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Electronics-I Lecture-3

Electronics-I Lecture-3. A discrete diode is a simple PN Junction , enclosed in a case and connections for the interfacing. The positive side(P-Type part) of the diode is called anode(A) and negative side(N-Type part) , cathode(K) .

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Electronics-I Lecture-3

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  1. Electronics-ILecture-3

  2. A discrete diode is a simple PN Junction , enclosed in a case and connections for the interfacing. • The positive side(P-Type part) of the diode is called anode(A) and negative side(N-Type part) , cathode(K). • A diode can have three models (a representation of a circuit with alternative circuit components whose combined behavior closely resembles the actual device) • Ideal Model(A forward biased diode is replaced by a short circuit and reverse biased with an open circuit) • Practical Model(A forward biased diode is replaced by a battery of 0.7 V for Si diode and 0.3 V for a Ge diode and reverse biased with an open circuit) • For this course we will only be using practical diode models. Semiconductors Diode

  3. An ideal diode acts like a switch. • When it is forward biased it allows current through it while voltage across it remains zero. This translates to Zeroresistance or a Short circuit. • When it is reverse biased it allows no current through it while the voltage across it increases linearly with applied voltage. This corresponds to Infinite resistance or an Open circuit. • An ideal diode is a model to which practical devices are compared to for performance. Ideal Diode Characteristics

  4. A practical diode can’t have zero volts across it when it is conducting. It requires a voltage of 0.7 V in case of Si and 0.3 V for Gi diode. • In practical models the short circuits of ideal diode models are replaced by batteries of 0.7V and 0.3V for Si and Ge diodes respectively. Practical Diode Model

  5. In real diodes, the forward biased diode voltage does not remain fixed when applied potential is increased. It increases with the applied voltage a little. • This increase in potential is due to • Semiconductor body resistance • Contact resistance • Consequently, as the applied voltage is increased, the resultant increase in current increases the voltage across the device in accordance with of Ohm’s law (V=IR) • Similarly, when a reverse voltage is applied across the real diode, the current across it is not zero. There is always some current flowing due to minority charge carriers and is called the reverse leakage current. Real DiodeModel

  6. Real DiodeModel

  7. For real diodes, the current equation is given by, • ID=IS(ekvd/T-1) • Where Is= reverse saturation current • k=11,600/ when =1 for Ge and high current values in Si • =2 for small values of current in Si • Vd= voltage across diode • T=Tc+273 Current equation and V-I characteristic curve of Real Diode

  8. In essence there are couple ways to solve circuits containing diodes. • Using Load lines and characteristics curve • Using diode equivalent t models only • The easiest way to solve diode circuits is by using the second technique and it follows these steps • Check whether the diode is forward biased or reverse biased in a particular circuit • Replace diodes with hypothetical resistors • Find the direction of current through these resistors • If conventional direction of current is positive(in the direction of arrow) and the applied voltage is greater than diode forward biased voltages, the diode is forward biased else the diode is reverse biased. • If the diode is forward biased replace it with a battery of 0.7V for Si and 0.3V for Ge diode. If it is reverse biased open circuit it. • Solve the resultant circuit using circuit analysis techniques. How to solve circuits with diodes

  9. Example-1 Example-4 • Example-2 Example-5 Repeat example- 1 with diode reversed. • Example-3 • Example-6 Series diode circuits

  10. Example-1 Example-4 • Example-2 • Example-3 Parallel and series-parallel diode circuits

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