Successfully choose complementary bipolar transistors

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For circuit designs that use complementary bipolar transistors, you sometimes need to sort the NPN and PNP transistors to have matching dc-current gains (β). 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.

Figure 1This circuit makes it easy to test and match the current gain of complementary bipolar transistors. Matched transistors will cause the voltmeter to read 0V.

Transistors Q₁ and Q₂ are the devices being tested to see if they are matched. In the test fixture, Q₁ and Q₂ 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 R₁ and R₂ should be equal.

To give the transistors a bit more headroom, an additional voltage drop is introduced between the transistors’ base connections. A voltage differential of a few volts is desirable, so a blue LED is a good choice for D₁. Its presence helps to set the base voltage for Q₁ (V_B1) to about half of the supply voltage (V_S). Using an LED in the place of D₁ is preferable to using a zener diode due to the sharper knee at the low currents. Moreover, you can see the glow of many blue LEDs at currents below 10 μA; the glow indicates the presence of base current, which means the circuit is working properly. Equation 1 is used to determine the needed supply voltage:

A typical blue LED will have a forward voltage of about 3.5V; assuming V_BE1=V_BE2=0.7V, you get a value for V_S of about 9.8V.

Resistor R₁ sets the emitter current of Q₁; it is calculated using Equation 2:

Figure 2 For a simpler version, replace the voltmeter with inverse-parallel-connected red LEDs.

You should select an emitter current that matches the application in which the transistors will be used, because beta varies with emitter and collector current. With matching transistors (β₁=β₂) installed in the test fixture, the voltage drops across R₁ and R₂ are equal, and the voltmeter will show 0.

The circuit in Figure 2 is functionally equivalent but uses a simpler method to indicate when the circuit is in balance. With matched gains, neither of the red LEDs (D₂ and D₃) will be on.