The transistor was discovered in 1947 at Bell laboratories. Starting with 1960s, the transistor takes on a spectacular ascending ramp, and it continues to do so these days. Even more, the future looks very bright for transistors, since we are moving now towards the "printed" electronic circuits--this means, electronic circuits printed on paper, and other materials, using special semiconductor/conductor inks.
In this page are presented just a few instances of using transistors, some schematics, a few graphs, and a general classification. The structure employed is:
1. Biasing PNP and NPN Bipolar Transistors
2. Bipolar Transistors Functions
3. Biasing JFET Transistors
4. JFET Transistors Functions
5. Types of Transistors
The basic notions highlighted in this page are related to a few electronic design topics presented in the first part, Hardware Design, of LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN.
The Bipolar Junction Transistors (BJT) work in two modes:
1. as amplifiers
2. as digital switches in the saturation/cutoff states
Many designers do not understand this: bipolar transistors are current controlled electronic devices. Of course that we do need specific voltages to bias a bipolar transistor, except those voltages have the polarity, and the required magnitudes, according to the currents they need to generate.
That misunderstanding is nobody's fault, because there are many books where this issue is unclear and/or incorrectly presented. To start, let's analyze a few voltage biasing schematics, though please remember that all biasing voltages are generated by the needed currents. Transistors' graphs corresponding to the two functioning modes mentioned above are related to Ib, Ie, and Ic only, according to the formula:
Ie = Ib + Ic
Ie = emiter current
Ib = base current
Ic = collector current
To start, let's see how we saturate transistors. Please note: the saturation/cutoff digital states represent the only situation when transistors behave similar to voltage controlled relays. Unfortunately, this situation brings some confusion, therefore any analogy to transistors' "voltage control" should better be avoided.
We have seen the BJT saturation and the cutoff states (the particular digital states of transistor's functionality). That also defines bipolar transistors as being perfect current controlled relays--which is also their main function. Further, BJT transistors are DC logic elements, and they are also the very building bricks of all logic ICs (including processors).
It needs to be pointed out that bipolar transistors (BJT) have nicer switching characteristics compared to the MOS-FET transistors. Even more, the Isolated Gate Bipolar Transistors (IGBT) have the nicest switching characteristics of all transistors--no ringing, or the minimum amount of al transistors.
Now, bipolar transistors have been used, since they were invented, in analog circuits as amplifiers in linear mode. There are three main schematics used to wire bipolar transistors as amplifiers--they are presented, again, only for the NPN case. What we are looking for is:
1. voltage gain
2. current gain
3. power gain
The three most common schematics used are:
1. Common-Emitter, for voltage, current, and power gain
2. Common-Base, for voltage and power gain
3. Common-Collector, for current and power gain
The FET (Field Effect Transistor) is a high-input impedance (100 MOhms and better), low noise, voltage controlled, solid-state semiconductor device. The first FET discovered was JFET (Junction Field Effect Transistor) followed a few years later by IGFET (Isolated Gate Field Effect Transistor) which was later renamed MOS-FET (Metal Oxide Semiconductor Field Effect Transistor).
The MOS technology is very cheap and perfectly suited for mass production, therefore it is used in most ICs today. For hardware designers, however, FET are rather expensive to procure, and they may be easily damaged by a simple hand touch (by electrostatic voltages).
More problematic is biasing the FET transistors; therefore, we will try presenting a few schematics.
Before working with transistors (BJT or FET), you need to study their output curve. For that, you have to get their Data Sheets. Particularly to FETs, their output curve is fairly complex (not presented here). You need to get one, because it is possible there will be a few (unexplained) references to it.
FET transistors behave similar to voltage controlled relays.
Three schematics are commonly employed to bias N-JFET transistors:
1. Self Biased
2. Universally Biased
3. Two Power Supply
All FET transistors have three main functions. They are used as:
2. analog/digital switches
3. voltage-controlled resistors
If you want digital FET switches/relays in your application, please consider one of the biasing schematics presented above. The voltage-controlled resistor function is left for you to discover. Further is presented only the amplification function.
It is possible transistors are the electronic components coming in the wildest variety possible (this is, considering their technical characteristics, not their shape which is standard), which clearly indicates they are very much used. Above are presented only the BJT and the JFET ones. More or less, all other existing types of transistors are similar in functionality.
It is possible we will attempt to develop this "Types of transistors" topic one glorious day. Meanwhile, we do encourage you to read LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN. In addition to presenting all schematics you need to start working with dsPIC controllers (or with any other type of Microchip controller), this book presents 12 firmware and 7 software source-code applications, each of them being a practical working project, fairly easy to understand.
Please believe this: complete, working source code programs in a book is unheard of! Even more, the really exceptional aspect is, all firmware and software programs presented in LHFSD are the essence of simplicity, and truly logic. Always remember that it is the firmware/software that drives the hardware.
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Page last updated on: August 24, 2012
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