Diode volt-ampere characteristic curve
1. When U<0, the diode is in the reverse cut-off state. When the reverse voltage is not very large, the current passing through the diode is very small, which is called reverse saturation current. When the reverse voltage threshold is exceeded, the diode breaks down and the current flowing through the diode increases. This breakdown may be avalanche breakdown or Zener breakdown, which is related to the doping concentration of the PN junction and the manufacturing process. Generally speaking, the voltage of Zener breakdown is smaller than avalanche breakdown.
2. When U>0 and less than the turn-on voltage Uon, the current through the diode is very small and can be ignored to 0.
3. After U>Uon, the diode current increases as the voltage across it increases, and the slope becomes larger and larger.
The volt-ampere characteristic curve of the diode has the following expression:
Id is the forward current flowing through the diode, Ud is the voltage of the diode, UT is a fixed value, which is 26mV at room temperature, and Is is the reverse saturation current.
Use matlab to draw the volt-ampere characteristic curve of the diode:
%% 画出二极管伏安特性曲线
%%Id = Is*(e^(Ud/Ut)-1)
%%参考这个二极管：1N41481
% https://www.cnblogs.com/initcircuit/p/10894211.html
Is = 25*10^(-9);
Ud = -0.1:0.001:0.5;
Ut = 26*10^(-3);
Id = Is*(exp(Ud/Ut)-1);
plot(Ud,Id)
After knowing this equation, the AC impedance rd between transistors b and e can be calculated, also called dynamic resistance.
Find the derivative:
d(Id)/d(Ud) = Is*(1/Ut)*e^(Ud/Ut), which is approximately equal to Id/Ut.
rd(Id0) = 0.026/Id ohms.
The temperature rises: the reverse saturation current increases and the forward voltage drop decreases. The characteristic curve moves to the lower left.