The ratio of the current to base vs the current to the collector

I'm doing an experiment while reading the self-study book by Kybett "electronics self teaching guide" In the experiment I measure the current to the base and collector of a NPN transistor then observe that the ratio is basically constant and consistent with the manufacturer's specs.

But my ratio isn't constant. In addition I'm having trouble locating "beta" , the ratio on the datasheet. I see one mention of "beta=10" this is nothing like any of the values from my test.

Here are my data. What am I missing?

Rb=base resistance Vc=base collector voltage Ib= current base Ic=current collector

Rb                  Vc            Ib                Ic            Ic/Ib
1Mohms         6.89 V          8microA            1.51mA         189
690kohms       6.25 V         12microA            2.17mA         197
470kohms       5.24 V         17microA            3.20mA         188
220kohms       1.89 V         36microA            6.57mA         182
194kohms       1.07 V         41microA            7.40mA         180
147kohms        .247V         54microA            8.24mA         152??
94kohms        .173V         85microA            8.31mA         97.76???
47kohms        .137V        171microA            8.35mA         48.83?????


What is going on?

• where did you see 10? you are looking at h_fe on the datasheet right? – user92289 Dec 2 '15 at 3:25
• it's in fig3, but I don't think that's the right place to look? – futurebird Dec 2 '15 at 3:27
• The figure means that the h_fe or beta is equal to 10 for that particular test. h_fe values are given under the electrical characteristics and the conditions for each value are given in the same row. – user92289 Dec 2 '15 at 3:31

1 Answer

For a collector current of 10 mA, the Electrical Characteristics state that the Hfe can be between 100 and 300 - you have about 190, so that seems fine to me.

In the last three lines of your table, the transistor is in or near saturation - the Emitter-collector voltage is very small, so the Hfe calculation is meaningless - the transistor can't possibly increase the collector current beyond about 8.3 mA.

Try the experiment again with a lower value collector resistor, or higher collector voltage, so you can get a higher collector current.