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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 ftl ters(MF-20 1,202) 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
(Ie 703 - 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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