1, the ideal model - ideal switch

Ideal diode V & ndash; I characteristic as shown in (a) blue line, the dotted line represents the actual diode V & ndash; I characteristics. Figure (b) is representative of its symbols.

Figure (a) shows that in the forward bias, the pressure drop is 0V, and when the diode is reverse biased, that it's resistance is infinite, the current is zero. In the actual circuit, when the power supply voltage than the diode voltage drop is large, the method has been applied to the approximate analysis is feasible

2, constant model - the forward voltage drop of 0.7V (silicon tube)

The basic idea is that when the diode is turned on, its pressure drop held constant, and does not change with the current, typical value 0.7V, but this only when the diode current iD is approximately equal to or greater than the right of 1mA. the model provides a reasonable approximation, so applications are wide

For example: Zener diode

Zener diode at work should reverse, and in series with a resistor. One plays the role of the resistance limiting the role to protect the regulator; followed when the input voltage or load current changes, by varying the voltage drop across the resistor, remove the error signal to adjust the operating current regulator, which plays the role of regulator.

The easiest Zener diode voltage regulator circuit composed as shown. From a zener diode characteristics, so if the regulator is always working in its regulation area, the VO. Basically stable at around Vz.

When the grid voltage increases, to maintain the output voltage constant, the voltage drop across the resistor R should be increased, that is, the current flowing through R increases. This increases the current from the zener diode accommodate its operating point by point to the C point b, this time by a characteristic curve indicated Vo & asymp; Vz essentially unchanged.

If the diode voltage regulator circuit load resistance decreases to maintain constant output voltage, load current is large. Since VI remains unchanged, the current flowing through the resistor R constant. Then the load is necessary to increase the current from the regulator out of adjustment, its operating point by point to a point b. Therefore, the regulator can be considered by adjusting the amount of current flowing through itself (essentially the same terminal voltage) to meet the change of the load current and the current changes and current limiting resistor R complex will be converted to voltage changes to adapt to the grid voltage changes.

Zener diode regulator circuit problem: the grid voltage constant, the load current is varied IZ adjustment range (several tens of mA), which limits the range of variation of the load current I0. How can we expand the range of IO.