Sencore TF151 - Dual Gate FET Testing; Dual FETs (Matched Pairs); MOSFET;IGFET Safety Precautions; Handling and Testing Precautions

1. Measure the lOSS with the TF1Sl
2. Multiply the current in step one by the value of the drain load resistor.
3. Subtract the voltage across the load resistor from the drain supply voltage.
4. Multiply the voltage across the FET from step three by the lOSS measured
in step one. 1llis is the dissipation across the FET in watts.
DUAL GATE FIELD EFFECT TRANSISTORS
The dual gate FET can easily be checked with the TF1Sl in or out of circuit for Gm,
and out of circuit for leakage. When connecting MOS or IG dual gate FETs to the
TF1Sl, the 'secondgate is connected to the TF1Sl with the GATE 2lead . The yellow
lead is for gate 1 and the blue lead for gate 2. The ' first test listed in the Sencore
Reference Book is for the first gate. The second &est, listed as GATE 2, is for the
second gate. Make the tests on Gate 1 and then set the function switch to the GATE 2
tests and repeat the test to completely check out the dual gate FET. Most dual gate
FETs are of the insulated gate type. If rhe dual gate FET is a junction type, such
as 3N124, it must be checked as a regular, single gate FET. The first and second
gate leads of the FET must be shorted together and connected to the TF1Sl with the
yellow
lead (Gate 1). The blue lead (Gate 2) is not used for this rest.
DUAL FIELD EFFECT TRANSISTORS
There are also dual FETs, that is, one package rhat conrains a marched pair of FETs
with independent leads for each element. To check these properly, both FETs sliould
be checked and the readings compared. If they do not march within 10%, the entire
unit is probably defective and should be replaced . These units are used in different-
ial amplifiers and bridge circuirs where rhe FETs must be marched.
SAFETY PRECAUTIONS WITH TI- IE MOSFET OR IGFET
The insulated gate, ofren called the IGFET or MOSFET are very delicate devices
out of the circuit. In circuit, they are jusr as rugged as the junction rype FET. Our
of circuit, rhe insulared gate type is subject to damage from static charges when
handled. The insula red gate FET is generally shipped with the leads all shorted to-
gether to prevent damage in shipment and handling . To test the insulated gate out of
circuit, the leads from the TF1Sl should be connected before the short is removed
from the FET leads. 1lle following points should be followed when checking the
MOSFET or insulated gare FET.
1. When a MOSFET is to be unplugged from a unit for testing, your body should be
at rhe same potential as the unit. This can be easily accomplished by placing
one hand on the chassis before you unplug rhe FET . Before the FET is conn e· ::: te.:i' :
to the TF1Sl, put the hand holding the FET against the front panel of the TF1Sl
and connect ttje source lead from the TF1Sl to the source of the FET. 1llis pro-
cedure prevents possible damage from static charges to the FET. A lead clipped
from the TF1Sl case to your watch band or ring is quite useful when a large
quantity of FETs are to be tested.
2. When handling the insulated gate FET, the leads must be shorted together. 1llis
is generally done in shipment by a shorting ring or piece of wire. Connect the
test leads of the TF1Sl, with the source lead being first, to the FET. 1lle shorting
ring or wire may then be remov ed without fear of damage.
3. When soldering or un solde ring the insulated gate FET, the iron tip must be at
ground potential. Connect a clip lead from the barrel of the soldering iron to the
TF1Sl case or to conduit ground. Do not use a sold e ring gun.
4.
Turn the power to the circuitoff before inserting or removing anFET. The vol-
tage transients that are generated may damage the FET.
MATCHING OF TRANSISTORS, FI ELD EFFECT TRANSISTORS AND DIODES
The TFlSl is ideal for matChing of transistors, field effect transistors and diodes.
In such circuits as a ring de modulator, audio output stage, bridg e circuits and othe rs,
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