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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
Q 6 0 7 .
I
+t
ineut sisnarl
Fis. G
Fig. E
Fig. F
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