1. Cct. Models for semiconductor diodes
Diode Models
Large Signal
Small Signal
Piecewise Linear
Ideal Diode
Offset Diode

2. 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

3. 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).

4. Example 1 Given offset diode with Vg = 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

5. Example (cont.) Try solving assuming that diode is off.
Note that offset diode voltage is not similar to the open cct. voltage.

6. 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.

7. Cct. Models for semiconductor diodes
Diode Models
Large Signal
Small Signal
Piecewise Linear
Ideal Diode
Offset Diode

8. 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.

9. 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!!

10. 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.

11. Load-Line Analysis of Diode Circuits

12. Example 1- Load-Line Analysis

13. Example 2 - Load-Line Analysis

14. Load-Line Analysis of Complex Circuits
What if we have more than one source and one load ??
 Use the Thevenin Equivalent

15. Example 3

16. Example 3 (cont.) (IDQ, VDQ)
The Q point (Operation point) is the solution of the system.
(IDQ, VDQ)

17. Example 4
Determine the operating point of the diode shown in the following circuit.
The I-V char. Curve is given below.

18. Example 4 (cont.)

19. Cct. Models for semiconductor diodes
Diode Models
Large Signal
Small Signal
Piecewise Linear
Ideal Diode
Offset Diode

20. 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.

21. Piecewise-Linear Diode Models
Now, diode voltage is the sum of resistance voltage and cutoff frequency voltage.