Experiment 6: Diodes

Electronic InstrumentationExperiment 6: Diodes

* Part A: Diode I-V Characteristics

* Part B: Rectifiers•Part C: PN Junction Voltage Limitation•Part D: Zener Diode Voltage Regulator

Diodes

A diode can be considered to be an electrical one-way valve.

They are made from a large variety of materials including silicon, germanium, gallium arsenide, silicon carbide …

ANODED1

DIODE

CATHODE

Diodes

In effect, diodes act like a flapper valve• Note: this is the simplest possible model of a

diode

Diodes For the flapper valve, a small positive pressure

is required to open. Likewise, for a diode, a small positive voltage

is required to turn it on. This voltage is like the voltage required to power some electrical device. It is used up turning the device on so the voltages at the two ends of the diode will differ.• The voltage required to turn on a diode is typically

around 0.6-0.8 volt for a standard silicon diode and a few volts for a light emitting diode (LED)

Diodes

10 volt sinusoidal voltage source

Connect to a resistive load through a diode• This combination is called a half-wave rectifier

V1

FREQ = 1k

VAMPL = 10V

0

R1

1k

D1

D1N4002

Diodes

Sinusoidal VoltageV1

FREQ = 1k

VAMPL = 10V

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(D1:1)

-10V

-5V

0V

5V

10V

Diodes

Half-wave rectifier

0

VV

R1

1k

D1

D1N4002V1

FREQ = 1k

VAMPL = 10V

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(D1:1) V(D1:2)

-10V

-5V

0V

5V

10V

At the junction, free electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle of the diode called the depletion zone.

Part A: Diode i-v Characteristics

Diode V-I Characteristic For ideal diode, current flows only one way Real diode is close to ideal

Ideal Diode

Diode Characteristics

A very large current can flow when the diode is forward biased. For power diodes, currents of a few amps can flow with bias voltages of 0.6 to 1.5V. Note that the textbook generally uses 0.6V as the standard value, but 0.7V is more typical for the devices we will use in class.

Reverse breakdown voltages can be as low as 50V and as large as 1000V.

Reverse saturation currents Is are around 1nA.


The iD-vD relationship (excluding breakdown) can be written simply as:

Note that for vD less than zero, the exponential term vanishes and the current iD

is roughly equal to the saturation current. For vD greater than zero, the current increases

exponentially.

i Iv

n VD sD

T

ex p 1


Recall that the i-v relationship for a resistor is given by Ohm’s Law: v=Ri

If we plot this expression, we obtain

v

i

The slope of the straight line is given by the resistance R

i-v Characteristics PSpice can be used to obtain such plots

0

V115V

R1

500

R2

1k

V(R1:1) - V(R1:2)

-6.0V -4.0V -2.0V 0V 2.0V 4.0V 6.0VI(R1)

-10mA

-5mA

0A

5mA

10mA

V-I Characteristic of a 500 Ohm Resistor

i-v Characteristics

Both the simulated current vs. voltage and the characteristic equation for the diode are plotted

V

0

R1

1kV2

5VdcD2

D1N4148

V

V(D2:1)

-16V -14V -12V -10V -8V -6V -4V -2V 0V 2VI(D2) (7e-9)*(exp( V(D2:1)/(.05107))-1)

0

4m

8m

12m

16m

19m

iDi Iv

n VD sD

T

ex p 1

i-v Characteristics

In this experiment, you are asked to find the parameters for the equation

That is, you need to find the constants in this equation so that it matches what PSpice determines. Note that VT=25mV, so you need to find n and Is

i Iv

n VD sD

T

ex p 1

i-v Characteristics

The i-v characteristic can be checked by building the circuit and measuring the same two voltages shown on the diode circuit.

From these voltages and the value of the resistance, both the current through the diode and the voltage across the diode can be determined.

Part B: Rectifiers

Rectifiers

As noted above, the main purpose of diodes is to limit the flow of current to one direction.

Since current will flow in only one direction, even for a sinusoidal voltage source, all voltages across resistors will have the same sign.

Thus, a voltage which alternately takes positive and negative values is converted into a voltage that is either just positive or just negative.

Rectifiers

If a time-varying voltage is only positive or only negative all of the time, then it will have a DC offset, even if the original voltage had no offset.

Thus, by rectifying a sinusoidal signal and then filtering out the remaining time-varying signal, we obtain a DC voltage from an AC source.

Diodes – Recall from Previous Slide

10 volt sinusoidal voltage source

Connect to a resistive load through a diode• This combination is called a half-wave rectifier

V1

FREQ = 1k

VAMPL = 10V

0

R1

1k

D1

D1N4002

Diodes

Sinusoidal VoltageV1

FREQ = 1k

VAMPL = 10V

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(D1:1)

-10V

-5V

0V

5V

10V

Diodes Half-wave rectifier

0

VV

R1

1k

D1

D1N4002V1

FREQ = 1k

VAMPL = 10V

Time


-10V

-5V

0V

5V

10V

Diodes Note that the resulting voltage is only

positive and a little smaller than the original voltage, since a small voltage (around 0.7V) is required to turn on the diode.

Time


-10V

-5V

0V

5V

10V0.7V

Diodes Filtering can be performed by adding a

capacitor across the load resistor

Do you recognize this RC combination as a low pass filter?

You will see how this looks both with PSpice and experimentally

0

R1

1k

D1

D1N4148

V2C1

47uF

Diodes

The rectifier we have just seen is called a half-wave rectifier since it only uses half of the sinusoidal voltage

A full-wave rectifier uses both the positive and negative half cycles of the sinusoid

Full-Wave Rectifier

Shown are the original voltage, the rectified voltage and the smoothed voltage Capacitor

Discharging

Full Wave Rectifier

Note path of current when source is positive

0

R4

10k

D6

D8

V

D5

D1N4148

V+

R3

50

V-

D7

D1N4148

V2

FREQ = 1kVAMPL = 10VOFF = 0

Full Wave Rectifier

1.4V (2 diodes)

Time

110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0msV(D5:2) V(R4:2,D7:1)

-10V

-5V

0V

5V

10V

Full Wave Rectifier With Smoothing

D3

D4

D1N4148

C1

0.1uF

R1

50

D2

R2

10k

D1

D1N4148

0

V1


Full Wave Rectifier With Smoothing

Time

110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0msV(R1:2) V(R2:2,D1:1) V(R4:2,D7:1)

-10V

-5V

0V

5V

10V

Smoothed Voltage

Part C: PN Junction Voltage Limitation

Voltage Limitation

In many applications, we need to protect our circuits so that large voltages are not applied to their inputs

We can keep voltages below 0.7V by placing two diodes across the load

0

V1

R1

1k

D1

D1N4148

D2

D1N4148

A B

Voltage Limitation

When the source voltage is smaller than 0.7V, the voltage across the diodes will be equal to the source

When the source voltage is larger than 0.7V, the voltage across the diodes will be 0.7V

The sinusoidal source will be badly distorted into almost a square wave, but the voltage will not be allowed to exceed 0.7 V

You will observe this with both PSpice and experimentally

Voltage Limitation

Case 1: The magnitude of the diode voltage is less than 0.7 V (turn on voltage)

0

V1

R1

1k

D1

D1N4148

D2

D1N4148

A B

0

R1

1k

V1100mVdc

Diodes act like open circuits

Voltage Limitation

Case 2: The magnitude of the diode voltage is greater than 0.7 V (turn on voltage)

0

V1

R1

1k

D1

D1N4148

D2

D1N4148

A B

Diodes act like voltage sources

Voltage Limitation

Case 2: The current drawn by the diode is given by the resistor current

IV

Rm A

1 0 0 7

1 0 0 09 3

..

0

R1

1k

V2

0.7Vdc

V110Vdc

Voltage Limitation

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(R1:1) V(R1:2)

-10V

-5V

0V

5V

10V

(1.2420m,718.277m)

0

D1

D1N4148

VV3

FREQ = 1kVAMPL = 10VOFF = 0 D2

D1N4148

V

R1

1k

Input Protection Circuits

More than one diode can be connected in series to increase the range of permitted voltages

Part D: Zener Diodes

Zener Diodes

Up to this point, we have not taken full advantage of the reverse biased part of the diode characteristic.

Ideal Zener Diode I

-VZ

V

Zener Diodes

For the 1N4148 diode, the breakdown voltage is very large. If we can build a different type of diode with this voltage in a useful range (a few volts to a few hundred volts), we can use such devices to regulate voltages. This type of diode is called a Zener diode because of how the device is made.

Zener Diodes

You will again find the i-v characteristic with PSpice and experimentally.

Such circuits can be used in combination with the rectifier and filtering to obtain a well regulated DC voltage.

0

V11V

R1

1k

D1

D1N750

BA

Zener Diodes

Note that, for a real Zener diode, a finite current (called the knee current) is required to get into the region of voltage regulation

VZ is the Zener Voltage

Knee Current

Zener Diodes Note the voltage

limitation for both positive and negative source voltages

D1

D1N750

V1


V V

0

R1

1k

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(R1:1) V(D1:2)

-10V

-5V

0V

5V

10V

Zener Diode Voltage Regulation

Full Wave Rectifier

Note stable voltage

Time

110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0msV(D10:1) V(R8:1,R8:2)

-8.0V

-4.0V

0V

4.0V

8.0V

0

C4

1mF

R5

50

D1N4148

D10

D1N4148

R7

10

V+

V3


V-

D9

D1N4148

R8

10k

V

D14

D1N750

D12

D1N4148

Experiment 6: Diodes

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