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1、Diffusion resistance01Diffusion capacitance02Small-signal admittance 03Small-signal equivalent circuit model 04Small Signal Model of the pn JunctionDC static working point：A small sinusoidal current is added to the dc IQD.When sinusoidal current is small enough,the increased conductance is the slope
2、 of IV curve.Diffusion resistanceIQD is static dc current Diffusion ResistanceDiffusion CapacitanceFor dc forward-biased pn junction,the total voltage is the sum of dc voltage and ac voltage.Diffusion Capacitance:the charge and discharge process of holes in the N-region and electrons in the P-region
3、.The diffusion capacitance of forward-biased pn junction is much larger than barrier capacitance of reverse-biased pn junction.Diffusion CapacitanceAdmittance of pn junction is the ratio between total ac vector current and vector voltage.ThusUsing low frequency approximation AC small-signal admittan
4、ce in pn junction gddiffusion conductance，Cddiffusion capacitance001()()DQdpnttIgIIVV00001()()2dppnntCIIVCd is about 1.9 nF,which is much larger than 0.366 pF of junction capacitance.pn junction admittancepn junction low-frequency and small signal admittance Diffusion Capacitance00001()()dppnntCIIVD
5、iffusion capacitance:with changing voltage,Q is in charge or discharge.The ratio of minority charge amount and V is diffusion capacitance.Diffusion CapacitanceEquivalent CircuitddYgj CForward-biased pn junction small-signal equivalent circuit.Barrier capacitance is parallel to diffusion resistance a
6、nd diffusion capacitance,and a series resistance is added.Ideal small-signal equivalent circuit The all small-signal equivalent circuitConsidering series resistance,the total voltage is Vapp and the voltage on pn junction is Va To obtain the same current,the increased voltage is needed due to the series resistance.Vapp=Va+IrsEquivalent Circuit