![]() The gain is not strictly a constant, but it can vary. The DMMs usualy measure Hfe as the large signal quantety and not hfe als the small signal parameter. I think you will find that hFE values are to be interpreted carefully. And if you have various transistors of the same type, it is also interesting to repeat the above test with a number of them. Then note the corresponding collector currents, and calculate Ic/Ib=hFE You may be surprised of the variation. Now adjust the power supply voltage so that the base current ranges say from 0,2mA upwards to 0,7mA or so. When measuring a power transistor like a 2N3055 you need lower values for the resistors: I took Rc=270ohms/5W, Rb=27K and emitter directly to GND. In addition, I measured the hFE using a TC1 component tester, and I can say the value shown by this tester was way out of line ! I would encourage you to do the same tests as well. Then for these various values, divide de collector current value by the base current value, to obtain the hFE You will find that hFE will vary with the collector current (as was already stated by XFDDesign). When you vary the base current from 10♚ -> 20♚ -> 30♚ -> 40♚ -> 50♚ by increasing the supply voltage, note down each time the corresponding collector current. For a BC547, you can use a Rc=2K7, Rb=390K and Re=56ohms. It is very interesting to put a transistor in a simple test configuration using 3 resistors and an adjustable power supply, and measure simultaneously the collector current and the base current. This allows the 10 ohm shunt to remain connected on the "low" side, requiring nothing extra in the way of switch contacts. Technically for the NPN configuration, it would be wrong to measure the emitter current, but with only 10uA of base current, and a 10 ohm shunt, the error is only one count on the display. So the reading on the meter represents the approximate ratio of collector current to base current at a 10uA base current level. The voltage across this shunt is fed directly to the 200.0mV full-scale input of the ADC, but with the decimal point turned off. A 10 ohm shunt to ground is used in the emitter leg for the NPN socket, or the collector leg for the PNP socket. This regulated voltage, along with a 220K base resistor, yields a base current of about 10uA for a silicon device with a 0.6V Vbe junction voltage. The ICL7106 clone chips in a basic 2000-count DMM have a built-in voltage reference that regulates the analog ground at 2.8V below the positive battery terminal. To add a little detail to Sean's answer, the hFE test on a a typical cheap DMM uses one socket and three basic 5% resistors.
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