Philips pm2521 - 6.4.1.7. Diode measurements (+)

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