We have created the form of our DC model for our transistor in the last post, “The Basic DC Model for a NPN Transistor“. However, there are lots of unknown values that we need to determine to build a model for the actual transistor we are going to use. We need to know values for Iceo, Vo, and Beta for our active region and cut off models. We will also need to determine Rsat for the saturation model and that will require knowing a Vce and Ic for some point while the transistor is saturated. In addition, once we start designing we will need to know the Maximum voltages (Vceo,Vcbo, and Vebo) and current (Ic) the transistor can tolerate as well as the maximum power the transistor can dissipate before turning into sizzling silicon.

We will obtain that information from the transistor datasheet. I pretty much randomly picked a general purpose NPN transistor and chose a 2N3904. It will be necessary for you to download the datasheet for this transistor. Please download the Fairchild 2N3904 datasheet so we are working from the same piece of paper.

On the first page of the datasheet we have many of the values we are concerned about in the Absolute Maximum Ratings table in the middle of the page. Please note that all these values are given with the ambient temperature (Ta) at 25 °C. (77°F). This will be important as we estimate some of the other values.

The Absolute Ratings Table and The Thermal Characteristics Table will give us:

Symbol | Description | Value | Importance |
---|---|---|---|

V_{CEO} |
Maximum Collector Emitter Voltage | 40 V | Make sure we do not exceed this |

V_{CBO} |
Maximum Collector Base Voltage | 60 V | Make sure we do not exceed this |

V_{EBO} |
Maximum Emitter Base Voltage | 6.0 V | Make sure we do not exceed this |

I_{C} |
Maximum Continuous Collector Current | 200 mA | Do not exceed CONTINUOUSLY |

P_{D} |
Total Device Dissipation | 625 mW | Derate 5 mW/°C above 25°C |

Now that we have started reading the datasheet it is time to talk a little about the convention they use on some of those symbols. The three letters after the Voltage V symbol and later a current symbol stand for: the “from” terminal; the “to” terminal; and the condition of the third terminal. For example: V_{CEO} is the voltage From the C, collector, terminal to the E, emitter, terminal with the base open. This means the positive (+) lead of a voltage source connected to the C, and the negative (-) lead connected to the E.

The other thing to point out now is we have seen our first case where the temperature is important because of the derating of the power dissipation value above 25°C.

These values are not part of the actual models, but they will be important once we start designing.

On the second page of the datasheet we start finding values to put in the actual model. The first value we find is I_{CEX}. This is the value we call I_{CEO} and is 50nA. Now you will start to see how imprecise this whole process is. To calculate Rsat for the saturation model we need V_{CE(sat)} and I_{C} and we use Ohms law to calculate Rsat. We have two values to choose from: Rsat = 0.2/.01 = 20 or Rsat = 0.3/.05 = 6. I would choose the 2nd value because it is probably more near where I would actually be operating the transistor if I was using it in saturation. If you look on page 3 at graph at the upper right corner there could be many points to calculate the value.

About now you are probably asking the question: “How the heck am I going to calculate anything meaningful if the ‘fixed’ values of my model are not fixed at all?” To that I will reply; Remember first that this is not our final model, we will have a much better model to use for the final calculations. Second, Our goal is to get an understanding of the various things affecting the transistor and wading through this inconsistent model variables is actually helping us develop that appreciation.

Now the bad news… it will get worse before it gets better.

The next value we need to determine is Vo. Vo is not actually shown on the datasheet. However, on page two we have a value called V_{BE(sat)} and the middle graph on the right side of Page 3 shows a graph of V_{BE(on)} for various temperatures and collector currents. For silicon transistors we are always going to assume a value of Vo of 0.65V.

The final value we need to know is Beta,(β). On datasheets this is calle h_{FE} and can be found on the second page of the datasheet. Things just went from bad to worse. I see six different values we could choose. My “gut feeling” is to choose the value at the point where they display both a maximum and minimum value. So, for Beta I chose a value of 200 because it was half-way between the 100, minimum, and 300, maximum values.

For those of us that build things and spend a long time in the lumberyard making sure the wood we choose is straight, working with a device that can have a critical number anywhere between 100 to 300 is not leaving a good feeling. It will be Ok..eventually. We will come up with a work-around. However, first we will put our model to use in a simple circuit to practice what we have learned. The model is drawn in the normal symbols in the first picture and the last picture shows the model drawn using Qucs.

I am always making the analogy of baby-steps. We have taken our first 3 or 4 baby steps. Our legs are wobbly because our “tolerance for ambiguity” is saying “this stuff ain’t gonna get it”. But it will…. trust me.

Gary

Fairchild is a trademark name and the datasheet is copyrighted by Fairchild Semiconductor Corporation.

“Reading a Datasheet for our DC Transistor model parameters” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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