Crown PSA-2 - Page 95

crown
possible.
All
heatsinks
in
the
amplifier
are
electrically
hot!
Q402-405
are
the
output
devices
and
are
selected
types
whose
Safe
Operating
Area
is
not
voltage
derated
within
the
operating
range
of
the
amplifier.
The
driver
(Q401)
is
a
high
Ft
(Gain
Bandwidth)
type
having
power
handling
capability
sufficient
to
eliminate
it
from
the
protection
design.
The
collector
of
the
driver
is
grounded
to
provide
additional
voltage
when
the
output
stage
is
driven
to
saturation.
As
such,
the
driver
does
not
saturate.
The
pre-driver
(Q400)
is
prevented
from
saturating
by
the
diode
clamp
circuit
of
D401
and
D402.
D400
is
part
of
the
bias
circuit
of
the
output
stages
and
is
thermally
joined
to
the
predriver
which
at
first
characteristically
cools
as
the
drive
to
the
output
stage
becomes
large.
This
is
due
to
the
fundamentally
class
AB
nature
of
the
output
stage
accompanied
by
the
high
current
gain
of
the
driver
and
output
devices.
It
should
be
noted
that
the
amplifier
is
usually
biased
for
class
AB
operation
rather
than
class
AB+B
as
the
other
Crown
products.
It is
possible
to
bias
the
unit
for
AB+B
if
the
higher
efficiency
is
desired.
The
heatsinking
capacity
@::
large
enough
to
minimize
any
such
need.
C400
constitutes
the
local
voltage
feedback
loop
immediate
to
the
output
stage.
(This
is
functionally
identical
to
the
capacitor
(typically
200pf)
which
is
placed
from
the
collector
of
the
last
voltage
amplifier
to
ground
in
the
other
amplifiers.)
Note
that
since
in
those
units
the
Vcc
supplies
and
ground
are
common
that
this
is
the
same
as
making
a
connection
to
the
Vcc
supply
as
is
done
here.
Current
degeneracy
and
phase
compensation
are
achieved
in
the
pre-driver
circuit
by
R401
parallelled
by
C401.
D403
serves
as
a
flyback
diode
as
in
all
other
Crown
amps.
U400
along
with
R408
and
R407
constitute
an
IC
absolute
temperature
sensor
to
measure
the
heatsink
temperature.
This
is
needed
to
provide
the
protection
system
with
knowledge
of
the
absolute
temperature
of
the
transistor
junctions
of
the
output
devices.
The
current
flowing
in
U400
is
proportional
to
absolute
temperature:
one
microamp
=
one
degree
Kelvin.
R408
is
selected
to
obtain
this
constant
as
determined
by
grading
of
the
ICs.
R407
is
selected
to
protect
U400
from
large
signals
and
as
Bauer
its
value
is
not
critical.
The
NPN
output
stage
is
similar
to
the
PNP
as
indicated:
PNP NPN
402-405 302-305
Q401 Q301
400 Q300
D401, 402 D301, D302
D400 D300
C400 C300
C401, R401 C301, C302
D403 D303
U400 U300
R408 R309
R407 R308
The
topology
of
the
NPN
stage
is
different
than
any
used
in
previous
Crown
products.
Its
principle
advantage
at
this
time
is
that
the
driver
of
this
configuration
must
Operate
with
minimum
Vce
which
is
less
than
that
of
the
PNP
stage,
and
this
suggests
the
use
of
the
PNP
driver
which
retains
its
Ft
with
lower
voltages
than
the
NPN
part.
If
PNP
outputs
were
available
to
complement
the
rugged
NPN
devices,
It
would
be
possible
to
use
the
topology
of
the
PNP
stage
for
the
NPN
stage
as
well
and
obtain
a
similar
advantage
for
the
PNP
driver
by
grounding
its
collector.
The
tendancy
of
three-stage
Darlington
amplifier
stages
to
oscillate
at
positive
clip
is
often
tracable
to
an
Ft
problem
in
the
driver
as
its
Vee
becomes
small.
B. Balanced Gain Stage - PSA-2/SA2
Continuing
to
follow
the
signal
backwards
through
the
amplifier
we
encounter
the
balanced
gain
stage.
It
is
composed
of
two
differential
stages
of
complementary
type.
The
differential
amplifiers
are
constructed
with
monolithic
dual
bipolar
NPN
and
PNP
transistors.
Resistor
networks
are
used
to
balance
the
collector
currents
in
the
pairs
such
that
the
necessary
current
flows
thru
the
bias
network
between
the
bases
of
the
pre-drivers
of
the
output
stages.
This
bias
network
is
composed
of
the
previously
mentioned
components,
D300,
R300,
and
D400.
The
path
of
the
current
from
Q102
(collector
pin
7)
to
Q103
(collector
pin
1)
in
channel
|
also
flows
through
D113,
D301, D401,
D118,
D117,
and
D115.
Diodes
D113
and
D115
are
used
to
isolate
the
driving
stages
in
case
of
catastrophy
from
the
output
stages.
If
the
output
supplies
were
to
propogate
to
the
+
15VDC
supplies,
the
overall
damage
would
be
great.
7-28