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3 - THEORY
MODEL 5000 SERIES MANUAL
Version 1.23
3-5
3.4 Oscillation Suppression
3.4.1 5000 Series Mainframe
Oscillation is a particularly difficult problem for a test system that operates over a
broad spectrum of devices and power levels. Transistor current gain is probably the
most difficult parameter to make stable over a broad range of collector currents. A
TIP29 transistor may be perfectly stable at 200mA while a 2N3662 may have a
tendency to oscillate for gain measurements at a collector current of 8mA.
Precautions have been taken to minimize these effects in the 5000 Series Tester. In
general, transistors with current gain tests made at 50mA or greater will not require
stabilization components at the fixture. Gain tests made at 10mA or less may require
stabilization at the fixture. All test fixtures supplied contain a series toroid inductor in
series with the base (G) lead with a resistor bypass (see fixtures parts list). TO-5
fixtures have higher toroid inductance than in-line fixtures. It may be necessary to
increase the inductance in series with the base by as much as 100µH. Bypassing
the fixture with a capacitor up to 1000pf from collector to emitter or from base to
emitter may help reduce unwanted oscillations. These measures are most effective
for low current, high frequency devices and are not generally required for higher
current devices.
Check for oscillation using a scope attached at the collector of the device. Attach the
scope ground to the jack below the power switch.
Do not attach scope
ground to any pin of the device in test
.
Run an exact value
measurement of h
FE.
Run in the repetitive mode. The test takes half a second to run
and the collector can be observed to determine the end of the test. There should be
no oscillations at the end of the half-second test cycle. Oscillations may occur prior
to the end of the cycle as the device is overdriven and the systems zeroes in on the
final value. This procedure is generally necessary only for low current, high
frequency devices.
When using the test system with a handler, the same general precautions should be
observed. Do the bypassing when required at or near the device-under-test.
Thyristors and 2 lead devices seldom require special treatment. Attach and test.
Bypassing or other oscillation suppression is not generally required.
3.4.2 Lo Current Deck
If you have a Lo Current Deck (5000 Series L), a general precaution against
oscillation is to attach a capacitor at the R
GK
terminals of the Lo Current Deck. To
activate connection of the capacitor from the base to emitter, program R
BE
external.
The h
FE
program provides this R
BE
external entry.
ANET and GNET can also be used to attach external networks. These networks are
attached to the front panel whether a Lo Current Deck is used or not. The R
BE
and
R
GS
external networks are connected at the Lo Current Deck when a Lo Current Deck
is used.