BJT Transistor hFE Test Fixture


Circuit Description:

The text fixture above measures the ratio of collector current (Ic) to base current (Ib) of bipolar transistors. A 3 position switch selects one of 3 base current conditions, 1mA, 0.5mA and 0.025mA. Collector current is measured with a DMM connected across points A and B, and the hFE or gain factor is determined by multiplying the DMM current reading by a factor of 1, 2, or 4 depending on the switch setting. For example, when the switch is in the X1 position, the base current will be about 1mA and so the hFE value will be the DMM reading directly. When the switch is in the X2 position, the base current will be half a much, so the DMM reading is multiplied by 2, or 4 if the switch in the 4X position. The supply voltage should be close to 9 volts for fairly accurate measurements. Also, the transistor should be removed from the fixture after each test to avoid excessive battery drain.

Base current is established with a series resistor which drops the supply voltage minus the emitter to base voltage or about 9 - 0.6 = 8.4 volts. The resistor value is worked out by dividing the resistor voltage (8.4) by the desired base current. If the desired base current is 0.25 mA, the resistor value will be 8.4/.00025 = about 33K which corresponds to the center open switch position and labeled X4. When the switch is in the X2 position a second 33K resistor is added in parallel for twice as much current. In the X1 position a 11K resistor is added in parallel with one of the 33K resistors for a total resistance of 8.25K which yields a base current of about 8.4/8.25K = 1.02mA. The 51 ohm resistor is used to avoid blowing the DMM fuse (as I did) in case the DMM is accidently connected across the supply while measuring current. The 2 diodes are used so that either NPN or PNP transistors can be tested and the collector voltage adjusted with the pot without the need of a second switch. The supply connections need to be reversed for testing PNP transistors. The 1K pot in series with the collector allows adjustment of the collector voltage so that readings can be made using various values. For example, the data sheet of the 2N3904 shows a hFE of 100 minimum at 10mA collector current and 1 volt collector voltage. If the transistor is used in a switching application, the collector voltage will be lower at around 200 millivolts and the hFE was measured to be 52 at 13mA. The collector voltage was adjusted to 200 millivolts using the 1K pot with the DMM measuring voltage across points C and D. A few measurements made are listed below.


Transistor   Multiplier     Collector     Collector      hFE
                             Voltage     Current (mA)

2N2219A        X1            2            125            125
NPN            X1            0.2          84              84
               X2            5.5          65             130
               X2            0.2          46              92
               X4            6.6          30             120
               X4            0.2          26             104

2N2222A        X1            1.1          153            153
NPN            X1            0.2          98              98
               X2            3            113            226
               X2            0.2          64             128
               X4            5.4          67             268
               X4            0.2          41             164

2N3906         X1            2.9          101            101
PNP            X1            0.2           43             43
               X2            5             65            130
               X2            0.2           28             56
               X4            6.6           34            136
               X4            0.2           18             72

2N3904         X1            3.4           94             94
NPN            X1            0.2           34             34
               X2            4.8           65            130
               X2            0.2           21             42
               X4            6.2           39            156
               X4            0.2           13             52

2N3053         X1            1.5          130            130
NPN            X1            0.2           71             71
               X2            4.5           69            138
               X2            0.2           38             76
               X4            6.4           34            136
               X4            0.2           19             76

2SA1015        X1            2.2           119           119
PNP            X1            0.2            50            50
               X2            4              80           160
               X2            0.2            35            70
               X4            5.6            49           196
               X4            0.2            26           104

2N4123         X1            3.9            82            82
NPN            X1            0.2            46            46
               X2            5.1            59           118
               X2            0.2            30            60
               X4            6.4            34           136
               X4            0.2            18            72

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