diode equivalent circuit -...

Module: Electronics I Module Number: 610/650221-222 Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky

Lecturer: Dr. Omar Daoud Part I -II

Philadelphia University

Faculty of Engineering Communication and Electronics Engineering

Diode Equivalent Circuit

DC Analysis:

- Load Line Analysis:

The shown circuit is the simplest diode

configuration.

The intersection of the load line on the

characteristics can be determined by

applying kirchoff's voltage law.

0=−− RD VVE

• If DV is set to zero and solve for

DI we have the magnitude of DI on

the vertical axis.

VVD

R

DREI

VE

0|

0

==⇒

=−

• Now set AI D 0= , the value of DV on the horizontal axis could be determined.

AID

D

DEV

VE

0|0

==⇒=−

• After finding the intersection values of both axes, the load line could be drawn

and then the point of operation as shown below.



Ex. 2.1: For the given diode configuration in the figure below, determine both of the

Q-values and VRRR.

Ex.2: Determine the diode voltage and current in a series configuration using the

piece-wise model, if R=2kΩ, E=5V, VRγR=0.6V and rRfR=10Ω.

Ex.3: For the same circuit shown in Fig.2, determine the diode voltage and current

in a series configuration, if the saturation current equals 0.1pA.



Diode's Configurations: Ex. 2.7: Determine both of VRoR and IRDR for the given circuit.

Ex. 2.8: Determine VRD1R, VRD2R and IRDR for the given circuit.

Ex. 2.9: Determine I, VR1R, VR2R and VRoR for the given circuit.



Ex. 2.11: In this example there are two LED's that

can be used as a polarity detector. Apply a

positive source voltage and a green light results.

Negative supplies result in a red light. Find the

resistor R in this combination to ensure a current

of 20mA through the ON diode. Both diodes have

a reverse breakdown voltage of 3V and an average

turn-on voltage of 2V.

What will happen if the Green LED is changed

with a Blue on of 5V turn-on voltage?



Other Diode Types:

- Solar Cell: - Photo diodes:

- Light Emitting Diodes:

- Schottky Barrier Diode:

- Zener Diode:

At some point of applying the reverse-bias voltage, the break-down occurs. This

means that a very high current passes through the diode in the opposite direction

of the forward current.

This element could be designed to provide specific break-down voltages, and

then can be operated by limiting the current to a value within the capability of

the device (has a constant voltage reference; VRzR.).



Diode Applications:

- Rectifier Circuit: • Half-Wave Rectifications:



( )m

Km

V-VV

≥≅

PIV318.0Vdc

• Full-Wave Rectifications:

- The DC level obtained from a sinusoidal input can be improved 100%

using a process called full-wave rectification.

- ( )Km-VV636.0Vdc ≅

- mV2PIV ≥

- The most familiar network for performing such a function is shown below

and known as a Bridge Network.



Ex. 2.16:

a- Sketch Vo and determine the dc level of the output for the network shown

below.

b- Repeat part a if the diode is replaced by a Si one.

c- Repeat both of a and b if the input voltage is increased to be 200V.

Ex. 2.17:

Determine the output waveform for the network shown below; calculate the output

dc level and the required PIV of each diode.



- Clipper Circuit:

Clippers are networks that diodes to "clip" away a portion of an input signal without

distorting the remaining part of the applied waveform.



- Clamper Circuit:

A clamper is a network constructed of a diode, a resistor and a capacitor that shifts a waveform to a different dc level without changing the

appearance of the

applied signal.



- Zener Diode Circuit:

• The analysis of the Zener diode is quite similar to the semiconductor diodes.

• The use of Zener Diode as a regulator

must follow one of the following

considerations:

1) VRiRU and R Fixed: - Determine the state of the

Zener diode by removing it

from the network and

calculating the voltage across the remaining open circuit.

<≥

⇒

+==

OFF is diodezener the, ifON is diodezener the, if

z

z

L

LiL

VVVV

RRRV

VV

- Substitute the equivalent circuit and solve for the desired unknowns.



- The ON state is shown in

the figure. Here,

zzz

LiR

L

LRz

LzR

zL

VIPR

VVR

VRV

RV

I

IIIVV

=

−=

−=⇒

+==

But,

Ex.2.26: For the Zener diode network shown, determine VRLR,VRRR, IRzR and PRzR.

Repeat you solution with RRLR=3kΩ.

2) UFixed VURiRU and variable RURLRU: - Due to the offset voltage range VRzR, there is a specific range of a resistor

values that will ensure the Zener is in the ON state.

- Determining the minimum load resistance value that will turn the Zener

diode to the ON state.

minmax

min

L

z

L

LL

zi

zL

L

LizL

RV

RV

I

VVRV

R

RRRV

VV

==⇒

−=⇒

+==



- Once the diode is in the ON state, the voltage across R remains constant

and then the current through it.

LRz

RR

ziR

IIIR

VI

VVV

−=

=

−=

- Since IRzR is limited to IRzMR as provided in the datasheet, the

min

min

maxL

zL

zMRL

IV

R

III

=

−=

Ex.2.27: For the shown Zener

diode network, determine the

range of RRLR and IRLR that will

result in VRRLR being maintained

at 10V. Then determine the

maximum wattage rating of the

diode.

3) UVariable VURiRU and fixed R URLRU: - For fixed values of RRLR, the voltage VRiR must be sufficiently large to turn the

Zener diode ON.

zRi

LzMR

L

zLi

L

LizL

VRIVIII

RVRR

V

RRRV

VV

+=

+=⇒

+=⇒

+==

maxmax

max

min)(

Ex.2.28: Determine the range of

values of VRiR that will maintain the

Zener diode in the ON state.

diode equivalent circuit -...

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