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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.

    clipboard_eded671b9989805a5866eea6454cb6dd2.png

    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 \(\PageIndex{1c}\).

    clipboard_e5a8a904fe49673248494aaae61a74253.png

    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.

    clipboard_ed1df67aadfe58b0e110ac526d913a6a7.png

    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.