How to Interpret Transistor Data
Transistors are semiconductor devices with at least three terminals, where a small current or voltage through one terminal is used to control the current flow through the others. They may be viewed as electronic valves. Their primary uses are as switches and amplifiers.
Transistors come in several types, with bipolar ones being the most popular. Bipolars have either NPN or PNP layers, with a lead attached to each one. The leads are the base, the emitter, and the collector. The base is used to control the current flow through the other two.
Transistors are modeled as back-to-back diodes. For an NPN, the base-emitter behaves as a forward-biased diode, and the base-collector behaves as a reverse-biased diode.
To create basic circuits, it is not necessary to understand all of the technical details described on a transistor's data sheet. The characteristics that should be studied include the gain, the dissipation, and the maximum ratings. These are usually found on the transistor's package, on the data sheets from the manufacturer and in some electronics textbooks or handbooks.
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Things You'll Need
- One 2N3904 or NPN transistor
- Transistor data sheet
- Introductory electronics text
Instructions
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1
Read the generalized description of the transistor. The transistor's main function will be described as that of amplifying, switching, or both.
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2
Read the dissipation rating of the device. Transistors are described as power or small-signal. Power transistors typically can dissipate a watt or more of power. Small-signal ones, in contrast, dissipate less than a watt. For example, the maximum dissipation for a 2N3904 is 350 mW (milliwatts), so it is properly characterized as small signal.
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3
Study the parameter Hfe, which is the current gain. It is a gain because a small signal at the base produces a much larger signal at the collector. Hfe has minimum and maximum values, though both may not be listed. For the 2N3904, Hfe minimum is listed as 100. The formula is Icollector = Hfe*Ibase. Therefore a base current of 1 volt yields a minimum of 100*1 V = 100 volts at the collector. In some texts, Hfe is known as Beta(dc).
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4
Examine the maximum breakdown voltages. The breakdown voltage is where the transistor will stop operating or be destroyed if it is given an input voltage of that amount. In reality, transistors should not operate near the breakdown voltages as their lifespan will be shortened. Vcb is the voltage between the collector and the base. Vceo is the voltage between the collector and the emitter with the base open, and Veb is the voltage from the emitter to the base. The Vcb breakdown voltage for the 2N3904 is listed as 60 V. The remaining values are 40 V for Vceo, and 6 V for Veb. These are amounts to be avoided.
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Review the maximum current ratings. The maximum current the collector can handle is Ic, and for the 2N3904 this is given as 200 mA. Note that the ideal temperature for transistors is specified or assumed as room temperature or as not to exceed 25 C.
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6
Summarize your findings. For some 2N3904 transistors at room temperature with a collector current of less than 200 mA, and where the power rating is not exceeded, their gain will be as low as 100 or as high as 300. Most 2N3904 transistors, however, will have a gain of 200.
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Tips & Warnings
The data sheet for PNP transistors will have characteristics similar to that of NPN ones.
References
Resources
- Photo Credit Arnold Reinhold
