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Diode Models and Circuits

Ideal DiodePN Junction as a DiodeApplications of Diodes

EEMB 205 – Electronics I

Dicle University, EEE Department

Mehmet Siraç ÖZERDEM, PhD

After we have studied in detail the physics of a diode, it istime to study its behavior as a circuit element and its manyapplications.

Dr. MS ÖZERDEM

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Diode’s Action in The Black Box (Ideal Diode)

Diode’s Action in The Black Box (Ideal Diode)

The diode behaves as a short circuit during the positivehalf cycle (voltage across it tends to exceed zero), and anopen circuit during the negative half cycle (voltage across itis less than zero).

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Ideal Diode

• In an ideal diode, if the voltage across it tends to exceed zero, current flows.

• It is analogous to a water pipe that allows water to flow inonly one direction.

Diodes in Series

Diodes cannot be connected in series randomly.

For the circuits above, only a) can conduct current from A to C.

Dr. MS ÖZERDEM

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I/V Characteristics of an Ideal Diode

Diode : shortDiode : open

Anti-Parallel Ideal Diodes

If two diodes are connected in anti-parallel, it acts as ashort for all voltages.

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D1 D2

VA > 0 ON OFF

VA < 0 OFF ON

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Diode-Resistor Combination

The IV characteristic of this diode-resistor combination iszero for negative voltages and Ohm’s law for positivevoltages.

If VA > 0 then IA=VA/R1 If VA < 0 then IA=0

Diode Implementation of OR Gate

• The circuit above shows an example of diode-implementedOR gate.

• Vout can only be either VA or VB, not both.

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Input/Output Characteristics

• When Vin is less than zero, the diode opens, so Vout = Vin• When Vin is greater than zero, the diode shorts, so Vout = 0

Diode’s Application: Rectifier

•A rectifier is a device that passes positive-half cycle of a sinusoid andblocks the negative half-cycle or vice versa.

•When Vin is greater than 0, diode shorts, so Vout = Vin ; however, when Vin

is less than 0, diode opens, no current flows thru R1, Vout = I×R1 = 0.

complete rectifier

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Signal Strength Indicator (Average or DC value)

To compute the average, we obtain the area under Vout and normalize the result to the period:

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Diode’s applications

• The purpose of a limiter is to force the output to remain below certain value.

Shifted to the right

Limiter

Rectifier

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Limiter: When Battery Varies

• An interesting case occurs when VB (battery) varies.• Rectification fails if VB is greater than the input amplitude.

Terminal Characteristics of Junction Diodes

Forward Bias

IS Saturation current (proportional to surface area of pn junction)VT =kT/q Thermal voltagek Boltzmann’s constant=1.38 × 10-23 joules/KelvinT The absolute temperature in kelvins =273+Cq the magnitude of electronic charge = 1.60 × 10-19 coulomb

Three distinct regions1. vD > 0

2. vZK < vD < 0

3. vD < vZK

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Temperature Dependence of the Diode Forward Characteristics

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Different Models for Diode

Thus far, Diode models include the ideal model of diode,the exponential, and constant voltage models.

the ideal model of diode,

the exponentialmodel of diode,

the constantmodel of diode,

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Input/Output Characteristics with Ideal andConstant-Voltage Models

The circuit above shows the difference between the ideal and constant-voltage model; the two models yield two different break points of slope

Example - Exponential Model

D1 and D2 have different cross section areas but are otherwise identical

Determine the current flowing through each diode.

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Input/Output Characteristics with a Constant- Voltage Model

Example

ExampleInput/Output Characteristics with a Constant- Voltage Model

Diode circuit

illustration for very negative inputs

equivalent circuit When D1 is off

input/output characteristic

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ExampleInput/Output Characteristics with a Constant- Voltage Model

Large-Signal and Small-Signal Operation

• Our treatment of diodes thus far has allowed arbitrarily large voltage and current changes, thereby requiring a “general” model such as the exponential I/V characteristic. We call this regime “large signaloperation”.

• However, as seen in previous examples, this model often complicates the analysis, making it difficult to develop an intuitive understanding of the circuit’s operation.

• Furthermore, as the number of nonlinear devices in the circuit increases, “manual” analysis eventually becomes impractical.

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Example (Cell Phone Adapter)

• Vout = 3 VD,on is used to charge cell phones. • However, if Ix changes, iterative method is often needed to

obtain a solution, thus motivating a simpler technique.

a) Determine the reverse saturation current IS1 so that Vout=2.4Vb) Compute Vout if the adaptor voltage is in fact 3.1 V

Io=0 (Neglect)

Example (Cell Phone Adapter)

a)

b) If Vad increases to 3.1V, we expect that Vout increases only slightly. First suppose Vout remains constant and equal 2.4VVR1=0.7V then Ix =7mA

Very small difference

we conclude thatVad=3V then Vout=2.4VVad=3.1V then Vout=2.411V

The constant-voltage diode model would not be useful in this case.

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Small-Signal Analysis

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Small-Signal Analysis

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Example

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A diode is biased at a current of 1 mA. (a) Determine the current change if VD changes by 1mV.(b) Determine the voltage change if ID changes by 10%.

Small-Signal Incremental Resistance

Since there’s a linear relationship between the small signalcurrent and voltage of a diode, the diode can be viewed asa linear resistor when only small changes are of interest

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Example (Cell Phone Adapter) Repeat part (b)

VD =

Now suppose that the load pulls a current of 0.5 mA, determine Vout.

Example (Cell Phone Adapter)

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Applications of Diode

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Half-Wave Rectifier

• A very common application of diodes is half-wave rectification, where either the positive or negative half of the input is blocked.

• But, how do we generate a constant output?

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Diode-Capacitor Circuit: Constant Voltage Model

If the resistor in half-wave rectifier is replaced by acapacitor, a fixed voltage output is obtained since thecapacitor (assumed ideal) has no path to discharge.

Diode-Capacitor Circuit: Ideal Model

The voltage across the diode.

Input and output waveforms of the circuit with an ideal diode

Vin, only shifted down

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Diode-Capacitor with Load Resistor

A path is available for capacitor to discharge. Therefore, Vout will not be constant and a ripple exists.

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Behavior for Different Capacitor Values

For large C1, Vout has small ripple.

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Peak to Peak amplitude of Ripple

t1=0

Peak to Peak amplitude of Ripple

To ensure a small ripple RLC1 must be much greater than t3-t1; thus, noting that

• Ripple voltage becomes a problem if it goes above 5 to 10% of the output voltage.

Ripple voltageVR

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t4

t1

Tin

∆T

Peak to Peak amplitude of Ripple

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Example• A laptop computer consumes an average power of 25 W with

a supply voltage of 3.3 V. Determine the average current drawn from the batteries or the adapter.

• If the laptop is modeled by a resistor, RL

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Example• A transformer converts the 110-V, 60-Hz line voltage to a

peak-to-peak swing of 9 V. A halfwave rectifier follows the transformer to supply the power to the laptop computer. Determine the minimum value of the filter capacitor that maintains the ripple below 0.1 V (VD,on = 0.8V)

This is a very large value

Full-Wave Rectifier

• A full-wave rectifier passes both the negative and positive half cycles of the input, while inverting the negative half of the input.

• As proved later, a full-wave rectifier reduces the ripple by a factor of two.

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Full-Wave Rectifier: Bridge Rectifier

The figure above shows a full-wave rectifier, where D1 and D2 pass/invert the negative half cycle of input and D3 and D4 pass the positive half cycle.

Input/Output Characteristics of a Full-Wave Rectifier(Constant-Voltage Model)

The dead-zone around Vin arises because Vin must exceed 2 VD,ON to turn on the bridge.

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Complete Full-Wave Rectifier

Example• Design a full-wave rectifier to deliver an average power of 2W

to a cellphone with a voltage of 3.6 V (Vout,p) and a ripple (VR)of 0.2V (VD,on = 0.8V).

Vin,p=Vp and Vout,p=Vp - 2VD,on

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Summary of Half and Full-Wave Rectifiers

Full-wave rectifier is more suited to adapter and chargerapplications.

Dr. MS ÖZERDEM

Voltage Regulator

• The ripple created by the rectifier can be unacceptable tosensitive load; therefore, a regulator is required to obtain a very stable output.

• Three diodes operate as a primitive regulator.

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Regulation With Zener Diode

• Voltage regulation can be accomplished with Zener diode.• Since rd is small, large change in the input will not be

reflected at the output.

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Line Regulation VS. Load Regulation

Our brief study of regulators thus far reveals two important aspects of their design:

The stability of the output with respect to input variations“line regulation”

The stability of the output with respect to load current variations“load regulation”

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Example: Line Regulation VS. Load Regulation

Line regulation

Load regulation

Evolution of AC-DC Converter

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Limiting Circuits

• The motivation of having limiting circuits is to keep thesignal below a threshold so it will not saturate the entirecircuitry.

• When a receiver is close to a base station, signals are large and limiting circuits may be required.

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Input/Output Characteristics

• Note the clipping of the output voltage

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Limiting Circuit Using a Diode:Positive Cycle Clipping

As was studied in the past, the combination of resistor - diodecreates limiting effect.

Limiting Circuit Using a Diode:Negative Cycle Clipping

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Limiting Circuit Using a Diode:Positive and Negative Cycle Clipping

General Voltage Limiting Circuit

Two batteries in series with the anti-parallel diodes control the limiting voltages.

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Non-idealities in Limiting Circuits

The clipping region is not exactly flat since as Vin increases, the currents through diodes change, and so does the voltage drop.

Capacitive Divider

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Waveform Shifter: Peak at -2Vp

• As Vin increases, D1 turns on and Vout is zero.• As Vin decreases, D1 turns off, and Vout drops with Vin from zero.

Waveform Shifter: Peak at 2Vp

Similarly, when the terminals of the diode are switched, a voltage doubler with peak value at 2Vp can be conceived.

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Voltage Doubler

The output increases by Vp, Vp/2, Vp/4, etc in each input cycle, eventually settling to 2 Vp.

Current thru D1 in Voltage Doubler

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Another Application: Voltage Shifter

Voltage Shifter (2VD,ON)

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