3.4: BJT Data Sheet Interpretation

The data sheet for a common NPN transistor, the 2N3904, is shown in Figure $\PageIndex{1}$. This model is available from several different manufacturers. First off, note the case style. This a TO-92 plastic case for through-hole mounting and is commonly used for small signal transistors. Under the maximums we find the device has a maximum power dissipation of 625 mW in free air (ambient temperature of 25$^{\circ}$C), a maximum collector current of 200 mA and a maximum collector-emitter voltage of 40 V. Obviously, the device cannot withstand maximum current and voltage simultaneously.

Figure $\PageIndex{1a}$: 2N3904 data sheet. Used with permission from SCILLC dba ON Semiconductor.

In Figure $\PageIndex{1b}$ we find a variety of characteristics including nominal values for $\beta$ (listed here as $h_{FE}$) under various conditions. At particularly small or large collector currents $\beta$ tends to drop off. Also, note the wide 3:1 variance at 10 mA. Perhaps more illustrative are the graphs from the third page, Figure $\PageIndex{1c}$.

Figure $\PageIndex{1b}$: 2N3904 data sheet (cont).

The upper-most graph depicts the variation of $\beta$ with both collector current and temperature. The normalized $\beta$ is plotted on the vertical axis. That is, this is not the expected value but is a ratio used to compare $\beta$ under varying conditions.

Figure $\PageIndex{1c}$: 2N3904 data sheet (cont).

For example, at room temperature and 10 mA, the normalized value is 1.0. The second page indicated a range of 100 to 300 for the 2N3904's $\beta$ under these conditions. Let's say we measure one particular transistor to have a $\beta$ of 200. If we were to operate this transistor at a lower current, say 0.2 mA, the $\beta$ would drop. From the graph, the normalized $\beta$ value at 0.2 mA and 25$^{\circ}$ C is 0.7. Therefore, the $\beta$ under these conditions would be 0.7/1.0 $\cdot$ 200, or 140. The graph also shows that, generally speaking, $\beta$ tends to increase with increasing temperature.

The middle graph plots the collector-emitter saturation voltage, or $V_{CE(sat)}$, for various current conditions. This is an important parameter when dealing with transistor switching circuits. We shall refer back to this graph a little later in this chapter.