Agilent Technologies B1500A Training Manual page 102

Gummel Plot With
Base-Collector Self Oscillation
This is a common occurrence when high frequency bipolar transistors are tested with jumper lead
connections. When RF couples in the air to the base lead, there can be enough DC rectification to
increase the base current bias. This causes positive feedback. At some point along the curve, there
will be enough feedback to abruptly turn on the transistor (steep increase in base and collector
current).
Separating the leads can help marginally. Threading one or two ferrite beads onto the base jumper
lead usually solves the problem. Use ferrite beads with any type of 4145, 4155, 4156 jumper lead.
Using the beads on a jumper lead involves prying open one end of the plastic connector. The
connector is temporarily unsoldered from the lead to slip on several beads.
For severe oscillation, the RF transistor must be tested in an RF fixtured designed to maintain RF
isolation between input (base) and output (collector) ports.
Fix by isolating
base-collector leads
Fix by using ferrite
bead on base lead
6-30
Module 6
Low Current Measurement