Sony TA-E88 - Overall Amplifier Stage

1-2-9, Overall Amplifier
Although the TA-E88/E88B consists of ten de ampli-
fier stages, all of these stages are basically the same.
The output amplifier is described here as an example
of one of these stages,
|
TA-E88/E88B
TA-E88/E88B
|
Circuit Description
@
When
observing
the
signal
levels,
the
0.2Vp-p
(about
—20dB)
input
signal
applied
to
the
gate
of
Veelp
characteristics of Q404
low temperature
Fig.
11
shows
the
relevant
signal
levels
when
S5
is
Vee
|
VBE
set
to
OdB,
and
the
input
level
is
adjusted
to
obtain
——
;
.2Vp-p).
+
an
output
level
of
apoue
OdBrms
.
p-p)
The
;
Noh
iene
and
—
signs
refer
to
the
signal
polarity
at
that
point.
8B
re
Iptic
However,
the
(—)
sign
on
the
right
hand
gate
(NFB
input)
of
the
first
stage
Q401
indicates
that
this
was
+)
originally
a
negative
polarity.
Fig.
70
2
23
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ourput
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era
Teal
aaa
7
dee
as
sf,
C408
[
R403
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0405 f
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C410
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Fig. 17
fevel
G407
collector
{ d8)
@
f
95 6.5, eee
i iis Q411, 413
Q407
gate
|
emitter
—~20 { oe
—~20dB
i
NFB
77dB
(0.21 Vp-p) '
{
40 +
i
i
‘ Q407 voltage gain:
77d8
60
i
-
I 0405
i voltage gain:
hd 77dB
80
1
——
jx OQ408R
Q401 gain 1308 ~ —84d8 collector
O401L drain
CS
sales
Ge
Rna
he
Maes
ure
Ota
Se
Ge
de
Gs
att
as
~700
—97dB
Q407 gate
(with no NFB)
Fig. 42 ls
N
Lgain when applying
| NFB: 20dB
J
open-loop gain:
$7dB
Note:
The —6.5dB {evel (dotted line in figure) in
addition to the OdB level (theoretical value)
at the 0407 collector, is an example of the
error incurred due to the load effect when
measuring: with an ac voltmeter of 1MQ input
resistance.
The 7dB shown as the gain (between base and
collector) for Q405 is the voltage gain. This is
important because since Q405 employs the
emitter of O407 (low input. resistance) as
collector load of G405.. This gain should
really be measured as current ratio.
Q401
is
not
measured
at
the
drain
of
Q401
(12).
However,
2.2Vp-p
(about
OdB)
signal
appears
at
the
collector
of
Q407,
and
is
extracted
from
the
emitter
follower.
The level diagram is shown in Fig. 12.
An
NFB
of
77dB
is
applied
to
the
amplifier
which
has
97dB
open-loop
gain.
Since
the
gain
of
an
NFB
amplifier
is
reduced
only
by.
the
amount
of
NFB
applied
to
that
stage,
the
level
at
the
drain
of
the
Q401
becomes
—84dB.
This
is
the
reason
why
the
signal
level
is
too
low
to..be
measured
at
the
intermediate
points
of
amplifier.
D401, Q403, Q404 and D402 form a thermally-
compensated
constant
voltage
circuit,
which
operates
as
follows
(the
voltages.
referred
to
here
are
relative
to
the
B=
voltage):
D402
(EQBO1-05)
is
a
SV
zener
diode
which
maintains
the
emitter
of
Q404
at
+5V.
level.
To
obtain
a
flat
base
voltage
(VBR).
from
the
collector
of
Q404,
the
base
and
collector
are
connected.
as
shown
in
Fig.
10.
The
characteristics
referring
to
this
connection
are
also
shown
in
Fig.
10.
When
the
value
of
Vpp
reaches
the
“shoulder”
of
the
curve,
it
suddenly
becomes
constant,
and
subsequently
varies
only
with
changes
of
tempe-
rature.
Any
temperature-related.
variations
‘in
VBE
of
Q404
are
fed
to
Q402,
but
since
Q402
and
Q404
are
identical
transistors,
these
variations
are
automatically
compensated
for
by
the
other
transistor.
Hence,
Q402
and
Q404
serve
to
comen-
sate
for
any
voltage
variations
caused
by
changes
of
temperature.
Q403
is
a
2SK42
N-channel
depression
type
FET.
As
shown
in
Fig.
13,
EG
=
ES
when
gate
and
source.
are
connected,
resulting
in
the
flow
of
a
constant
current
of
less
than
SmA.
In
other
words,
Q403
serves
as
the
load
resistance
for
Q402.
!p characteristics of depression type FET
(EG = ES)
Fig. 13
Q402
has
an
emitter
resistor
with
sufficiently
high
resistance
to
obtain.
constant.
base
voltage,
which
means
that
the
emitter
current
(collector
current)
always
remains
constant.
A
bipolar
transistor
type
constant.
current
circuit
such
as
this
can
provide
highly-constant
current
at
low.
supply
voltages.
Q401
is
a
cascode
connection
with
the
polarity
from
the
lower
gate
to
the
upper
drain
as
shown
in Fig. 11,
@
Q405,
Q406
and
Q407
form
the
second
cascode-
connection
differential
amplifier.
Q408,
Q409
and
Q410
(current-mirror
circuit)
serve
as
the
load
resistance
for
Q406
and
Q407.
The
output
from
this
differential
amplifier
appears
single-ended
at
the
collector
of
Q407,
while
the
signal
on
the
collector
of
Q406
is
passed
from
Q408;
0410
to
Q409,
Q407
and
Q409
drive
Q411
and
Q412.
Furthermore,
although
Q409
also
operates
as
a
constant
current
circuit,
and
the
Q407
collector
current
is
passed
through
R418
and
R419
to
Q411
and
Q412,
signals
of
almost
equal
amplitude
are
passed
to
these
two
transistors
(Q411
and
Q412).
In
addition,
the
thermal
compensation
for
O410R
and
Q409
is
achieved
by
Q410L
and
0408.
@
Q411,
Q412,
Q413
and
0414
form
a
cross
coupled
current
driver
circuit,
comprising
pairs
of
NPN,
PNP
emitter-followers.
CAUTION
The
transistors
have
a
resistor
to
their
collector
or
emitter
circuits.
These
transistors
will
be
not
damaged
if
any
of
transistor
terminals
are
short-circuited,
The
exception
is
Q403
which
supplies,
constant
current
to
D401,
Q404
and
D402.
Ifthe
drain
and
source
of
Q403
are
short-circuited,
D401,
Q403,
Q404
and
D402
will
be
completely
destroyed.
1-3. Parts Information
1-3-1. Small Resistors
The
TA-E88/E88B
uses
many
small
resistors,
similar
to
the
type
shown
in
Fig:
14.
These
resistor:
are
“%W
metal-oxide
with
an
accuracy
of
1%.
Notethat
this
accuracy
rating
has
been
omitted
in
the
sclema-
tic
diagrams.
(The
“4W.
and
%W
carbon
resstor
accuracy
ratings
are
indicated),
shows multiplier
18
00
Gu.
represents
15k
€2
metal-oxide resistor.
Fig. 14
1-3-2. Square Tantalum Capacitors
The
capacitors:
employed.in
the.
TA-E88/E88B
(as
shown
in.Fig:.
15)
are
the
same.
square.
tantauna
capacitors
used
in
pulse
circuit
power
supplies,
stc:
These
capacitors-are
especially
used
in
the
B+
andp—
bus
where
their
greater
by-pass
effect
is
neeed..