Page 29
The
output
of
this
part,
AOUT,
is
routed
to
the
transmit
amplifiers
by
PTT
switch
S1.
The
transmit
signal
is
amplified
from
about
20mW
to
about
200mW
by
transistor
Q2.
Resistors
R24
and
R25
bias
the
base
of
this
tran
‐
sistor
to
3.0V,
which
puts
the
emitter
at
2.3V.
The
emitter
re
‐
sistors
R28
and
R29
cause
about
45mA
of
collector
current
to
flow
(2.3V/51.7
ohms).
Because
of
lim
‐
iting
resistor
R27,
the
collector
voltage
is
11.1V—(.045*47)
=
9.0V.
R26
and
C32
provide
a
lot
of
negative
feedback
to
keep
the
part
from
oscillating
and
to
lower
the
input
impedance.
Nor
‐
mally,
the
input
impedance
of
Q2
would
be
the
parallel
combination
of
its
bias
resistors
(R24
and
R25)
in
parallel
with
4.7*beta=470
ohms
(the
emitter
resistor
times
the
transistor’s
beta
of
about
100).
Thus,
the
in
‐
put
impedance
would
be
about
285
ohms.
Feedback
networks
tend
to
lower
the
actual
impedance.
The
AD603
has
more
than
enough
drive
capability
to
handle
this
de
‐
crease
in
load
impedance.
Q2’s
AC
gain
is
the
load
imped
‐
ance
seen
by
the
collector
(as
reflected
by
transformer
T2),
di
‐
vided
by
the
4.7
ohm
emitter
re
‐
sistance.
RFC1
and
C33
assure
that
the
top
side
of
T2
is
at
AC
ground.
T2
has
a
9:3
turns
ratio,
which
causes
an
impedance
step
down
to
the
next
stage.
The
2SC2166
final
amplifier
has
a
very
low
input
impedance
of
about
five
ohms
due
to
the
grounded
emitter
configuration
and
the
feedback
network.
That
five
ohms
is
reflected
back
to
the
collec
‐
tor
by
a
factor
of
the
square
of
the
turns
ratio,
or
9,
yielding
a
collector
impedance
of
about
45
ohms.
The
gain
of
this
stage
is
thus
45/4.7,
or
almost
10.
A
voltage
gain
of
10
is
equivalent
to
a
power
gain
of
20dB
(10logE*E).
In
reality,
the
tran
‐
sistor
operates
at
its
maximum
possible
gain
of
about
11dB.
This
means
that
a
base
input
signal
of
600mVp
‐
p
results
in
a
collector
swing
of
about
6Vp
‐
p.
The
2SC2166C
final
operates
in
much
the
same
manner
as
the
pre
‐
vious
stage
except
that
its
out
‐
put
is
fed
to
the
antenna
via
a
low
pass
filter
with
50
ohms
in
‐
put/output
impedance.
The
output
transformer
has
a
3:5
turns
ra
‐
tio,
so
the
50
ohm
filter
load
is
reflected
back
as
an
18
ohm
col
‐
lector
impedance.
With
the
col
‐
lector
current
resting
at
about
386mA
(.58V/1.5
ohm),
about
772mAp
‐
p
(386mApk,
273mArms)
is
available
before
saturating
the
transistor
and
causing
distor
‐
tion.
With
a
collector
impedance
Содержание HT-7
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