PPT is about the PN diode resistance levels

1. Diode Resistance Levels
Dr.G.Puvaneswari
Assistant Professor(SG)
ECE, Coimbatore Institute of
Technology, Coimbatore

2. PN DIODE RESISTANCE LEVELS
• The resistance of a PN diode changes when
the operating point moves from one region to
another
• The resistance level of a PN diode is
determined by the type of applied voltage or
signal
• Three different levels:
– DC or Static Resistance
– AC or Dynamic Resistance
– Average AC Resistance

3. DC or Static Resistance (RD)
• Application of a dc voltage to a diode circuit
will result in an operating point that will not
change with time
• Resistance measured at the at the operating
point of diode circuit is defined as DC
resistance.
• Static resistance is defined as the ratio of the
voltage drop (VD) across a diode and the
current flowing through it (ID)
RD = VD / ID

4. DC or Static Resistance contd.

5. FORWARD BIAS:
dc resistance levels at the knee and below:
• Will be greater than the resistance levels
obtained for the vertical rise section of the
characteristics
• resistance levels in the reverse-bias region will
naturally be quite high

6. • Reverse Bias:
– resistance levels in the reverse-bias region will be
quite high
• the lower the current through a diode the
higher the dc resistance level.

7. AC or Dynamic Resistance
• If a sinusoidal input is applied,
– A specific change in current and voltage can be
observed
– This is due to shifting of the instantaneous
operating point up and down a region of the
characteristics
• With no applied varying signal
– the point of operation would be the Q-point
determined by the applied dc levels
– Q-point (quiescent or still or unvarying)

8. AC Resistance at a Q-Point
• Draw a straight line tangent to the curve
through the Q-point
• Obtain a particular change in voltage and
current that can be used to determine the ac
or dynamic resistance for this region of the
diode
• Note: keep the change in voltage and current
as small as possible and equidistant to either
side of the Q-point

10. • Dynamic resistance of a diode in the vertical-
rise region of its characteristic is very small
because of an abrupt change in characteristics
• ac resistance is much higher at low current
levels
• In general, lower the Q-point of operation,
higher the ac resistance

11. AC resistance and Diode current
equation

13. • dVD/ dID = 0.026V/ ID
• The above equation shows that the dynamic
resistance can be found simply by substituting
the quiescent value of the diode current into
the equation and with out the need for the
characteristics curve
• Resistance is accurate only for values of ID in
the vertical-rise section of the curve (ɳ=1)
• For lower values of , ɳ =2 (silicon) and the
resistance value obtained must be multiplied
by a factor of 2

14. Dynamic Resistance (ac resistance)
• Including body resistance effect (resistance of
the diode material) and the contact resistance
(resistance due to the connection between
the semiconductor material and the external
metallic conductor), the dynamic resistance is,
• rB can range from 0.1ohm for high-power
devices to 2ohm for low-power general-
purpose diodes
ohms
r
I
r B
D
d 

026
.
0
'

15. Average AC Resistance
• For a large signal input (broad swing), the
resistance associated with the device for the
vertical region of the characteristics is called
the average ac resistance.
• It is defined by a straight line between the
limits of operation

17. References
• Robert L. Boylestead and Louis Nasheresky
"Electron Devices and Circuits: Theory and
Practice", Prentice Hall of India, 11th Edition,
2013.
• David A. Bell, "Electronic Devices and Circuits",
Prentice Hall of India, 5th Edition, 2010.