Crown bca Manual Suplementario
] Balanced Current Amplifier
The simplest combining method is to join the output sig-
nals at a single circuit node. This is the method used in
all power stages that are referred to in the jargon as a
totem-pole, half-bridge or single-ended push-pull de-
sign. While combining at a node is the simplest method,
it was not the method first used to produce push-pull
power amplifiers.
nals at a single circuit node. This is the method used in
all power stages that are referred to in the jargon as a
totem-pole, half-bridge or single-ended push-pull de-
sign. While combining at a node is the simplest method,
it was not the method first used to produce push-pull
power amplifiers.
The second and original method of combining the push-
pull output signals was to use a magnetic device, a trans-
former with a center-tapped primary, to perform the
differencing. Transformers had been in use previously
to adapt the high output impedance of vacuum tubes to
lower impedance loads. Power output was obtained at
such a high cost that it was rarely permissible to oper-
ate a power stage with impedance mismatching.
pull output signals was to use a magnetic device, a trans-
former with a center-tapped primary, to perform the
differencing. Transformers had been in use previously
to adapt the high output impedance of vacuum tubes to
lower impedance loads. Power output was obtained at
such a high cost that it was rarely permissible to oper-
ate a power stage with impedance mismatching.
Push-pull operation using a transformer with a center-
tapped primary as the combiner was particularly attrac-
tive as it also solved a problem implicit to transformer-
coupled designs. It was now possible to minimize (can-
cel) the DC magnetizing force produced by the quies-
cent bias currents in the primary. The output transformer
became smaller and less expensive as a consequence.
Larger low-frequency outputs were possible before core
saturation would cause significant distortion.
tive as it also solved a problem implicit to transformer-
coupled designs. It was now possible to minimize (can-
cel) the DC magnetizing force produced by the quies-
cent bias currents in the primary. The output transformer
became smaller and less expensive as a consequence.
Larger low-frequency outputs were possible before core
saturation would cause significant distortion.
The invention of the half-bridge power stage was not
documented until the 1940’s. Vacuum tubes were still
the only power devices available, but applications and
circuit design were continuously becoming more diverse.
Fundamental things were slowly but surely coming to
the fore.
documented until the 1940’s. Vacuum tubes were still
the only power devices available, but applications and
circuit design were continuously becoming more diverse.
Fundamental things were slowly but surely coming to
the fore.
PARADIGMS
The push-pull paradigm is part of the Crown BCA de-
sign while the class-B paradigm is not. The distinction is
that the class-B paradigm is taken to include both the
push-pull concept and the notion of strict time alterna-
tion of the active devices used to effect the push-pull
power stage.
sign while the class-B paradigm is not. The distinction is
that the class-B paradigm is taken to include both the
push-pull concept and the notion of strict time alterna-
tion of the active devices used to effect the push-pull
power stage.
Push-pull operation is possible with the output of a power
stage being a signal statistic and not an expression of
the immediate state of the power devices used to create
the signal statistic. Such is the case within the Crown
BCA power stage.
stage being a signal statistic and not an expression of
the immediate state of the power devices used to create
the signal statistic. Such is the case within the Crown
BCA power stage.
Note that push-pull concepts in no way imply the oper-
ating efficiency of a power stage. The first push-pull am-
plifiers were very inefficient because they were class-A
ating efficiency of a power stage. The first push-pull am-
plifiers were very inefficient because they were class-A
linear designs which by nature have large quiescent
power losses. Loy Barton’s class-B designs were
still inefficient by switch-mode standards as the
vacuum tubes were conductivity modulated to
effect the power output of the stage. Large
voltages were evident on the tubes during
most of their conduction cycle and thus
they dissipated (wasted) much power.
When the output signal becomes a
signal statistic as in a PWM power
stage, it is not necessary to re-
tain the class-B paradigm to re-
tain push-pull operation.
tain push-pull operation.
So then is there a class desig-
nator for the BCA paradigm?
This author suggests class-I as
a possible designator. Such a
choice has a mnemonic tag in
that the Crown BCA is implicitly
an interleaved power stage. The
chosen name for the overall
nator for the BCA paradigm?
This author suggests class-I as
a possible designator. Such a
choice has a mnemonic tag in
that the Crown BCA is implicitly
an interleaved power stage. The
chosen name for the overall
technology where interleaving is taken to its logical con-
clusions has been named OCIA (Opposed Current In-
terleaved Amplifier) technology.
clusions has been named OCIA (Opposed Current In-
terleaved Amplifier) technology.
BCA OPERATION
The following figures show the fundamental operation of
the basic Crown BCA power amplifier. The switch com-
mutation sequence is depicted by the Sp and Sn wave-
forms and the Vp and Vn waveforms are the switched
voltages which are input to the main output inductors Lp
and Ln. The currents Ip and In are the currents flowing
in Lp and Ln respectively.
the basic Crown BCA power amplifier. The switch com-
mutation sequence is depicted by the Sp and Sn wave-
forms and the Vp and Vn waveforms are the switched
voltages which are input to the main output inductors Lp
and Ln. The currents Ip and In are the currents flowing
in Lp and Ln respectively.
The quiescent switching sequence is for both output
switches to enable and disable in unison (50% duty
switches to enable and disable in unison (50% duty
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