06283-2
QI04
and
QI05
are
current
limiters
for
the
voltage driver
stages
of
QI06
and
Ql07.
As
the
signal
level
increases turning on Q 107
harder,
a
voltage
drop
appears
across
R125 and R126.
When
the
drop across R125
reaches
approximately
0.6 volts
this
turns
on
Ql05,
thus removing
the
drive
to
Ql07.
The
action
of
Ql04
and
QI06
is
the
same.
Q 108
is
a
Vbe
multiplier
it
provides bias
voltage
for
the
output
stage
and keeps idle
current
at
a
constant
level as
the
temperature
changes.
Ql10
and
Qlll
form a
current
mirror
stage
that
acts
as loads for
Ql06
and
Ql07.
The
current
mirror
stage
uses
the
base
emitter
diode only
of
QllO
to
match
base
emitter
diode of
Qlli.
As
Ql07
is
turned
on
harder,
an
equal and opposite amount
of
current
is removed from
QllO.
Hence,
the
current
of
Ql06
plus
the
current
of
Q
107
remains
constant.
The
output
circuit
consisting
of
Q1l4
through Q127 is a
quasi-complementary
output
circuit.
Q1l4
and
Q1l6
are
connected
common
collector
(Darlington)
to
provide
the
current
gain
necessary
to drive
the
output
transistors
Q1l8,
Q120, Q122, Q123 and Q126
completing
the
positive
output
circuit.
Ql15
is
connected
as a common
emitter
and drives
Q1l7.
Q1l7
is
connected
as a
common
collector
to
provide
the
current
gain
necessary
to
drive
the
output
transistors
Ql19,
Q12l,
Q123, Q125 and Q127, this
output
has 100%
feedback
to
the
emitter
of
QlU.
The
stages
Q1l5,
Q1l7, Q1l9,
Q12l,
Q123, Q125 and Q127
act
like one
large
PNP power
transistor.
Q1l2
and
Q1l3
are
positive and
negative
VI
limiter.
The
VI
limiter
action
allows
small
amounts
of
current
to
flow when
the
voltage across
the
output
transistors
is
great
Oarge voltage)
~d
allows
larger
amounts
of
current
to
flow when
the
voltage
of
the
output
transistor
is lower (small voltages).
Ql12
is
connected
in a common
emitter
configuration
and
is biased
OFF.
With an
output
signal
there
is
current
flow
in
Q1l6
and
Q1l8,
this produces a
voltage
drop
across R148, and this
will forward bias
Q1l2.
As
Q1l2
turns
on
it
robs
the
drive
current
from Q 114 and limi
ts
the
maximum
output
current
through Q 116 and Q 118.
The
tum
on
threshold
of
Q1l2
is
small if
the
output
voltage
swing is low
or
if
driving a
short
circuit.
R13'
provides some
pre
bias signal
so
that
the
short
circuit
current
will
be
lower. As
the
output
voltage
swing
increases,
DIll
is
forward
biased and forms a voltage divider consisting
of
R136,
DIll
and R137 reducing
the
tum
on
signal
to
Q1l2,
this
allows a
larger
amount
of
output
current
to
flow
before
Q1l2
tums
on and limits
the
drive signal. R136,
C1l2
and
C1l3
form a
delay
circuit
around
Qll2.
This delay allows
transients
to
be handled
accurately
and
prevents
false
triggering
of
the
limiter
circuitry.
The
negative
limiter
works in
the
same
manner.
The
output
appears across flyback clipping diodes D 114
and
D 115,
then
passes
through compensation networks
LI0l/R160
and
R16l/C120
and
appears
at
the
output
terminals
Jl03
and
JI04.
To
maintain
overall amplifier
stability,
linearity,
and low
distortion,
degenerative
feedback
is
used throughout
the
amplifier.
Voltage divider consisting
of
R12l,
R122 and
Cl09
applies
the
correct
amount
of
feedback
to
the
inverting
input
of
QlOl.
Except
for
the
input,
and
feedback
loop,
the
amplifier
uses
direct
coupling
throughout.
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