Philips pm3232 Instruction Manual 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.