Diode Approximations | Diode Models

You might come across the terms ideal and practical diodes. As a beginner, these terms might confuse you. So, I think it is important to discuss the properties of an ideal diode and a practical diode.

Outline

• Discuss different diode approximation or diode models
• How does practical and ideal diode model differ from each other?
• Why do we need to learn the behaviour of an ideal diode?

Diode Models And IV Characteristics:

The current-voltage characteristics curve is shown in figure 7. Look at the graph, how much a practical diode deviates from ideal behaviour. An ideal diode is a piecewise linear device, while a practical diode is a non-linear device. Linear device means its current-voltage graph is a straight line. A diode is non-linear because its current-voltage graph is a non-linear curve. For voltage values less than 0.7V, the current is zero. Just above 0.7V, the current increases rapidly. Current doesn't increase proportionally to an increase in voltage.

First Approximation Or Ideal diode:

This is the simplest approximation. The graph is simple and piecewise linear. Zero resistance when forward biased, infinite resistance when reverse biased. It is analogous to a mechanical switch. Because the switch has zero resistance when close. And a switch has infinite resistance when open. The voltage across the diode is 

VD = 0

Figure 1 Ideal diode and its equivalent

Figure 2: Ideal diode IV Curve

Second Approximation Or Practical diode Model:

The graph is piecewise linear. Look at the graph, the diode doesn't conduct until the voltage reaches 0.7.

According to this approximation, a diode is analogous to a switch in series with a barrier potential of 0.7V. Look at the figure, the diode becomes forward bias when the applied potential is at least 0.7V (close switch). Hence current increase rapidly.

For applied voltage below 0.7V, it remains reverse bias (open switch). The voltage across the diode is

VD = 0.7 V

Figure 3 Non-ideal diode and its equivalent

Figure 4 Non-ideal diode, IV Curve

Third Approximation Or Detailed Model:

According to this approximation, a diode is analogous to a switch, a barrier potential and a resistor (Rf), connected serially. Rf shows the internal resistance of semiconductors.

Now examine the effect of Rf. The diode turns on as the applied voltage is 0.7V or above. Thereafter, the current increases as an increase in voltage (Rf is ohmic resistance). We can apply Ohm's law and find voltage and current through it. The voltage across the diode is

VD = IDRf +0.7

Where

VD = diode voltage

ID = diode current

Rf = forward resistance of diode or bulk resistance

Figure 5 Third approximation of diode

Figure 6 Third approximation IV Curve

Difference between ideal and practical diode:

Figure 7 Practical diode Vs Ideal diode IV Curve

You have seen all three diode approximations. The figure above is the graph obtained from a real diode. In this graph, we consider all the effects of ohmic resistance, threshold voltage, leakage current and breakdown region.

Forward Biased

Ideal Diode

• It behaves as a perfect conductor
• An ideal diode is like a closed switch. It has 0Ω resistance between anode and cathode
• No need for threshold voltage
• It has zero resistance, and hence infinite current through the diode. (I = V/R)

Real Diode

• It also acts as a conductor. Due to imperfections, it offers a small forward resistance Rf
• It is also like a closed switch, except a small forward resistance Rf
• Real diodes need a little voltage called knee voltage or threshold voltage to overcome barrier potential
• Due to small forward resistance Rf, there is a voltage drop at the diode and hence finite current through the diode

Reverse Biased

Ideal Diode:

• It behaves as a perfect insulator
• An ideal diode is like an open switch in reverse bias conditions. No current can flow from anode to cathode
• Breakdown region is not possible at any magnitude of applied voltage

Real Diode:

• It behaves as an insulator. There is a voltage limit called the breakdown voltage. The diode can not operate beyond this limit
• Real diodes deviate from ideal behaviour because of minority carriers. Due to minority carriers, it has some reverse current flows from cathode to anode. This current is called reverse saturation current or leakage current
• They do have a breakdown voltage. If the applied voltage exceeds the rated voltage, results in junction breakdown

Need of Ideal Diode Behaviour:

You have read the features of an ideal diode. An ideal diode can not possible to produce. It has infinite resistance in reverse biased conditions, and hence no current. It has zero resistance in forwarding biased conditions, and hence infinite current. Such conditions are not possible practically.

Which diode model commonly use during analysis?

Most of the time we consider the third approximation.

Why do we consider an ideal diode?

We consider the ideal diode at the first stage of our analysis and troubleshooting. It is easy to understand the circuit analysis with an ideal diode. As a beginner, it is better to start with ideal diodes, to keep circuit analysis as simple as possible.

Diode As A Switch

Diode As A Switch:

Have a closer look at diode symbol. It represents an arrow, which shows the direction of current. It allows current to flow from anode to cathode. As you know, it is a voltage controlled two terminal device. It has characteristics of a switch. In one direction it allows current to flow, while in other direction it blocks the current. In other words, during forward biased conditions, diode has ideally zero resistance. It behaves as a closed switch. While during reverse biased it behaves as an open switch. Diodes use in switching applications are also known as 'signal diodes’.

Figure 1 Diode as a switch. Practical versus ideal diodes

Apply voltage positive V at anode. When anode is more positive than anode, it behaves as a closed switch. Input is directly coupled to the output. And hence current flows from positive to negative terminals.

Now apply positive voltage at cathode. See figure 1. It means cathode is at greater potential than anode. In this situation the diode acts as an open switch between input and output terminals. Consequently no current flows through the diode.

• Signal diodes are common in mixing circuits (mixers)
• Logic gates can also be implemented with the help of switching diodes.

Positive Logic AND Gate Using Diodes:

The figure below (figure 2) is two input positive logic AND gate circuit. It is made up of diodes. Here diode is using as a digital switch.

Let's understand the working of circuit. Input A and input B is tied to cathode. While both anodes are connected to VCC through resistor R1. The output VOut depends on both inputs. If either input is zero output is zero.

VA is voltage at input A

VB is voltage at input B

VA < VCC

VB < VCC

Vout = 0 (logic 0)

At this condition, both diodes are forward biased (closed switch) because both input terminals (cathodes) are at  lower potential than anode. The current flows from diodes and hence output is low.

VA > VCC

VB < VCC

Vout = 0

At this condition, diode D1 is reverse biased. While diode D2 is forward biased. The current flows from diode D2 and hence output is low.

VA < VCC

VB > VCC

Vout = 0

At this condition, D2 is reverse biased. While D1 is forward biased because input A (cathode) is at lower potential than anode. The current flows from D1 and hence output is low.

VA > VCC

VB > VCC

Vout = 1 (logic 1)

At this condition, both diodes are reverse biased (opened switch). No current flows from diodes and hence output is high.

A	B	Out
0	0	0
0	1	0
1	0	0
1	1	1

Figure 2: AND gate using diode

Positive Logic OR Gate Using Diodes:

Look at figure 3. It is two input OR gate, implemented from diodes only. A and B are two inputs, where output is taken across resistor R1. Truth table is given as well.

Let's understand the working of circuit.

Look at the diode position. Input A is at its anode. While the cathode is tied to resistor R1, which is grounded. Same for input B.

When anyone input A or B is positive (logic high) the output is high. That is positive logic (voltage) at anode, makes the diode forward bias. Hence input appears at the output.

VOut = VA = VB ...for ideal diode

VOut = VA - 0.7 … for practical diode

Where VA is voltage at input terminal A

Vout is voltage across resistor R1

A	B	Out
0	0	0
0	1	1
1	0	1
1	1	1

Figure 3 Diode OR gate

Cockcroft Walton Voltage Multiplier | Diode Capacitor Multiplier

Diode as a voltage multiplier:

A very sophisticated method of obtaining high voltage with the help of simple circuit is Cockcroft-Walton voltage multipliers.  It consists of diodes and capacitors only. There are many possible ways to design voltage multipliers, with the help of diodes and capacitors. This article is intended to provide basic information about Cockcroft-Walton voltage multipliers.

Voltage multipliers are used to produce high voltages ( hundreds and thousands of volts). They are commonly used in high voltage low current device like cathode ray tubes, Lasers etc.

Diode capacitor voltage multiplier takes AC as an input, and produce DC output which is multiple (2,4,6) of AC peak voltage.

VO = 2*N*VP    Equation 1

VO = 2*N*VP - Vdrop     Equation 2

Where

N = number of stages,

VP = peak input voltage

Vdrop = diode drop

Equation 2 >> we consider the diode drop.

For example from a three stage multiplier, we get

VO = 2*3*VP

VO = 6*VP

A single stage Cockcroft Walton multiplier consists of 2 diodes and 2 capacitors. Each stage produce an output which is multiple of 2. Hence, the number of diodes and capacitors stages increase as the voltage increases.

For example at input if you have 10V (AC) at input, each stage produce 20 V. For example, we have 3 stages we get 60 V (DC) at output. It means each stage produce 20V. (20+20+20 = 60). And the voltage multiplier circuit contains 6 capacitors and 6 diodes.

VO = 2*N*VP

VO = 2*3*10 = 60V (neglect diode drop)

Basically voltage multiplication depends on charging and discharging of capacitors. Each capacitor charges upto 2VP except  C1 , which charges upto VP. The upper rail of capacitors clamps the signal, while the bottom rail of capacitors smooths the DC.

Diode Capacitor Voltage Doubler | Single Stage Cockcroft Walton Multiplier:

During negative half cycle D1 is forward biased and charges the capacitor C1 to peak voltage Vp, while D2 is reverse biased. During positive half cycle D2 is forward biased. C2 will try to charge up to 2Vp, because C1 and source are in series. See figure below, C2 tries to charge upto 2VP, after several cycles it will reach upto 2VP. The output is taken across capacitor C2.

Figure 1 Single stage multiplier

Look a figure 2. Input 10V (AC)

Output 20V (DC) approximately (we are considering practical diodes)

Voltage at capacitor C1

Figure 2 Output of Single stage multiplier (ideally output is 20 V, practically it is a little bit less because of diode drop as shown in the figure)

Voltage Quadrupler | Two Stage Cockcroft Walton Multiplier:

Figure 3 Two stage voltage multiplier

The voltage quadrupler circuit consists of two stages of Cockcroft-Walton multiplier. The upper rail of capacitors stores and clamps the signal. The output is observed across lower rail of capacitors.

During first negative half cycle D1 and D3 are forward biased. D1 charges the capacitor C1 to peak voltage Vp, while D2 and D4 remain reverse biased. During first positive half cycle D2 and D4 are forward biased. C2 will try to charge up to 2Vp, because C1 and source are in series.

During second negative half cycle, again D1 and D3 are forward biased. C1 already charged upto VP. Apply KVL and calculate the voltage across capacitor C3. It is 2VP. Look at figure 4. During second positive half cycle D2 and D4 are forward biased. Capacitor C4 charges upto 2VP. C2 has already charged upto 2VP. The output is taken across the two capacitors.

Figure 4 Working of voltage quadrupler

Look a figure 5: Waveforms obtained at various points. You can visualize easily. Voltage across C1 is shifted or clamped. Also, voltage across C3 is also clamped. I discussed earlier, upper rail of capacitors clamp or shift the waveforms. While, the lower rail of capacitors produce DC at the output.

Input 10V (AC)

Output 20V (DC) approximately (we are considering practical diodes)

Voltage at capacitor C1

Voltage at capacitor C3

Figure 5 Output of voltage quadrupler