Sencore TF151 - Testing Regular Transistors; Beta and Leakage Parameters

5. IOSSMAX MA is the maximum zero bias drain current for the listed FET. This
is the maximum current and should not be exceeded when the lOSS check is made
on the f<'ET. lOSS measurements are not made on positive biased FET's .
6.
GAIN Gm (uMHOS) shows the Gm limitations of the device. The minimum and
maximum figures, when shown, are the manufacturing limits. The AVE figure
is an average or typical gain figure. All three figures are listed when available.
7. LEAKAGE IGSS MAX uA is the maximum leakage current of the FET as read on
the LEAKAGE
scale of the TF151 meter when making the GATE leakage test.
Any reading above this figure can be considered as bad and the FET rejected.
If any transistor is not listed in the book, the information was not available at the
time·of printing. The Sencore FET and Transistor Reference Book is printed once
each year. If you wish to receive each copy as it is printed, sign the warranty card
and return it to the Sencore factory as soon as possible. Your name will be put on
an automatic mailing list to receive the new transistor book for the TF151 as they
are printed. A nominal charge will be made for each book.
TESTING REGULAR TRANSISTORS (BI-POLAR)
What should a regUlar transistor be tested for? The two most important parameters
of a transistor are the beta, and leakage between the base and collector (lCBO)' Beta
is the current amplification factor and can be either DC or AC beta. DC beta is the
ratio of collector current divided by the base current and is referred to as hFE in the
transistor manual. AC beta is the ratio of change in the collector current divided by
the
change in base current while holding the collector voltage constant and is referred
to as hfe in the transistor manual. AC beta measured at low frequencies and DC
beta are very nearly the same. However, AC beta is more revealing because it is
measured under dynamic conditions.
The beta limits, or spread as it is called, is the extreme that may be expected with
a
particular transistor. This can be a ratio of as much as 2;1 or more for a given
transistor. If only a single figure is given in a transistor manual, it is generally the
minimum figure. In the Sencore Reference Book, the column the figure is in will tell
you if it is minimum or an average figure.
The application of a transistor generally fits a beta pattern and the chart below is
given as a rough gUide to beta values of various transistors found in these circuits.
A more exact figure can be found in the Sencore Reference book or a transistor
manual.
~
' RF- IF
Power
Audio
Beta Range
2-50
10-100
40-400
Leakage current or ICBO is probably one of the most important parameters of a tran-
sistor. This is the current that flows between the base and collector in the reverse
direction. This reverse current flow will cause the AC beta of the transistor to de-
crease and up~ set the circuit the transistor is used in, if it becomes excessive. 1l1is
leakage is similar to grid leakage in a vacuum tube.
Leakage current can increase with transistor age, especially if some slight im-
purities were left in the transistor at manufacture. Leakage will cause the most
problems in circuits that operate at high and low temperatures, because leakage
current will increase with heat. The current will approximately double for each 10
degrees centigrade increase for germanium devices and for each six degrees centi-
grade increase for silicon devices.
B