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EENG 3510 Chapter 3

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EENG 3510 Chapter 3

Diodes

Chapter 3 Homework

3.2 (c & d), 3.3 , 3.9, 3.19, 3.23

diode circuit symbol

i–vcharacteristic

equivalent circuit in the forward direction

equivalent circuit in the reverse direction

an external circuit to limit the forward current

the reverse voltage

AND gate (in a positive-logic system)

OR gate

1. Both diodes are conducting.

Voltage at B is zero.

ID2 = 10 V -0 V / 10 k = 1 mA

I + ID2 = (0 – (-10) V ) / 5 k = 2 mA

I + 1 = 2mA

I = 1 mA

5.V = 0

- Assume both diodes are conducting.
- VB = 0
- ID2 = (10 V – 0 V) / 5k = 2 mA
- I + 2 = (0 – (-10)) V / 10 k
- I = - 1 A This is not correct.
- Assume D1 is off and D2 is on.
- ID2 = (10 V – (-10 V) )/ 15k = 1.33 mA
- V = VB = -10 V + (10 k X 1.33 mA)
- V = -10 V + 13.3 V = 3.3 V

I = 5 V / 2.5 K = 2 mA

V = 0, Why ?

V

I = 0 A, Why?

V

V = 5 V, Why ?

V

I = 0 A, Why?

V = 5 V, Why ?

V

I = 5 V / 2.5 K = 2 mA

V = 0, Why ?

I = 3 V / 1 K = 3mA

V = 3 V, Why ?

I = 4 V / 1 K = 4mA

V = 1 V, Why ?

V = forward voltage

Silicon diodes conduct when the forward voltage = 0.7 volts

Germanium diodes conduct when the forward voltage = 0.3volts

Given: A forward biased diode, forward voltage drop is 0.7 V at 2 mA,

n = 1 at 0.6 V

Find : the current i2

Graphical Analysis

Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation

Development of the constant-voltage-drop model of the diode forward characteristics. A vertical straight line (B) is used to approximate the fast-rising exponential. Observe that this simple model predicts VD to within 0.1 V over the current range of 0.1 mA to 10 mA.

The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation.

- A voltage regulator is a circuit whose purpose is to provide a constant dc voltage between its output terminals
- The output voltage is required to remain as constant as possible in spite of
- Changes in the load current drawn from the regulator output terminal
- Changes in the dc power-supply voltage that feeds the regulator circuit

Circuit symbol for a zener diode.

- In recent years, zener diodes are replaced in voltage-regulator design by specially designed integrated circuits (ICs) that perform the voltage regulation function much more effectively and with greater flexibility than zener diodes.

120(N2/N1) V

Coils wound

around an iron core

Remove pulsation

Remove ripple

Half-wave rectifier

Transfer characteristic of the rectifier circuit

Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model.

Input and output waveforms, assuming that rD!R.

- Two important parameters:
1) Current-handling capability: the largest current the diode

is expected to conduct

2) Peak inverse voltage (PIV): the diode must be able to

withstand without break

Full-wave rectifier utilizing a transformer with a center-tapped secondary winding

transfer characteristic assuming a constant-voltage-drop model for the diodes;

input and output waveforms

Most Popular Rectifier Circuit Configuration

The bridge rectifier

input and output waveforms

A simple circuit used to illustrate the effect of a filter capacitor.

Note that the circuit provides a dc voltage equal to the peak of the input sine wave. The circuit is therefore known as a peak rectifier or a peak detector.

Input and output waveforms assuming an ideal diode.

Waveforms in the full-wave peak rectifier

Simplified physical structure of the junction diode.

(Actual geometries are given in Appendix A.)

Two-dimensional representation of the silicon crystal. The circles represent the inner core of silicon atoms, with +4 indicating its positive charge of +4q, which is neutralized by the charge of the four valence electrons. Observe how the covalent bonds are formed by sharing of the valence electrons. At 0 K, all bonds are intact and no free electrons are available for current conduction.

At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.

The concentration of free electrons n, and the concentration of holes p

Ex: phosphorus

A silicon crystal doped by a pentavalent element. Each dopant atom donates a free electron and is thus called a donor. The doped semiconductor becomes n type.

Ex: boron

A silicon crystal doped with a trivalent impurity. Each dopant atom gives rise to a hole, and the semiconductor becomes p type.

Equilibium: Is= ID,

Maintained by the barrier voltage V0

(a) The pn junction with no applied voltage (open-circuited terminals). (b) The potential distribution along an axis perpendicular to the junction.

Anode

Cathode

Depletion width: increases

Barrier voltage v0: increase

I = IS-ID

The pn junction excited by a constant-current source I in the reverse direction. To avoid breakdown, I is kept smaller than IS.Note that the depletion layer widens and the barrier voltage increases by VRvolts, which appears between the terminals as a reverse voltage.

Anode

Cathode

Depletion width: decrease

Barrier voltage v0: decrease

I = ID -IS

The pnjunction excited by a constant-current source supplying a current I in the forward direction. The depletion layer narrows and the barrier voltage decreases by V volts, which appears as an external voltage in the forward direction.