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42
1.4. 4. 2.
Impedance
convertor
The
output
signal
from
the
a.c.
attenuator
is
passed
via
switch contact
V~
3
to
an
impedance
convertor
which
converts the high input
impedance
signal
to
a
low impedance
to
match
the input of the R.M.S. convertor
A302, which would
otherwise
draw
current
from
the
signal.
The
gate
of
FET
V302
gives
the
high
impedance
input to the
stage,
which
is
fed
from
a
current
source supplied
by V306.
In
other measuring functions than
a.c.,
control
signal
RE
P switches
on
FET
switch
V301, which
short-circuits
the
a.c.
path to
earth.
The impedance
convertor input
is
protected
against excessive
input
voltages
by:
—
Diode-connected
V350
which
acts
as a
zener diode
for negative voltages
—
R304
and
the
gate-drain
junction of
V302
for
positive voltages.
For
the input range
from
—IV
to
-i-1
V
on
the gate of
V302,
the
output
at
V305
collector
lies
between
OV
and
2V;
i.e.
it is
always
positive.
IMPEDANCE CONVERTOR
(TRIGGER LEVEL RMS/CONV.)
V301
Fig.
30.
Impedance
convertor
-10V
ST 3382
81
1130
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