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Cct. Models for semiconductor diodes
Diode Models
Large Signal Small Signal Piecewise Linear
Ideal Diode Offset Diode
Offset Diode Model
Internal resistance =0
Cutoff frequency (c)>0 (Vγ=0.6 volts for Silicon)
Reverse current (Is) =0
Can be modeled as Ideal diode in series with Vγ
I-V characteristic curve
Offset diode accounts for the offset voltage (better approximation).
If VD < Vγ it will be OFF (open circuit in series with Vγ.
If VD> Vγ it will be ON (short circuit series with Vγ).
Given offset diode with V = 0.7V, Calculate ID.
we assume it is ON and then check current direction if it is consistent.
By inspection it is clear that current is consistent with our assumption
4 0.70.825
4
PSV VI
RV V
I mAk
Example 1
Example (cont.) Try solving assuming
that diode is off. Note that offset diode
voltage is not similar to the open cct. voltage.
Example 2
If VB = 2 V; R1 = 1 k; R2 = 0.5 K ; Vγ = 0.6 V. Determine Minimum value of V1 so D1 conducts.
Sol: we assume diode OFF.
Cct. Models for semiconductor diodes
Diode Models
Large Signal Small Signal Piecewise Linear
Ideal Diode Offset Diode
Large Signal Vs. Small signal Models
In small signal model, the exact I-V char. Curve is used to determine diode voltage and current
it is used to determine accurate operating point of the diode.
Large signal models use approximating I-V char. Curve and thus approximate results of diode voltage and current.
Small Signal Model
No approximation is used. Diode cannot be replaced
with linear element (open or short).
We need to deal with the non-linear properties of the diode.
KVL, KCL can be applied Nodal and Mesh analysis
cannot be used. Superposition only deals
with Linear circuit elements!!
)1( T
D
V
v
sD eII
The Operating point of the Diode
The operating point (or Q-point) is the point that determines the exact value of diode voltage and diode current.
Load-Line Analysis of Diode Circuits
equations. KVL or KCL writeto difficult is It
1Vn
vexpIi: diode a is there whenbut
,...dt
diLv ,
dt
dvCi iR,v use can We
T
DsD
Analysis" Line-Load"
the perform can we
given, is diode the
of curve V-I the If
viRV
:gives KVL
shown,circuit the For
DDSS
Example 1- Load-Line Analysis
point)-(Q point operating the at
voltage and current diode the :Find
diode the of curve V-I the
,Ω1kR 2V,V :Given
shown,circuit the For
SS
mA 1.3i V, 0.70V
point operating the at
analysis line-load perform
vi 10002
i.e., ,viRV
DQDQ
DD
DDSS
Example 2 - Load-Line Analysis
point operating the at
voltage and current diode the :Find
diode the of curve V-I the
,k 10R V, 10Vss :Given
shown,circuit the For
mA 0.93i V, 0.68V
point operating the at
analysis line-load perform
vi 10k10
i.e., ,viRV
DQDQ
DD
DDSS
Load-Line Analysis of Complex Circuits
What if we have more than one source and one load ?? Use the Thevenin Equivalent
Example 3
SS L
L S
Given: diode I-V curve, V 24V, R 1.2k , R 6k
Find: the load voltage v and source currents I
Applying Thevenin Equivalent
20 ,
1
- 0 ( _ _ _ )
20 1000( ) 0
2020 ( )
1000 1000
LT SS
L
LT
L
T T D D
D D
DD D
RV V V
R R
R RR k
R R
V R i v DC Load Line Equation
i v
vi v mA
20 ( )D Di v mA
L DQ
DQ
S SS L
v v 10.0 V
I 10
I (V -v )/R 11.67 mA
mA
Example 3 (cont.)
The Q point (Operation point) is the solution of the system.
(IDQ, VDQ)
Determine the operating point of the diode shown in the following circuit.
The I-V char. Curve is given below.
Example 4
Cct. Models for semiconductor diodes
Diode Models
Large Signal Small Signal Piecewise Linear
Ideal Diode Offset Diode
Piecewise linear Diode Model For this model we
assume the following:
Cutoff frequency >0
Internal resistance (rD) >0 (but constant!!)
Reverse current =0.
The approximation mainly in the value of the internal resistance.