Sencore TF151 - FET Modes of Operation; Measuring FET Transconductance (Gm); Gm Measurement Setup and Procedure

PARAMETEH TRANSISTOR FET
TUBE
Input
Impedance Low High
High
Gain measured in Current out = Beta Current out = Gm Current out = Gm
Current in Voltage in Voltage in
Warm up time
Short
Short
Long
Power Consumption Small Small Large
From the chart, you can see that the FET combines the advantages of the bi-polar
transistor with that of the vacuum tube. To test the FET, the gain must be measured
in Gm or micromhos and the gate leakage which is similar to grid leakage in a tube,
must be m eas ured.
DEPLETION AND ENHANCEMENT MODES IN FIELD EFFECT TRANSISTORS
111e field effect transistor is unlike the regular or bi-polar transistor in many ways.
One of the differences is in the mode of operation or how the applied bias effects the
drain current of the device. 111ere are three modes of operation of the FET, the
depletion mode, the enhanc ement mode, and the depletion e nhancement mode. A
depletion mode F ET is the junction type. It is biased just like a tube, with the gate
at the opposite polarity of the drain potential. At zero bias, the maximum drain
current will be flowing, much lik e a tube. The enhancement mode is just the opposite
of the tube, or much like that of the regular bi-polar transistor. The gate bias is
of the same polarity as the drain or a forward bias. At zero bias, the enhancement
FET it cut off and only a minute amount of drain current will. be flowing. To get rhe
proper bias on the FET for testing, the TF151 has a BIAS switch on the lower part
of the panel. There are two positions, NORM for the depletion type FETs and POS
for the e nhancem e nt type FETs. In the NORM position, zero bias is applied to the
FET and in the POS, a forward bias is applied. Although the depletion type is the
most popular and is widely used at present, the en hancem ent type will probably take
its pla ce among the l eaders because of the ease of bias and coupling .
MEASUHING Gm of the
FIELD EFFECT TRANSISTOR IN OR OUT OF CIRCUIT
Both junction and insulated gates, sometimes referred to as MOSFET or IGFET can
be checked in or out of the circuit with the TFlSl. If the FET is a dual gate type,
both gates may be checked independently for greater accuracy .
,
To oonnect and make t he test on an PET in or out of circuit:
1. Connect the Source, Drain, and Gate leads as shown in figure 5 or 6. 111e blue
!cad is co nn ected to the seco nd gate of dual gate FETs. (See the section on dual
gate field effect transistors when testing these devices).
2. Set the function switch to GAIN (Gm) Xl range on the Field Effect Transistor
side of the TF lSl and the type switch to the type to be tested, e ith er 'N' channel
or 'P' channel.
3. Set the BIAS sw it ch to NOHM for m ost junction and insulated gate FETs, or to
POS for the enhance m ent type FET. When in doubt, consult the Sencore Transistor
manual that comes with the TP151.
4. Hotate the Gm ZEHO co ntr ol until the meter reads zero (0) and then depress the
GAIN button a nd read the top scale of the meter directly in micromhos .
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