Successfully choose complementary bipolar transistors

摘要

For circuit designs that use complementary bipolar transistors, you sometimes need to sort the NPN and PNP transistors to have matching de-current gains (P). One example of a circuit requiring matching is the output stage of an amplifier. The circuit in Figure 1 shows a simple test fixture to achieve this match. Transistors Q_1 and Q_2 are the devices being tested to see if they are matched. In the test fixture, Q_1 and Q_2 share the same base current (I_B); since there is no additional path where the current can flow, no additional compensation is needed. Note, however, that β should be high enough that I_E≈I_C. With this detail in mind, resistors R_1 and R_2 should be equal.