6158C_Unit4 - UNIT 4 Power Conditioning and Amplifying The...

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Page 1 Version 1.09 UNIT 4 Power Conditioning and Amplifying: The basics of power regulation and operational amplifiers. Your Name Date of Submission CHEMISTRY 6158C Department of Chemistry University of Florida Gainesville, FL 32611 (Note: Much of the material in this handout was rewritten/updated in 2001 by graduate student Andrew K. Ottens.)
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Page 2 Version 1.09 Introduction: This unit will focus on power conditioning used in power supplies, and power amplification provided by the operational amplifier. Conversion between AC and DC power requires many components, which will be examined in greater detail. A major step in converting an AC waveform into a non-varying DC output is the use of diodes. As discussed in lecture, diodes are most commonly made from semiconductor materials. In lecture we explored the idea of p-n junctions, and discussed how a p-n junction when forward biased will allow for large current conduction, but when reverse biased allows no significant current to flow. The actual current flow when forward biased is described by the Shockley equation, ) 1 e ( I i kt / qV i = , where I i is the intrinsic current, and is minimal, q is the charge on an electron, V is the applied voltage, and T is temperature. From the Shockley equation it is possible to see the resulting exponential current characteristic of a diode in forward biased mode. To put it simply, diodes are semiconductor devices that generally pass current in one direction (forward bias) and not the other. Some specialized devices, such as photo diodes, require that the diode be reversed biased. Light emitting diodes, on the other hand, will produce a cascade of electrons being demoted from excited energy states when forward biased. The higher the energy state, the more energetic the photons released when electrons fall back down. This leads to the various colors of LEDS from red to yellow to green, and now even blue (colors in order of increasing photon energy). Transistors are the heart of most electrical circuits today. There are a variety of transistors; bipolar junction (BJT), junction field effect (FET), metal oxide field effect (MOSFET) are the most common classifications. Symbols for the various transistors and a description of their operation can be found in Supplement 3 of the textbook. A number of you would benefit from knowledge of transistors and their use in electrical circuits. Therefore, it is strongly suggested that you read through Supplement 3. There you will find detailed information on how transistors are used in various circuits. For example, on page 399 the internal components of an LF351 op amp are shown. You will use this op amp later on in this unit, and reading the text will help you understand the working of operational amplifiers. One idea you should keep in mind, which is further explained by looking at the op amp circuit, is that the output of an op amp derives its power from the power terminals of the chip. In the case of the LF351, very little power is taken from the two inputs due to the characteristic high impedance of the gate of a JFET.
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