# 4.4: BJT Data Sheet Interpretation

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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.

This page titled 4.4: BJT Data Sheet Interpretation is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by James M. Fiore via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.