Engineers teach you how to successfully screen complementary bipolar transistors to design circuits

For circuit designs that use complementary bipolar transistors , it is sometimes necessary to screen NPN and PNP transistors that match their DC gain (β) . An example of such a circuit that requires matching is the output stage of an amplifier. The circuit in Figure 1 shows a simple test setup that can achieve this matching.

Transistors Q1 and Q2 are the devices under test to see if they are matched. On the test equipment, Q1 and Q2 share a common base current (IB); since there are no other paths for the current to flow out, no additional compensation is required. However, it should be noted that β should be high enough so that IE≈IC. Another detail to remember is that resistors R1 and R2 should be equal.

To give the transistors some headroom, an additional voltage drop is introduced across the transistor base connections. A few volts is fine, so a good choice for D1 is a blue LED. This diode helps set the base voltage (VB1) of Q1 to about half the supply voltage (VS). Replacing D1 with an LED is better than using a Zener diode because it has a sharper knee at low currents. In addition, many blue LEDs can illuminate at currents of less than 10 μA; the light can be used to indicate the presence of base current, thus indicating that the circuit is operating properly. Equation (1) determines the required power supply:

The forward voltage drop of a typical blue LED is about 3.5V; assuming VBE1=VBE2=0.7V, the VS value can be about 9.8V. Resistor R1 sets the emitter current of Q1, which is calculated using formula (2):

The choice of emitter current should be appropriate for the intended transistor application, since the beta value varies with emitter and collector currents. When the test equipment is connected to matched transistors (β1 = β2), the voltage drops across R1 and R2 are equal and the voltmeter reads 0.

The circuit in Figure 2 has equivalent functionality but uses a simpler method to represent current balance. When the gains are matched, both red LEDs (D2 and D3) are off.