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TECHNICAL
DESCRIPTION
Super A power amplifier
A big problem has remained unsolved in the progress regarding
amplifier
technology:
how
can
class-A
operation
be made
compatible with high power efficiency? The class-A operation
is best in terms of low distortion; its critical drawback is such
that its power loss is greater than the available output power.
Because of this, class-B amplifiers have generally been applied
for power
amplification
because they offer both acceptable
distortion and power efficiency. However,
as long as class-B
amplifiers
are employed,
the occurrence
of crossover distor-
tion
and
switching
distortion
is inevitable.
Although
the
improvements
of circuit elements
have
realized
fairly good
class-B amplifiers, when their values of distortion are referred
to, the ''nature"
of distortion
is still detrimental
to the re-
produced
sound quality. To solve this problem by improving
circuit
technology,
JVC
has developed
its new
''Super-A"'
power amplifier.
The
Super-A
power amplifier consists of a voltage amplifier
stage whose
distortion
is minimized by compensating for the
non-linearity of semiconductors, and aclass-A power amplifier
stage which
includes a bias circuit for raising the power effi-
ciency to such a degree as is comparable to that of a class-B
amplifier. JVC's Super-A power amplifier offers the following
features:
1)
Power
efficiency
is extremely
high despite the class-A
operation, greatly reducing power loss and heat generation
compared with conventional class-A amplifiers.
2)
Switching distortion inherent in class-B operation is com-
pletely eliminated; it is perfectly zero.
3) AVecE distortion *1, ACob distortion *2 and A Vpe dis-
tortion
"3 are all extremely
low because of the unique
non-distortion circuit configuration.
"1
Distortion caused by non-linear variations of current gain
B due to variations of collector-emitter voltage VCE.
*2
Distortion
caused
by non-linear
variations
of amplifier
gain which are caused by non-linear variations of feedback
capacity Cob due to variations of collector-emitter voltage
VCE.
*3
Distortion caused by non-linear variations of base-emitter
voltage VBE of power transistor.
Distortion of measurement
system
Distortion of '"Super—A"
Fig. 1
Output and residual distortion waveforms of measur-
ing instrument
Fig.2
Output
and
distortion
waveforms
of Super-A
am-
plifier
Fig. 3
Output and distortion waveforms of class-AB ampli-
fier
Voltage amplifier stage utilizing ''Super-A"'
performance to the utmost
Fig. 4 shows
operating
characteristics of
a common-emitter
transistor; 8 varies as VCE varies, resulting in A Vce distortion
as described above.
Fig. 5 shows a variation of the feedback capacity Cop in re-
lation to VCE. This non-linear variation of Cop due to varia-
tions of VCE
causes
a non-linear variation of the amplifier
gain, leading to the A Cob distortion mentioned above. Fig. 6
shows operating characteristics of a common-base
transistor.
What
is noteworthy
about
this type
of operation
is that,
though its current gain is O dB, its output linearity in constant-
current drive is fairly good since a scarcely varies with varia-
tions of VcE (a= 1) as shown by the equal distance between
adjacent base current curves.
An
example
of the
cascode-connected
amplifier
circuit
is
shown
in Fig. 7. The features of this circuit are good frequ-
ency response (because of no mirror effect due to Cob) and
high output impedance. This means that this circuit eliminates
distortion
by
effectively
combining
the
advantages
of
common-emitter
and
base-emitter configurations. The driver
stage
of the
Super-A
power
amplifier
employs
an applied
circuit of this kind and a drastic reduction in distoriton is the
result.
Fig.4
Output characteristics of common-emitter amplifier
Fig.5
Cop characteristic
Fig.6
Output characteristics of common-base amplifier
Fig. 7
Cascode amplifier
Distortion of ''class AB"
f=10 kHz, THD=0.0008%
Fig. 1.
f=10 kHz, Output power=100W/8
QO,
THD=0.0009%
Fig. 2.
9
f=10 kHz, Output power=100W/8 Q
THD=0.0022%
Fig. 3.
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