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Barkhausen must be wrong.

S

Simon S Aysdie

Jan 1, 1970
0
POWER gain IS the requirement.

That's what I always thought. One doesn't, per se, care about "gain"
from the negative resistance viewpoint either.
 
T

The Phantom

Jan 1, 1970
0
Presumably one could cascade such circuits.

One could certainly cascade them (I'm assuming you mean without an
intermediate buffer), but would you expect the cascade to have voltage
gain? Care to guess what the result will be with the network we've been
discussing? I'll run the analysis and report the result.
 
T

The Phantom

Jan 1, 1970
0
That's what I always thought. One doesn't, per se, care about "gain"
from the negative resistance viewpoint either.

I think "power gain" is necessary, but not sufficient, to give oscillation.
If the RC network in the example under discussion is replaced with one
which doesn't have "voltage gain", then in spite of the substantial power
gain in the cascaded emitter followers, there will be no oscillation.
 
J

John Larkin

Jan 1, 1970
0
One could certainly cascade them (I'm assuming you mean without an
intermediate buffer), but would you expect the cascade to have voltage
gain? Care to guess what the result will be with the network we've been
discussing? I'll run the analysis and report the result.

Certainly the voltage gains would multiply, if the component values
were right. But the impedances would get crazy fast.

John
 
T

The Phantom

Jan 1, 1970
0
Certainly the voltage gains would multiply, if the component values
were right. But the impedances would get crazy fast.

When you said "such circuits", I assumed you meant the very circuit we've
been discussing, the one in the oscillator on the referenced web page. I
didn't realize you intended "such circuits" to mean something like "similar
circuits, but with varying impedance levels". With that meaning, I suppose
that cascading might give even more gain. But, Epstein showed in his paper
that the maximum gain that a passive network can have is 2. So, if one
cascaded networks with impedance levels going up by an order of magnitude
or so ad infinitum, one would think that the voltage gain would be
unlimited. What would cause the voltage gain to remain below 2?
 
J

John Larkin

Jan 1, 1970
0
When you said "such circuits", I assumed you meant the very circuit we've
been discussing, the one in the oscillator on the referenced web page. I
didn't realize you intended "such circuits" to mean something like "similar
circuits, but with varying impedance levels". With that meaning, I suppose
that cascading might give even more gain. But, Epstein showed in his paper
that the maximum gain that a passive network can have is 2. So, if one
cascaded networks with impedance levels going up by an order of magnitude
or so ad infinitum, one would think that the voltage gain would be
unlimited. What would cause the voltage gain to remain below 2?

Not having seen the paper, I can't say. Possibly a gain of 2 requires
an infinite output impedance, or something like that, which prevents
unlimited cascading.

Can anybody post the paper?


John
 
J

john jardine

Jan 1, 1970
0
The Phantom said:
When you said "such circuits", I assumed you meant the very circuit we've
been discussing, the one in the oscillator on the referenced web page. I
didn't realize you intended "such circuits" to mean something like "similar
circuits, but with varying impedance levels". With that meaning, I suppose
that cascading might give even more gain. But, Epstein showed in his paper
that the maximum gain that a passive network can have is 2. So, if one
cascaded networks with impedance levels going up by an order of magnitude
or so ad infinitum, one would think that the voltage gain would be
unlimited. What would cause the voltage gain to remain below 2?

Greater than 2 seems certainly doable. Did Epstein add some of those
weasely "yes but's" or "assuming ... "?.
 
J

John Larkin

Jan 1, 1970
0
Greater than 2 seems certainly doable. Did Epstein add some of those
weasely "yes but's" or "assuming ... "?.

You'd think that if you can get a stage gain of, say, 1.1 with maybe a
100:1 loading ratio, then 10 stages would get you above 2. Spice could
do that, but the real world probably can't.

100^10 is a bunch of ohms.

John
 
R

Robert Baer

Jan 1, 1970
0
John said:
The signal input to the right transistor is not from an emitter follower.
Could have fooled me; the two look exactly like a darlington emitter
follower.
 
J

john jardine

Jan 1, 1970
0
John Larkin said:
You'd think that if you can get a stage gain of, say, 1.1 with maybe a
100:1 loading ratio, then 10 stages would get you above 2. Spice could
do that, but the real world probably can't.

100^10 is a bunch of ohms.

John
I'm up at 1e11!.
For fun I'll put a sim pic on A.B.S.E.
 
F

Fred Bloggs

Jan 1, 1970
0
The said:
At this site:

http://www.4qdtec.com/singen.html

there is a schematic titled "A Practical Twin-T Oscillator".

In the text under the schematic we find:

"Now hold on a minute: an emitter follower has no voltage gain and surely
you've been taught that an R-C oscillator must have voltage gain? Well this
one works and has no voltage gain (of course it does have current gain)."

The schematic does show two emitter followers closing the loop. One would
think this couldn't work, but the poster says it does.

Is Barkhausen wrong?

Get real. I don't recall that the basic criterion for feedback
oscillation is called Barkhausen's Conjecture, it is called the
Barkhausen rule or something dumb like that.
 
J

John Fields

Jan 1, 1970
0
At this site:

http://www.4qdtec.com/singen.html

there is a schematic titled "A Practical Twin-T Oscillator".

In the text under the schematic we find:

"Now hold on a minute: an emitter follower has no voltage gain and surely
you've been taught that an R-C oscillator must have voltage gain? Well this
one works and has no voltage gain (of course it does have current gain)."

The schematic does show two emitter followers closing the loop. One would
think this couldn't work, but the poster says it does.

Is Barkhausen wrong?
 
P

Phil Hobbs

Jan 1, 1970
0
This is very well known. It's not difficult to make high order PLLs (for
example) that behave pretty well. Third order PLLs are especially
useful for situations involving constant frequency drift rates, e.g.
accelerating spacecraft, because third order loops have zero phase error
due to a linear rate of change of frequency.

You do have to make it act like a second-order loop during startup and
big transients, or it's liable to exhibit nonlinear oscillations.


Cheers,

Phil Hobbs
 
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