Cct Models for semiconductor diodes Diode Models Large

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

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

Offset Diode Model q Internal resistance =0 q Cutoff frequency (c)>0 (Vγ=0. 6 volts

Offset Diode Model q Internal resistance =0 q Cutoff frequency (c)>0 (Vγ=0. 6 volts for Silicon) q Reverse current (Is) =0 q Can be modeled as Ideal diode in series with Vγ

I-V characteristic curve q Offset diode accounts for the offset voltage (better approximation). q

I-V characteristic curve q Offset diode accounts for the offset voltage (better approximation). q If VD < Vγ it will be OFF (open circuit in series with Vγ. q If VD> Vγ it will be ON (short circuit series with Vγ).

Example 1 Given offset diode with Vg = 0. 7 V, Calculate ID. we

Example 1 Given offset diode with Vg = 0. 7 V, 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

Example (cont. ) q Try solving assuming that diode is off. q Note that

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

Example 2 If VB = 2 V; R 1 = 1 k; R 2

Example 2 If VB = 2 V; R 1 = 1 k; R 2 = 0. 5 K ; Vγ = 0. 6 V. Determine Minimum value of V 1 so D 1 conducts. Sol: we assume diode OFF.

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

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

Large Signal Vs. Small signal Models q In small signal model, the exact I-V

Large Signal Vs. Small signal Models q 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. q Large signal models use approximating I-V char. Curve and thus approximate results of diode voltage and current.

Small Signal Model q No approximation is used. q Diode cannot be replaced q

Small Signal Model q No approximation is used. q Diode cannot be replaced q q 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!!

The Operating point of the Diode q The operating point (or Q-point) is the

The Operating point of the Diode q 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

Load-Line Analysis of Diode Circuits

Example 1 - Load-Line Analysis

Example 1 - Load-Line Analysis

Example 2 - Load-Line Analysis

Example 2 - Load-Line Analysis

Load-Line Analysis of Complex Circuits What if we have more than one source and

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

Example 3

Example 3

Example 3 (cont. ) q The Q point (Operation point) is the solution of

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

Example 4 q Determine the operating point of the diode shown in the following

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

Example 4 (cont. )

Example 4 (cont. )

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

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

Piecewise linear Diode Model q For this model we assume the following: Cutoff frequency

Piecewise linear Diode Model q For this model we assume the following: Cutoff frequency >0 Internal resistance (r. D) >0 (but constant!!) Reverse current =0. The approximation mainly in the value of the internal resistance.

Piecewise-Linear Diode Models q Now, diode voltage is the sum of resistance voltage and

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