Sony STR-S5L - Power Amplifier Circuits

STR.55L
TPOWER
AMP]
Collector
Follower
Output
Circuit
The
output
stage
of
the
power
amp
is
usually
an
emitter
follower,
but
in
this
model
it is
a
collector
follower.
This
is
because
the
output
circuit
of the
driver
IC is
SEPP,
and
the
output
transistor
is
driven
by
this
collector.
Figure
D
is a
simplified
illustration
oi
tfri,
circuit.
Q604,
605
are
included
to
drive
high
voltage
output
stages
(maximum
rated
power voltage
for
IC602
is
20V,
t40V)'
This
is
also
favorable
to
'reducing
distortion.
Protector
circuit
Fig'
D
In a
circuit
like
the
one
shown
in
Figure
E, by
increasing
the
flow
of the
base
current
i1,
even
if the
transistor
is
saturated,
ir
will
increase
without
limit'
(In
a
conventional
emitter
follower
output
circuit,
the
emitter
potential rises
and
this
does
not
occur')
In
this
model,
as shown
in
Figure
F, a
diode
is in-
serted
to
limit
the
base
current
before'the
transistor
is saturated.
When
this
circuit
is
operating,
in
other
words'
when
if
is
flowing,
base
potential at
@
is l
'8V'
This
l.8V
is
a
result
of
the
voltage
of
0'6V
between
the
base
and
emitter
of
the
transistor
and
forward
direction
voltage
of
I
'2V
at
the
2
silicon
diodes.
By
increasing
ir',
the
potential at
(D
drops
to
1.2V.
i1
'
cannot
be
increased
any
further;
if it is
tried,
i2
will
flow'
By
limiting
if
in
this
way,
the
transistor
is
protected.
Also,
this
circuit
has
a
favor-
able
effect
on
distortion
resulting
from
clipping'
Legato
Linear
Circuit
Usually,
in
a class-B
amp,
switching
distortion
is
generated
when
the
positive half
cycle
and
negative
half
cycle
of
the
signal
switch.
This
occurs
each
time
the
output
transistor
cuts
off'
In
order
to remove
this
distortion,
therefore,
this
transistor
must
be
used
so
that
it
does
not
cut
off.
Figure
G is
the
output
circuit
of
this
set.
This
circuit
operates
so
that
R647
and
C623
do
not
cut
off
Q606
and
Q607'
This
is
an
input
signal,
but
during
the
positive half
cycle,
it
results
from
i1
f{owing
to
the
base
of
Q606
and
during
the
negative
half
cycle,
from
i2
flowing
to
the
base
of
Q607.
I
+t
ineut
sisnarl
Fis. G
Fig.
E
Fig.
F