ELECTRONIC DESIGN NOTES #7
Transistors
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| The transistor was discovered in 1947
at Bell laboratories. Starting with 1960s, the transistor takes on a spectacular development ramp, and it
continues to do so. Even more, the future looks very bright for transistors, since we are moving towards the
"printed" electronic circuits--this means, electronic circuits printed on paper and other materials,
using special semiconductor and conductor inks. In this page are presented just few instances of using transistors, few schematics, some 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 NOTE The basic notions highlighted in this page are related to electronic design topics presented in the first part Hardware Design of Learn Hardware Firmware and Software Design. |
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Bipolar Junction Transistors (BJT) work in two modes: 1. as amplifiers 2. as digital switches in saturation/cutoff states Many designers do not understand this: bipolar transistors are current controlled electronic devices. Of course, we need specific voltages to bias a bipolar transistor, but those voltages have the polarity and the required magnitude 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 those voltage biasing, though remember that all biasing voltages are generated by the needed currents. Transistors graphs corresponding to the two functioning modes 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 mode of functioning is the only situation when transistors behave similar to voltage controlled relays. This situation brings some confusion, therefore any analogy to voltage control should better be avoided.
There are few good methods to bias the transistor as they are listed further down, though only
for the NPN transistor. For PNP you should reverse polarities.
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We have seen the saturation and cutoff states (the digital mode). That defines bipolar transistors as being perfect current controlled switches, which is their main function. That also makes them DC logic elements, therefore transistors are the very building bricks of all logic ICs (including processors). It needs to be pointed out that bipolar transistors (BJT) have nicer switching characteristics than the MOS-FET ones. The Isolated Gate Bipolar Transistors (IGBT) have the nicest switching characteristics--no ringing, or the minimum amount possible. 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 article "Driving Automotive Injectors" (A25 in Amazing Articles) presents few topics about using power
transistors. |
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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 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 (electrostatic voltages). More problematic is biasing FET transistors; therefore, we will try presenting few schematics. ATTENTION Before working with transistors (BJT or FET), you need to study their output curve. For that you have to get their Data Sheet. Particularly to FET, their output curve it is fairly complex (not presented here). You need to get one, because it is possible there will be few (unexplained) references to it. ATTENTION 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
A practical way to work with FET transistors is, always test their output curve before using
them. FETs are third order semiconductors and it is not easy to control them. First, decide on using one or two
FET functions, for example switching and variable resistors, and then use a simulator program, or a
test stand to discover the right values need for the biasing resistors.
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All FET transistors have three main functions. They are used as: 1. amplifiers 2. analog/digital switches 3. voltage-controlled resistors If you want digital FET switches 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.
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It is possible transistors are the electronic components coming in the wildest variety possible, which clearly indicates they are very much used (and needed). 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 develop this "Types of transistors" topic one 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 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! The really exceptional aspect is, all firmware and software programs presented in LHFSD are the essence of simplicity, and incredibly easy to understand for beginners. Always remember that it is the firmware/software that drives the hardware. |
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comments regarding this page using support@corollarytheorems.com Page last updated on: March 15, 2008 © Corollary Theorems Ltd. All rights reserved. |
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