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Dynamic resistance of the diode is the resistance for an AC source deppending of the DC polarization of the diode.The DC polarization determines the dynamic resitance, a high diode DC current is a minimun dynamic resitance because the variation in the curve of diode is more high then less resistance.If the AC source is aproximated5mVpp the diode presents a dynamic resistance of Rd=25.9mV/Idwhere25.9mV is a constant and Id y the DC diode current.The AC source should be small to avoid great changes in Id because:Id(Total)=Id(AC)+Id(DC) [superposition principle]Example:with Id=5mA then Rd=25.9mV/5mA=5.18 OhmsId=0.5mA then Rd=25.9mV/0.5mA=51.8 OhmsThe term Dynamic is because the resistance change arround the working zone of the diode.I hope that the information evacuates yours doubts.
The Current-Voltage relationship of a diode is not constant (not a straight line) and hence the resistance cannot be measured. Due to this non-linear nature of the the curve, there exists a unique value of resistance at every point of the curve which is called dynamic resistance (not static of constant resistance). The dynamic resistance equals the incremental change in voltage divided by the incremental change in current, when the voltage is changed by a small amount. In other words it is the slope of the graph of voltage against current. The dynamic resistance is different at different current values.
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Dynamic resistance is derived from Shockley’s Diode Equation. It defines the diode resistive nature when an AC source which depends on the DC polarization of the PN junction diode is connected to it. If an external sinusoidal signal is given to the circuit consisting of a diode, the altering input will shift the instantaneous Q – point slightly from the current position in the characteristics and therefore it defines a definite change in voltage and current. When no external alternating signal is applied, the operating point will be the Q – point (or quiescent point) which is determined by the applied DC signal levels. The AC resistance of the diode is increased by lowering the Q-point of operation. In short, it is equivalent to slope of voltage – current of the PN diode.
rd = ΔVd / ΔId
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the dynamic resistance of diode is the resistance which depends on the AC frequency and ripples means changes in the AC supplied levels. which has deep effect on the DC output of the diode.
