Dear Group:

A hot button has been pushed. The Miller effect (not to be overly
confused with the Miller theorem) stems from an exhaustive analysis of the
amplification vs. frequency of very low mu triode amplifiers (the tubes
available at the time). The work was published just after WW-1! It was
noted that the effect on frequency response of the capacitance between grid
and plate was magnified by the amplification of the stage. Miller's paper
showed that the effect of the aforementioned capacitance could be
approximated by a capacitor in series with a resistance with the combination
placed in shunt with the grid-cathode. Several conditions need to be true
that were overlooked by later commentators!

Many years latter, Miller's work became corrupted into Miller's theorem
that erroneously contended that the effect of the feedback capacitor was the
same as a gain dependent capacitor in shunt with the input and output of the
amplifier. Various "proofs" have been published that "prove" Miller's
theorem. They remind one of HS proofs involving dividing by zero.

It is true that the frequency effect of capacitance between input and
output (note, those are nouns) of an amplifier is detrimentally increased as
the gain of the amplifier increases. [as gain goes up - the upper
half-power frequency goes down] [note the notion of approximately constant
gain-bandwidth product for well behaved amplifiers]
However, the simple modeling indicated by "Miller's theorem" only
occasionally estimates the relationship between feedback capacitance and
upper half-power frequency. Fortunately, the allege convenience of using
Miller's theorem in the slide rule days has now gone away. Today, one may
use PSpice (or the like) and receive close estimates of the expected
performance of an amplifier.

It is difficult for me to see gain at the end of an antenna's wire. The
apparent capacitance that exists at the end of an antenna's wire simply
exists and is not magnified by anything.

Lecture mode off. 73, Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Richard Harrison" wrote in message
...

snip
Energy from the magnetic field is converted to energy in the electric
field for an instant (Cecil`s famous conservation of energy). This
produces an insreased voltage at the open circuit end. This incresed
voltage has more capacitive effect, akin to the "Miller effect" caused
by the higher signal voltage on the plate of an amplifier vacuum tube
than on its grid.

snip


Best regards, Richard Harrison, KB5WZI