Page 97 - Troubleshooting Analog Circuits

a4 7. Identifying and Avoiding Transistor Problems

Figure 7.4. When using high-power amplifiers, there are certain problems you just never have if you use
a big-enough heat sink. This heat sink‘s thermal resistance is lower than 0.5 “CAW. (Photo
copyright Peggi Willis.)

Fabrication Structures Make a Difference

Another thing you should know when using bipolar power transistors is that there
are two major fabrication structures: the epitaxial base, and the planar structure pio-
neered by Fairchild Semiconductor (Figure 7.5) (Ref. 4). (See my comments a couple
paragraphs down concerning the obsolete single-diffused transistors.) Transistors
fabricated with the epi-base structure are usually more rugged and have a wider safe-
operating area. Planar devices feature faster switching speeds and higher frequency
response, but aren’t as rugged as the epi-base types. You can compare the two types
by looking at the data sheets for the Motorola 2N3771 and the Harris 2N5039. The
2N.5039 planar device has a current-gain bandwidth 10 times greater than the
2N3771 epi-base device. The 2N.5039 also has a switching speed faster than the
2N3771 when used as a saturated switch, but the 2N3771 has a considerably larger
safe area if used for switching inductive loads. You can select the characteristics you
prefer, and order the type you need . . .
But be careful. If you breadboard with one type and then start building in produc-
tion with the other, you might suddenly find that the bandwidth of the transistor has
changed by a factor-of 10 (or a factor of 0.1) or that the safe area doesn’t match that
of the prototypes. Also be aware that the planar power devices, like the familiar
2N2222 and 2N3904, are quite capable of oscillating at high frequencies in the
dozens of megahertz when operated in the linear region, so you should plan to use
beads in the base and/or the emitter, to quash the oscillation. The slower epi-base
devices don’t need that help very often.
When I first wrote these articles on troubleshooting back in 1988, you could still
buy the older “single-diffused’’ transistors such as 2N3055H and the old 2N3771. I