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ME 140L - Prelab 6

# ME 140L - Prelab 6 - PRELAB 6 DIODES AND TRANSISTORS 1...

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PRELAB 6 DIODES AND TRANSISTORS 1. OBJECTIVES square4 Understand the characteristics of ideal and real diodes. square4 Learn how diodes can be used in real applications. square4 Understand the basic operating principles of BJT transistors. square4 Use transistor equations in real applications. 2. GENERAL NOTATIONS C: Capacitance (Farads) I: Current (Amperes) R: Resistor (Ohms) V: Voltage (Volts) 3. DIODES 3.1 OVERVIEW The diode is the simplest non-linear circuit element in electronics: it switches ON and OFF, depending on the polarity of the voltage across it. The term “non-linear device” comes from the diode’s V-I characteristics. A diode is a two terminal device composed of a slab of p-type (holes) material and a slab of n-type (electrons) material put together. The behavior of the diode is determined by the particle-level interactions of the p-type and n-type material under an applied voltage. We will not go further into explaining the inner workings of this electronic component. There are two modes of operation of the diode: a forward-biased (positive applied voltage) and reverse-biased (negative applied voltage). We will take for example the ideal diode: if the voltage across the diode is positive, the diode acts as a short circuit. If, however, the voltage across the element is zero or negative, then there is no current flowing and the diode behaves as an open circuit. In simpler terms, the diode is a one- way street. This can be explained much better by the figures below. + V - - V +

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As you can see from the V-I characteristics of an ideal diode, there is no voltage drop or resistance associated with the ideal diode: it is simply a switch. For V ≤ 0 , the current is 0 , meaning that the diode is acting as an open circuit. At V > 0 , the diode conducts at the rated current. These are the basic rules for the operation of the ideal diode. However, as most of you probably have found out by now, nothing in this world is perfect, and that goes for electronic components as well. The ideal diode is a good stepping stone to understanding the operation of a real diode, but it will not be used in any of the circuits you are going to encounter in the real world. The equivalent circuit of a real diode is shown below, along with its V-I characteristics. diode ideal_diode 0.7 V Rd
As you can see from the figures above, the real diode is represented by an ideal diode in series with a resistor R d and 0.7 Volts voltage drop. The resistance R d is usually so small that it is neglected during circuit analysis. Notice, from the V-I characteristics of the diode, that the forward current of the diode does not start to increase until the voltage reaches that 0.7V mark. This means that in order for a diode to conduct current, the voltage across it has to be positive and it has to be greater than 0.7 Volts . In other words there is a voltage drop of 0.7 Volts. Another difference one can see between the ideal diode and the real diode is that at a high enough negative voltage, called the breakdown voltage, the diode conducts current the opposite way. As

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