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DESCRIPTION OF THE NEW CIRCUIT
1.
SAW filter
FM-IF section
of
this model
is
so
designed that 2 types
of
the frequency band characteristics(WIDE, NARROW)
can be selected.
Namely, when the IF band switch
is
set
to
WIOE, the
SAW
fIlter and 4-pole linear phase filter are used
to
sup-
press distortion
to
a minimum to improve the group
delay characteristics.
When
the IF band switch
is
set
to
NARROW, 2 ceramic ftlters(MF-201,202) are used in
addition
to
the above
SAW
filter and the 4-pole linear
phase filter
to
improve selectivity characteristics. In ad-
dition, amplification
is
performed by IF Amp (lC201)
to
compensate for losses caused
by
insertion
of
the two
ceramic
fIlters (MF-201, 202). Accordingly, positioning
at NARROW
is
convenient when there
is
interference
from a station close by when receiving a long distant
station.
This IF band selection
is
handled by switching the +ISV
OC
power supply using SS03; and the
output
at the
FM
front end
is
applied
to
the gates
of
FET Q201 and 202
as
shown in Fig.
1.
Assuming
that
IF band switch
is
set
to
WIDE, a +ISV
voltage
is
applied
to
the anodes
of
Q201 and 202,
by
means
of
SS03, and
0201
turns ON. IF signal
is
input
to
the next stage(lC202) through the path
of
WIDE,
and
0201
becomes reverse biased and the
path
of
NARROW
is
cut
off.
On the contrary, when SS03
is
set
to
NARROW, the
voltage
is
applied
to
Q202, IC201 and the anode
of
0201,
and the
path
of
NARROW
is
connected. The
path
of
WIDE
becomes reverse biased and the path
of
WIDE
is
cut off. The diodes(020
1,
202), which are used for
this electronic switch, are IS2076 having a small junction
capacitance. Separation adjustment also changes over
simultaneously with IF band SS03 switching,
to
obtain
the best stereo separation in both
WIDE
and NARROW.
2.
Dynaharmony
(Class
G Amplifier) output circuit
The level
of
the music source changes momentarily, the
percentage
of
high level (over 1/2
of
peak value)
is
very
small, it
is
only less than
2%
of
the total music signals.
Large
output
amplifiers have been used conventionally
to
playback at high levels without distortion; however,
in this case, high voltage
is
impressed to the
output
tran-
sistor,
so
the power consumption
is
large.
Studies have been made, how to obtain high
output
powers without the increase
of
the output transistors'
consumption in order
to
improve the efficiency. Thus,
the new dynaharmony output circuit (Class G Amplifier)
was developed.
The principle circuit
is
shown in
Fig.
2.
QI - Q4 are
drivers at the
output
stage, and
Q6
and Q7 operate with
normal small input signals. When the input signal
is
large,
QS
and Q8 operate in addition.
When
the positive input
signal at normal level
is
impressed, Q2 and Q6 turn
ON
and current flows from the
+B1
power source. When the
input signal level increases and reaches more than
+B1,
-9-
HITACHI
SR-2004
QI and
QS
also turn on. At this time, since the emitter
potential
of
QS
exceeds
+B1
, the current does
not
flow
from
+B1
but
only from +B2. When the input signal
level decreases,
Ql
and
QS
are turned off, current does
not flow from
+B2
and current
is
supplied from
+B1
again.
When
the input signal
is
negative,
Q2
and Q6 turn
off
and
Q3
and
Q7
turn on. The operation, when
input
signal
is
negative,
is
the same
as
that
of
positive.
3. Current mirror circuit
This set contains a current mirror circuit which drives
the output stage in push-pull operation
to
keep
dis-
tortion low.
The current mirror circuit
is
composed
of
Q703, 704,
705 and
0703
as
shown in Fig.
3.
When
a positive signal
is
impressed
to
Q704 and a ne-
gative signal
to
Q70S, the collector current
of
Q704 and
Q70S become
as
follows:
le704
=
10
+61
(1)
Ie
705 =
10
- 61 (2)
(10:
OC
bias current
of
Q704, Q70S)
The characteristics
of
the diodes between
0703
and B -
E
of
Q703 are equal and, in addition, R711 and R712
are equal,
so
the current flowing to
0703
and R714
(le704)
is
equal
to
the collector current
of
Q703.
(This
is
the origin
of
the name "current mirror circuit").
That
is
to say, the collector current
of
Q703 is:
le703
=
10
+61
(3)
and the
NPN
driver transistor
is
driven by current
of
261
(Ie703 -
Ie
705).
On
the contrary, when a negative signal
is
supplied
to
Q704 and a positive signal
to
Q70S, the description
is
the reverse
of
the above and the PNP driver transistor
is
driven by the current
of
261.
As
mentioned above, the current mirror circuit carries
out the push-pull operation and can drive the
output
stage with low distortion.
4. Constant current circuit
This constant current circuit stabilizes the bias
of
the
main amplifier circuit. In Fig.
3,
Q706 functions to
make the sum
of
the emitter current
of
Q701 and Q702
constant and Q701 and Q702 operate
as
a complete dif-
ferential amplifier.
Assuming
that
the current flowing to R706 increases due
to
fluctuation
of
the power voltage, etc., this current
is
divided in two by Q701 and Q702, and the voltage drop
increases in R708 and R709. Next, the base potential
voltages
of
Q704 and Q70S increase, the sum
of
the
emitter current
of
Q704 and Q70S increases and the
voltage drop
of
R716 increases. The increase
of
the
voltage drop
of
R716
is
fed back
to
the base
of
Q706,
the collector current
of
Q706 increases, and the col-
lector current
of
Q706 absorbs the surplus current from
R706 through R707. Also, when the current flowing
to
R706 decreases, the base potentials
of
Q704 and Q70S
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