Ultralinear UHF power transistors for CATV applications

An investigation of the cross-modulation performance of UHF power transistors and its correlation with the gain parameter curves S
21
(I
C
, V
CB
) shows that such transistors today exhibit an inherent linearity limitation and are not optimized for ultralinear applications. This means that using higher power transistors does not result in a larger output signal with low cross-modulation distortion. Presented measurements clearly demonstrate that the nonlinear products increase, not only with increasing emitter dc current, but also with collector voltage above certain limiting values I
CL
and V
CL
, respectively; I
CL
and V
CL
are relatively small compared with the corresponding maximum permissible values of I
C
and V
CB
. Their existence can be explained by current crowding effects, by the space-charge limitation of the collector current, by the nonuniform thermal conductance, by the temperature gradients, and by the unequal current density distribution across the (large) active areas of power transistors. A correlation between the V
C
gain parameter falloff effect and the "thermal distortions," has been established. Design and selection criteria for ultralinear transistors needed in CATV amplifers are given. A simple linearity test method is described.