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EE 3114 Lab 2 Semiconductor Diodes Rev A

EE 3114 Lab 2 Semiconductor Diodes Rev A - EE3114...

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EE3114 Fundamentals of Electronics I Lab 2 Rev A 09/28/10 © Polytechnic Institute of NYU EE3114 EXPERIMENT 2 Circuit Applications of Semiconductor Diodes I. Introduction Semiconductor junction diodes find numerous applications in electronic circuits. The large asymmetry in forward and reverse current-voltage characteristics explored in the previous experiment facilitates such large-signal applications as the voltage clipper, the voltage clamper, peak envelope detectors, various rectifier applications including the voltage doubler and digital logic circuitry. The reverse breakdown characteristic of the voltage reference, diodes is employed in voltage regulator, overvoltage protection and dc level shifting circuitry. The nonlinearity in the forward characteristic finds application in analog multiplier, mixing and harmonic generation devices. Finally, the voltage variable depletion capacitance characteristic of the pn junction finds use in frequency modulation and automatic frequency control circuit applications. Unfortunately, the applications are so extensive that a single laboratory session is inadequate for full coverage; hence, only a few of the large signal applications will be treated. IL Background An approximate large-signal piecewise-linear model of the semi-conductor diode was considered in the previous experiment. The three parameters needed for this characterization are (i) The forward resistance r f , typically in the range of ohms. (ii) The back resistance, r b , usually quite large. say ohms × 10 5 (iii) The cut-in or threshold voltage, V γ , typically 0.2-0.3V for germanium and 0.5-0.8V for silicon. In certain large-signal applications, signal voltages may be so large that the threshold appears to be negligible, while the circuit resistance in series with the diode is of such a magnitude that the resultant diode drop in the forward direction is ignorable and the
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EE3114 Fundamentals of Electronics I Lab 2 Rev A 09/28/10 © Polytechnic Institute of NYU reverse saturation current produces a very small drop across any series resistance, say R S when the diode is in the non-conducting state. These conditions may be stated succinctly as: γ V V signal >> , And b S f r R r << << Provided the above conditions apply, a simplified analysis may be made, usually with good results, in which the diodes of the circuit are considered to be "ideal," that is, they may be thought of as satisfying the conditions: 0 = γ V , 0 = f r , and b r . In graphic form, the current-voltage characteristic of the ideal diode is illustrated in Figure 1. Figure 1. Current-voltage characteristics of the ideal diode The circuit analysis is simplified as only two diode states need be considered, namely (i) The conducting (forward) state - no forward voltage drop regardless of the forward current and (ii) The non-conducting (reverse) state - no reverse current regardless of the magnitude of the reverse voltage.
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