Philips pm3232 - Page 25

67
We
regard
the
drift voltages
from
pre-amplifier
and control
amplifier as
being
due to
d.c.
voltage
sources
at
the
input (Vdl
and Vd2).
We
then
find:
<
CM
>
li
>
(1)
V2
=
-B
(VI
+
Vd2)
(2)
Vo
=
A(V2-Vi-Vd1)
(3)
It
follows
from
Vi
+
VI
=
(1
)
and
(3)
that:
A(V2-
Vi- Vdl)
A
2
V2-
Vdl
2
Substituting
this result in
(2)
gives:
V2
-
Vdl
V2
=
-B(
^
+Vd2)
V2
(1
+1
)
=
|vd
1
-
BVd2
V2
=
BVdl
BVd2
2.B
^^1
or
or
Substituting
this
result in
(3)
gives:
A
2tB
B
2
^,Vi.^Vd.*-^Vd2
2
or
or
Vo
A
=
Vi
+
1
Vdl
+
Vd2
The
last
two
terms
represent drift
voltages
which
are
equivalent to a
drift
voltage:
Vd
=
1
Vdl
+
Vd2
at
the
input.
We see
that
the drift
of
the
pre-amplifier is
reduced by
a
factor
2
drift
due to
the
control
amplifier,
which is
given
by
Vd2
4 I
^
g
(1
+
^
0*^ ^he other
hand, we
have
an
extra
The
drift
of
the
control
amplifier, however,
is so
low (because
of strong
thermal
coupling of
the
amplifier
transistors,
and
optimum
design) that
the term
2Vd2 can be
neglected.