i (mA) 5 4.5 3.5 3 2.5 1.5 0.5 o 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 v(V) (a) Volt-ampere characteristic of a constant-current diode i (mA) 5 4.5 4 3.5 2.5 1.5 0.5 o 0.5 I 1.5 2 2.5 3 3.5 4 4.5 5 v(V) (b) Volt-ampere characteristic of a light-emitting diode (LED). (c) (d) Figure P9.20
Several types of special-purpose diodes exist. One is the constant-current diode for which the current is constant over a wide range of voltage. The circuit symbol and volt–ampere characteristic for a constant-current diode are shown in Figure P9.20(a). Another special type is the light-emitting diode (LED) for which the circuit symbol and a typical volt–ampere characteristic are shown in Figure P9.20(b). Sometimes, the series combination of these two devices is used to provide constant current to the LED from a variable voltage shown in Figure P9.20(c). Sketch the overall volt–ampere characteristic to scale for the parallel combination shown in Figure P9.20(d).
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