Thursday 4 April 2013
SMALL SIGNAL MODEL OF DIODE
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Small-signal operation is that a time varying
signal with small amplitude “rides” on a DC value that may or may not be large.
The analysis of the circuit is then divided
into two parts:
1. DC
“bias”
2. AC
“signal” of small amplitude.
and the solutions are added
together using superposition.SMALL SIGNAL MODEL
where vd(t) is some time varying
waveform, perhaps periodic such as a sinusoidal or triangular signal.
The purpose of VD in this circuit
is to set the operation of the diode about a
point on the forward bias i-v characteristic
curve of the diode. This is called the quiescent point, or Q point, and the
process of setting these DC values is called biasing the diode.
The total voltage at any time t is the sum of
the DC and AC components
vD ( t ) = VD
+ vd ( t )
provided the AC signal is small enough that
the diode operates approximately in a linear region.The diode current is iD
( t ) >> IS such that
iD ( t ) ≈ IS e vD
( t ) /ηVT = IS e ( ( VD
+ vd ( t ) ) / ηVT
)
= ISe e ( VD/
ηVT ) .
e ( vd ( t ) ) /ηVT )
= ID . e ( vd ( t ) /ηVT )
Where ID is the DC diode current
By expanding the exponential term and
neglecting high order terms
iD ( t ) = ID . ( 1 + vd
( t ) ) /ηVT )
So, if vd(t) is small enough we
can see from this last equation that iD is the sum (or
superposition) of two components: DC and AC signals. What we’ve done is to
linearize the problem by limiting the AC portion of vD to small
values.
The term nVT / I D has
units of ohms. It is called the diode small-signal resistance:
rd = ηVT / I
From a physical viewpoint,
rd is the inverse
slope of the tangent line at a particular bias point along the characteristic
curve of the diode. Note that rd changes depending on the (DC) bias
This purely resistive AC model for the diode
works well when the frequency of the AC signals is sufficiently low.
HIGH FREQUENCY
HIGH FREQUENCY
At high frequencies, we need to include the effects
that arise due to these time varying signals and the charge separation that exists
in the depletion region and in the bulk p and n regions of the diode under
forward bias conditions.
As you’ve learned in electromagnetics, a time
varying electric field is a displacement current. The effects of a displacement
current are modeled by equivalent circuit capacitances
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3 Responses to “SMALL SIGNAL MODEL OF DIODE”
30 July 2014 at 14:55
thank u sir for helping as for doing our project
5 February 2017 at 09:04
Small signal low frequency model
5 February 2017 at 09:05
Small signal low frequency model
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