DYNAMIC RESISTANCE AND RECTIFICATION

AC or Dynamic Resistance
❑ From the previous discussion dc resistance is
given by: IF
(mA)
VFQ
RD =
I FQ
IFQ
❑ Therefore dc resistance is
independent of the shape of VD
the diode I-V characteristic
VFQ

Department of EE 1

, AC or Dynamic Resistance…
❑ What will be the resistance of the diode if a very
small ac signal is applied to the diode?

❑ In this case the ac signal is smaller than the
knee voltage

❑ Therefore we can not use the large signal diode
approximations, we must use small signal diode
approximation

Department of EE 2

, AC or Dynamic Resistance…
VF
IF


VDD
+1mV R
0 Uac
-1mV




Department of EE 3

, AC or Dynamic Resistance…
❑ We have two input sources, IF
therefore superposition theorem (mA)

has to be used to analyse the
circuit
❑ If we consider the dc source
alone (i.e. make the ac source = IFQ
0), the dc source will establish
the operating point (VFQ, IFQ)
❑ The operating point is also
known as Quiescent Point (Q- VFQ
point) which means Still or
Unvarying level
Department of EE 4