Heathkit IT -3120 User Manual

Page 36 11§j-: ff'=MiUHfi
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SET G OI 0
,
L- __ ,--~- ~
R II
1 5k 0.
HI
I. SV
Gm=O
Figure 12
R II
I S k 0.
B I
l. SV
Gm
Figure 13
fE r
B2 +
l. sV -T
SET Gm' O
L.. _______ ./r/~
RY
4 . 70.
GATE 1 I GATE 2
Figure 14
Gm = 0
Battery B1 and control R11 (Set Gm = 0) are placed in
series with
the meter and shunt resistor R7 as shown in
Figure 12. This
circuit forms a shunt type ohmmeter that
will be used to measure the source-to-drain resistance in the
Gm test. Control R 11 is adjusted here to place the meter
pointer over the 0 mark (full scale). Notice that the FET is
not connected into the circuit at this time, even though it
may be
connected to the Tester .
Gm
Because Gm (transconductance) is the reciprocal of
resistance, an ohmmeter type circuit can measure Gm . See
Figure
13 . Notice that the Gm meter scale is similar to a
typical
ohmmeter scale.
In this test, the drain and source leads of the FET are
connected in shunt with the meter. An F ET with a low
drain-to-source resistance has a Gm
that approaches 00. This
is because the FET shunts most of the meter current. If the
drain-to-source resistance is high, the Gm approaches O. This
is because the FET shunts very little meter current.
Typically, values of transconductance are expressed in
,umhos (the meter indication multiplied by 1000).

Heathkit IT -3120 Specifications

General

Brand	Heathkit
Model	IT -3120
Category	Test Equipment
Language	English