Diode Circuits and Applications

In this chapter, we will:

n Determine the operation and characteristics of dioderectifier circuits, which is the first stage of the process ofconverting an ac signal into a dc signal in the electronicpower supply.

n Apply the characteristics of the Zener diode to a Zenerdiode voltage regulator circuit.

n Apply the nonlinear characteristics of diodes to createwaveshaping circuits known as clippers and clampers.

n Examine the techniques used to analyze circuits thatcontain more than one diode.

n Understand the operation and characteristics of specializedphotodiode and light-emitting diode circuits.

Block Diagram for ac to dc Converter

The diode rectifier, filter, and voltage regulator are diode circuits.

Half-Wave Rectifier

Voltage Transfer Characteristics

Signals of Half Wave Rectifier

Input voltage Output voltage

Diode voltage

A load line is used in graphical analysis of nonlinear electronic circuits, representing theconstraint other parts of the circuit place on a non-linear device, like a diode or transistor. Itis usually drawn on a graph of the current vs the voltage in the nonlinear device, called thedevice's characteristic curve.

Load Line Analysis

Load Line Analysis

Load line when vS is at its maximum forward voltage.

Load line when vS is at its most negative value.

Load Line (con’t)

As vS varies with time, the load line also changes, which changes the Q-point (vD and iD) of the diode.

Half-Wave Rectifier as Battery Charger

Full-Wave Rectifier

Voltage transfer characteristics

Input and output waveforms

Full-Wave Bridge Rectifier

When vS is positive, D1 and D2 are turned on (a). When vS is negative, D3 and D4 are turned on (b).

In either case, current flows through R in the same direction, resulting in an output voltage, vO, shown in (c).

Full-Wave Bridge Rectifier

Output Voltage of Full-Wave Rectifier with RC Filter

The ripple on the ‘dc’ output isP

Mr T

ffRCVV

21 where

2==

Output Voltage of Full-Wave Rectifier with RC Filter

Diode conducts current for only a small portion of the period.

M

r

VV

Tt 21

π=

∆

Equivalent Circuit During Capacitance Charging Cycle

M

r

MpeakC

MC

VVt

tCVitCVi

2,

=∆

∆+=−=

ω

ωωωω

PSpice Schematic of Diode Bridge Circuit

Steady state output voltage for a 60Hz sine wave input with peak value of 13.4V.

Modulated input signal

Detector circuit

Demodulated output signal

Demodulation of Amplitude-Modulated Signal

Voltage Doubler Circuit

Equivalent Circuits for Input Cycles

Negative input cycle Positive input cycle

Voltage Regulator

The characteristics of the Zener diode determines VL.

LIZ

i

ZPSI

L

ZL

IIIR

VVI

RVI

−=

−=

=

The Zener diode begins to conduct when VPS = VZ. When VPS ≥ VZ: VL = VZ

IL = VZ/RL,, but VZ ≠ constantI1 = (VPS – VZ)/Ri

IZ = I1 - IL

Voltage Rectifier with nonzero Zener resistance

Voltage Transfer Characteristics of Limiter Circuit

2 Diode Circuit

Voltage transfer characteristics

Problem-Solving Technique: Multiple Diode Circuits

1. Assume the state of the diode. a. If assumed on, VD = Vγb. If assumed off, ID = 0.

2. Analyze the ‘linear’ circuit with assumed diode states.

3. Evaluate the resulting state of each diode.4. If any initial assumptions are proven incorrect,

make new assumption and return to Step 2.

Diode Logic Circuits: 2-Input OR Gate

V1 (V) V2 (V) VO (V)

0 0 0

5 0 4.3

0 5 4.3

5 5 4.3

Vγ = 0.7V

Diode Logic Circuits: 2-Input AND Gate

V 1( V )

V 2( V )

V O( V )

0 0 0

5 0 0

0 5 0

5 5 4.3

Vγ = 0.7V

Photodiode Circuit

Optoisolator

Design DC Power Supply Circuit

Diode Clippers

A clipper (or limiter) is a circuit used to eliminate some portion (or portions) of a waveform.n A series clipper is in series with its load.n A shunt clipper is in parallel with its load.

Series Clippers

Negative Shunt Clipper Operation

A Positive Shunt Clipper

SL

LinL RR

RVV+

=

When the diode is conducting:

When the diode is not conducting:

FL VV =

Biased Shunt Clippers

Diode Clampers

A clamper (or dc restorer) sets (or restores) the dc reference of awaveform.

Clamper Operation

Biased clampers

Biased clampers allow a waveform to shifted above or below a dc reference other than 0 V.n The dc reference is determined by the biasing voltage (VB)

and the setting of the potentiometer (R1).

Zener clampers

The diodes in (a) are in a common-cathode configuration. The diodes in (b) are in a common-anode configuration.

Voltage Doublers

n A voltage doubler provides an output that is twice its peak input voltage.

Half-Wave Voltage Doubler

n The term “half-wave” reflects the fact that the output capacitor (C2) is charges during one alternation of each input cycle and discharges during the other.

Half-Wave Voltage Doubler Operation

Full-Wave Voltage Doublers

n The term “full-wave” reflects the fact that the output capacitors are charged during alternate half-cycles of the input signal.

Voltage Tripler

n A voltage tripler provides a dc output voltage that is approximately three times the peak input voltage.

Voltage Quadrupler

n A voltage quadrupler provides a dc output voltage that is approximately four times the peak input voltage.

A Basic Dual-Polarity Power Supply

The output voltages are approximately equal to the peak values of the input waveform.

LED Level Indicators

n The LED in circuit (a) lights when the driver output is +5 V.

n The LED in circuit (b) lights when the driver output is 0 V.

Multisegment Display

n Multisegment display – A device used to display alphanumeric characters (numbers, letters, symbols, and punctuation marks).n LED displays contain some number of diodes that are

connected in a common-cathode or a common-anode configuration.

n A liquid crystal display (LCD) consists of segments that reflect (or do not reflect) ambient light when provided an active input.

Seven-Segment Displays

n The display uses LEDs that are arranged in a figure 8 configuration. n The display represented below is a common-cathode display.

Each LED lights when a positive voltage is applied to the appropriate pin.