be 10 times the driver stage, so driver THD may be halved or
doubled without greatly affecting the overall measurements.

By the paraphase connection does the mavellous thing of increasing
amplifier sensivity to a sensible figure and thus prevent the Quad-22
preamp having to produce such a high drive voltage which might be
higher han the power amps.

Everything needs some careful weighing of many factoids before any
simple conclusion may be made.

I would guess the schematic shown in RDH4, page 353 might be a dog of
a thing to get unconditional stability as easily as the text suggests.
There's no report there of square testing at low levels with various
cap loads...Much is damn well missing!

So, what's missing must be found out about and learnt about by real
world engineers like myself, not too much dementia yet, because I
don't assume and I still ask questions.

Patrick Turner.