Hi,

I need some help understanding the beta (hFE value of a transistor)

I am using a 2N 3904 transistor for an experiment to determine the beta value by observing the base current and collector current while changing the base resistor value.

VS was 9v. (using a 9v battery)
RC: 1K

RB BETA (Calculated using IC/IB)
1M 246.3
900K 242.85
800K 239.2
700K 230.5
600K 220
500K 211.76
400K 213.46
300K 321.56
200K 215.24
100K 108.4
50K 56.22
33K 37.377

Generally, the beata value I achieved is around 210-240 region. However, I also achieved a value of 321 when RB was at 300k. at 200k, the beta got back to 215.

At 100k and below, the beta value dropped dramatically.

I do not understand how the beta value can vary, I originally thought it to be a constant value if operating within the linear range of the amplifier.

Kindly advise.

Thank you.

 

Have a look at a typical datasheet: http://pdf.datasheetcatalog.com/datasheet/fairchild/2N3904.pdf

Page 3, first graph. \(h_{FE}\) is anything but constant. Moreover, these results are the main trend of a sample set, not all produced devices, from each manufacturer.

 

hfe is constant for small-signal AC applications where the signal level is not large enough to significantly change the bias point. But it will vary from unit to unit of the same type, due to manufacturing variations.

 

I encourage you to repeat your measurement. Because you use Rc resistor the collector current cannot be greater than
Ic_max ≈ 9V/1KΩ = 9mA So for low Rb resistor value smaller than 200kΩ your BJT could enter saturation region. For example for RB = 100K base current is equal to Ib ≈ (9V - 0.7V)/100K ≈ 83μA and if beta equal to 225 the Ic = 18.7mA. But you cannot measure this because your Rc resistor limits Ic to 9mA max.
So repeat your measurements without Rc resistor.

 

Aaronaxel, two questions:
1.) How did you calculate Ib?
2.) It is a known fact that the (small signal value of) beta slightly depends on the selected bias current (DC).

 

Yup! That's transistors for you. And that is what spec sheets are for. Tells you the test conditions and parameter range. Did you take that into account?