Diode Experiment

2014 Rev 1.5

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Table of Contents Glossary ....................................................................................................................................... 2

Introduction .................................................................................................................................... 3

Goals of this experiment ............................................................................................................. 3

Preparation ................................................................................................................................. 3

Background ................................................................................................................................. 4

Diode I,V Characteristics ............................................................................................................. 6

Charge Pumps for Voltage Multiplication ................................................................................... 8

Rectifiers: AC-DC Conversion ...................................................................................................... 9

Diode as a Small Signal Resistor ................................................................................................ 13

Lab Experiments ........................................................................................................................... 15

Part 1 Diode I-V Characteristics ............................................................................................. 15

Part 2 Charge Pumps for Voltage Multiplication ................................................................... 15

Part 3 Rectifiers ...................................................................................................................... 16

Part 4 Diodes as small signal resistors ................................................................................... 16

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Glossary

DAC Digital to Analog Converter

DMM Digital Multimeter

DPOT Digital Potentiometer (digitally controlled resistor)

DSW Digital Switch

GUI Graphical User Interface

IC Integrated Circuit

Op Amp Operational Amplifier

PCB Printed Circuit Board

STC Single Time Constant

SMD Surface Mount Device

TH Through-hole

VCCS Voltage-Controlled Current Source

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Introduction

The objective of this experiment is to understand the behavior of diodes in a variety of circuits.

This experiment relies on the NileDelta board, OpAmp board, and Diodes board, connected as

shown in Fig. 1. Each circuit in the experiment is explained and relevant sections of the 6th Edition

Sedra/Smith Microelectronics Circuits1 textbook (e.g. 2.3) are given2.

Goals of this experiment 1. Understand the functionality of different types of diodes.

2. Investigate the role of diodes in a variety of circuits on the Diodes board.

3. Correlate simulation results with experimental measurements.

Preparation

Checklist

Read the Background section, making sure you understand the operation of each circuit.

In all simulations use a diode model for the 1N4148 diode.

Bring standard headphones and an audio source with pre-loaded music of your choice

(laptop/cell-phone/MP3 player) to the lab.

Complete the preparation exercises below.

Install the GUI file for this experiment:

1. Download the Diodes Experiment subpanel file from

http://www.ele.utoronto.ca/~ot/aelabs/diodes/

2. Extract this file (Diodes.vi) into "/AUX

SubPanels/AELabs". If an AELabs directory does not exist, create one.

3. The NileDelta GUI will now be able to automatically recognize the Diodes board.

Preparation Exercises

PE1. Using the internet, find the datasheet for the p-n diode used in this experiment (1N4148WS

by Fairchild Semiconductor). Write down the forward voltage, VF, at 10mA, as well as the

breakdown voltage, -VZK, at 5A. Confirm the data from Table 1.

PE2. Devise a simulation to plot the I-V curve (ID versus VD) of the 1N4148 diode. Hint: you

should use a DC sweep to vary the voltage on the diode and plot the current. Using the

cursors, measure the small-signal resistance, rd, at ID = 0.02 mA, 0.2 mA and 2 mA.

(i) What is the relationship between the small-signal resistance and the bias current?

1 All images and figures have been used under permission from Oxford University Press. 2 Should your university use a different textbook for this introductory electronics course, the instructor is responsible for providing the alternative sections to those of Sedra/Smith.

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PE3. Construct a simulation of the diode charge pump in Fig. 7. Use the 1N4148 diode, 1 F capacitors and a pulse frequency of 1 kHz.

(i) Disconnect the load resistor, RP2_Load in Fig. 7. Using pulse signals of 2 Vp-p, 4 Vp-p,

and 10 Vp-p observe the output voltage and intermediate voltages by performing a

transient (time domain) analysis. Which of these input values produces an output that

is closest to the expected output and why?

(ii) Connect the load resistor. Using a pulse signal of 10 Vp-p, conduct a parameter sweep

by varying the load resistance in the range of 1 k to 1 M while using a transient

simulation to monitor the output voltage. How does changing the load affect the output

voltage and ripple? What is the minimum load this circuit can supply before the output

voltage ripple exceeds 10% of the peak output voltage?

PE4. Perform a transient analysis simulation of the variable attenuator in Fig. 15. You can

replace the VCCS with a DC current source.

Tip: For Vcarrier, use a 1 kHz sin wave of 0.5 Vp-p and chose an appropriate simulation time.

(i) Include a plot of the output waveform, in your lab notebook for a DC current of

ID = 0.02 mA, 0.2 mA and 2 mA.

(ii) Using eq. (2.5) and your result from part PE2, verify the attenuation factor from the

simulation in part PE4 (i).

Background You will experiment with four different diode circuits in this lab, each corresponding to one section

of the Diodes board. The controls on the left side of the GUI are enabled based on the selected

part. The OpAmp board is used to generate the stimulus waveforms used in this experiment, as

shown in in Fig. 1. For this reason the OpAmp board is also referred to as the input circuit, as shown in Fig. 2.b. As in the Op-Amp Experiment, this input circuit can be electronically

configured in numerous ways, but the most relevant configurations for the Diode Experiment are

(a) non-inverting amplifier with adjustable gain, (b) an oscillator, and (c) a summing amplifier. A

set of switches is used to route the signal from the OpAmp board to each of the circuits used in

parts 1-4 on the Diodes board.

PC with GUI

NileDelta Op Amp Board

R1

R2

-

+

PIC C

BNC 2

Audio

Power Management

BNC 1

12 V

Diodes Board

VinVout

Fig. 1: Diodes board setup used in this experiment .

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Fig. 2.a: The Diodes board.

Fig. 2.b: Input Circuit: configurable amplif ier/oscillator . Note that in this experiment the OpAmp board is used as the input circuit .

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Diode I-V Characteristics (Ref: 4.2 Terminal Characteristics of Junction Diodes) The terminal characteristic of a diode is shown in Fig. 3. The diode current, i, is given by:

1 TVvs eIi (1)

Fig. 3: Exponential characteristic of diodes .

The objective of this experiment is to measure the I-V characteristic of different types of diodes.

Four types of commonly used diodes are included, as listed in Table 1. Theoretically, the I-V

characteristic of Fig. 3 can be measured by imposing a voltage, VD, using a voltage source and

measuring the resulting diode current, ID. Given that the current changes exponential with voltage,

this approach is not safe and can easily lead to high currents that damage the diode. Instead, it is

recommended to sweep the current within a well-defined range and measure the resulting voltage.

The circuit shown in Fig. 4 demonstrates one possible way to safely sweep the current through a

diode. An op amp with negative feedback is utilized in order to control the voltage across a fixed

resistor, R1. Remember, there is a virtual short between the input terminals of the op amp. The

resulting current, ID, is passes through both R1 and the diode, D1, and can be expressed by:

1R

VI inD (2)

The op amp provides the appropriate voltage, Vo = Vin + VD, as long as VSS < Vo < VDD.

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+

-

ID = Vin / R1

ID

+

VD

-

D1

I = 0

Vin

R1

+

Vin

-

VDD

VSS

Vo

Fig. 4: Practical circuit for measuring the I-V characteristic of a diode .

Table 1: Types of diodes used in Part I.

Diode Type Forward Voltage Breakdown Voltage Component Part #

D1 Standard p-n junction diode ~ 1V @ 10 mA 75 V 1N4148WS

D2 Schottky barrier diode ~ 0.37V @ 10 mA 30 V RB751V40

D3 Zener diode ~ 0.9V @ 10 mA 2.5 V MMBZ5222BLT1G

D4 Light emitting diode (LED) ~ 1.8V @ 10 mA Unknown LTST-C171KRKT

The practical circuit shown in Fig. 4 is used as a building block for the actual I-V measurement

circuit implemented on the Diodes board, as shown in Fig. 5. Four switches are used to select

which diode is activated in the circuit. The resistor, Rcurrent, is used to control the diode current, up

to a maximum of ID_max = Vin_max / Rcurrent. Notice that Vin is related to ID by a constant factor of

Rcurrent, which means that ID can effectively be measured by probing Vin (or Vcurrent). The voltage

across the selected diode, VD, is amplified by a difference amplifier, and can be easily measured

at a probe point, Vdiode, with reference to ground.

The I-V characteristic of each diode can therefore be measured by sweeping Vin and measuring

Vcurrent versus Vdiode.

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SWP1_1 SWP1_2 SWP1_3 SWP1_4

D1 D2 D3 D4

Vamp

Vcurrent

Vdiode

Rcurrent

Difference Amplifier

Vin

+

-

+

-

Fig. 5: Part 1 Schematic: Measuring the I-V characteristics of different diodes.

Charge Pumps for Voltage Multiplication (Ref: 4.6.3 The Voltage Doubler) Charge pump circuits are commonly used to generate high voltages beyond the supply voltage.

The Dickson charge pump is shown in Fig. 6. It operates by transferring charge from left to right

along the diode chain, from one capacitor to the next. When Vclk is high (Vdd), diode D1 conducts

until the voltage across capacitor C1 is charged to Vdd. When Vclk is low (negative Vdd), diode D2

conducts and transfers charge from C1 over to C2. Eventually, the voltage across capacitor C2

settles to 2Vdd. When Vclk goes high (Vdd) again, the voltage at node 2 settles to 3Vdd. This process

is repeated at each stage, with the voltages at each node as shown in Fig. 6.

Theoretically, this circuit produces an output voltage of 5Vdd. In practice, however, there is a diode

voltage drop of VD at each stage and the final output voltage is 5 x (Vdd VD). For Vdd = 3 V and VD 0.7 V, the output voltage is about 5 x (3 0.7) = 11.5 V.

Part 2 of the Diodes board contains an implementation of the Dickson charge pump with the

addition of an optional load resistance, as shown in Fig. 7. The load resistance is used to

demonstrate the limitations of a practical charge pump circuit. Although the circuit is able to

increase the voltage beyond the supply voltage, the amount of current that it can output is very

limited.

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D1 D2 D3 D4 D5

C4 CoutC3C2C1

vclk

-Vdd

3Vdd

Vdd

5Vdd

3Vdd

0V 0V 0V0V

3Vdd

Vdd

0V

5Vdd

Fig. 6: Dickson charge pump.

D5 D6 D7 D8 D9

C3 C4 C5 C6 C7

VP2_1 VP2_2 VP2_3 VP2_4 VP2_5

SWP2_1

RP2_Load

Vin

Fig. 7: Part 2 Schematic: Charge pump with optional load resistance .

Rectifiers: AC-DC Conversion (Ref: 4.5 Rectifier Circuits) In this experiment you will investigate different types of rectifier circuits used for AC-DC

conversion. A conventional AC-DC conversion process is shown conceptually in Fig. 8. The basic

half-wave rectifier topology and associated waveforms are shown in Fig. 9. When a capacitor is

added at the output to reduce the ripple voltage, the circuit is known as a peak rectifier. The output

waveform of an un-loaded peak rectifier (i.e. R ) is shown in Fig. 10. The effect of adding a finite resistance, R, to the peak rectifier is shown in Fig. 11. If we neglect the voltage drop on the

diode, the voltage ripple on the output voltage, Vr, can be approximated by

fCR

VV

p

r2

(3)

10

where f is the frequency of the input sinusoid. The ripple voltage in DC power supplies is generally

kept below 5%. If the frequency is fixed (typically 60 Hz), this imposes a limitation on the

capacitance, C.

Fig. 8: Block diagram of a conventional DC power supply .

Fig. 9: (a) Half-wave rectifier. (b) Transfer characteristic of the half -wave rectif ier circuit. (c) Input and output waveforms.

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Fig. 10: (a) A simple circuit used to il lustrate the effect of a fi lter capacitor. (b): Input and output waveforms assuming an ideal diode. 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.

Fig. 11: Voltage and current waveforms in the peak rectifier circuit with CR >> T. The diode is assumed ideal.

A full-wave bridge rectifier circuit is shown in Fig. 12.a. The full-wave topology is generally

preferred for AC-DC conversion since energy can be drawn from the input source during both half

cycles of the sinusoidal input. When a capacitor is connected at the output for filtering, the full-

wave topology offers lower ripple voltage compared to the half-wave topology, since the

frequency is effectively doubled, as shown in Fig. 12.b.

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Fig. 12: The bridge rectif ier: (a) circuit; (b) input and output waveforms .

Fig. 13: Waveforms for full -wave peak rectifier (i .e. when a capacitor is added across R in Fig. 12).

The schematic for this experiment is shown in Fig. 14. The circuit can be configured either as a

half-wave or full-wave (bridge) rectifier by controlling the switches SWP3_1 and SWP3_2. A

peak rectifier can be implemented by enabling capacitors C9 and C10. The circuit uses a standard

two-winding step-up transformer with a primary-to-secondary turns ratio of N1:N2 = 100:131. An

AC coupling capacitance, C8, is connected at the input of the transformer to ensure that only AC

signals arrive at the primary side of the transformer.

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C10C9 R7D13D12

D11D10

C8

VP3_OUT

SWP3_2

SWP3_1

1:1.31

SWP3_4 SWP3_5SWP3_3

Vin

Fig. 14: Part 3 Schematic: Programmable half -wave/full -wave rectif ier .

Diode as a Small Signal Resistor (Ref: 4.3.7 The Small-Signal Model) While the diode is intrinsically nonlinear, when forward-biased with a constant current, the diode

can be approximated as a linear resistor for small variations in current and voltage. This linear

approximation of nonlinear devices about a DC operating point is called small-signal modeling.

Application: A Voltage-Controlled Variable Attenuator

The fact that the diode behaves as a resistor for small-signals can be used to create a variable

attenuator, as shown in Fig. 15. At frequencies well above DC, the capacitors in Fig. 15 can be

considered as short circuits. Thus, the amplitude of Vcarrier is attenuated by a voltage divider

consisting of R8 and the small-signal resistance of the two diodes D14 and D15, rd14 = rd15 = rd, which

are in parallel with load resistance, R9,

1||

||

891514

91514

RRrr

Rrr

V

V

dd

dd

carrier

out (4)

The small-signal resistance of diodes D14 and D15 is given by rd = VT/ID, where VT is the thermal

voltage (VT ~ 25 mV at room temperature) and ID is the DC current flowing through the diodes.

For the case where rd

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the case where Vcarrier is a high frequency sine wave, while Vin is a slowly varying signal. The

result is that Vin modulates the attenuation factor of the circuit, which produces an Amplitude Modulation (AM) signal at the output, as shown in Fig. 16. AM is commonly used in radio

communication systems to transit information signals wirelessly over long distances.

There is one additional point to mention about the variable attenuator circuit on the Diodes board.

You will notice that, as in the OpAmp board, there is a manual potentiometer (large black knob)

located in the bottom right corner of the Diodes board. This 200 k potentiometer controls the gain of an inverting op amp audio amplifier circuit that is connected between VP4_OUT and the

physical audio jack on the underside of the board. There are two primary reasons why this is

necessary: 1) It allows you to set the final output volume to a comfortable level, and 2) It ensures

that the circuit is able to supply enough current to the large headphone load (usually 8-16 ). For simplicity, this audio amplifier is not shown in Fig. 15.

VDD1

VcarrierANALOG4

C11 C12

D14

D15

R8

R9

VP4_OUT

VinRcontrol

Vin

Fig. 15: Part 4 Schematic: Variable Attenuator.

Fig. 16: Amplitude Modulat ion signals when Vin is a slowly varying signal.

Vin

Vcarrier

Vout

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Lab Experiments

Throughout this lab, use the NileDelta board, OpAmp board, and Diodes board connected as shown

in Fig. 1. Note, the signal labeled Vout on the OpAmp board is physically connected by the header

pins to the signal labelled Vin on the Diodes board. Therefore, you should probe the test-point

labelled Vout in order to measure Vin.

Part 1 Diode I-V Characteristics E1. Set up the function generator as a 100 Hz, 2 Vp-p triangle wave. After verifying the signal

on the oscilloscope, connect it to the BNC1 jack on the NileDelta board. Configure the

input circuit (OpAmp board) as a non-inverting amplifier circuit and then send the BNC1

signal to ANALOG2. Select Part 1 in the GUI in order to connect Vin to the Part 1

experiment, which is shown in Fig. 5. Select D1 using the switch SWP1_1. The objective

of this part is to plot the I-V characteristic of each diode on the oscilloscope. Based on eq.

(2), design the gain of the non-inverting amplifier on the OpAmp board such that the current

in the I-V characteristic circuit is varied over a range of -5 mA to +5 mA. Show the output

of the difference amplifier on CH1 of the oscilloscope, and voltage at the cathode of D1 on

CH2. Change the oscilloscope to XY mode and plot CH2 (y-axis = diode current) versus

CH1 (x-axis = diode voltage). Comment on the shape of the curve. Does the voltage range

match your expectation for D1, based on Table 1? What is the approximate on-state voltage

of this diode?

E2. Repeat this experiment for D2, D3, and D4 by enabling one diode at a time while noting the

difference in the I-V characteristics. For D2, comment on the major difference between this

Schottky diode and a regular p-n diode. For D3, comment on the breakdown behaviour of

this zener diode. For D4, reduce the frequency of the triangle waveform to 5 Hz and observe

the change in brightness of the LED. Note that the brightness of an LED is proportional to

its forward current, not voltage. What are the main differences between these four types of

diodes? What is the approximate on-state voltage of each type of diode?

Part 2 Charge Pumps for Voltage Multiplication E3. Click on the Part 1 switch in the GUI to deactivate Part 1. Set all four of the ANALOG

channels to OFF (in this experiment you will generate your own input signal). Configure

the input circuit (OpAmp board) as an oscillator (see Fig. 12: Astable Multivibrator in the Op Amp Experiment handout) and design the frequency to be 1 kHz. Modify the circuit

such that the amplitude of the oscillation is 10 Vp-p (think about what needs to be adjusted).

E4. Click on the Part 2 switch in the GUI to send the output of the oscillator to the charge

pump, as shown in Fig. 7. Observe the output at each node and compare with Fig. 6. Does

the output voltage match the theoretical value? What is the voltage difference between each

successive stage? Measure the ripple on the output voltage with the load disconnected.

How can you reduce the ripple?

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E5. Close the SWP2_1 switch to connect the load. Measure the voltage and its ripple. How

does adding the load affect the output? What is the reason behind this?

E6. Vary the oscillator frequency between 500 Hz and 2 kHz and measure the range of the

output voltage.

E7. Vary the oscillators amplitude between 10 Vp-p and 20 Vp-p and measure the range of the charge pump output voltage.

Part 3 Rectifiers E8. Ensure that VDD1 is set to 10 V. Click on the Part 2 button to disconnect the oscillator.

Configure the input circuit (OpAmp board) as a non-inverting amplifier with a gain of

2 V/V. Set up the function generator to provide a 20 kHz, 2 Vp-p sine wave on ANALOG2.

Now click the Part 3 button to send the 4 Vp-p output to the rectifier circuit.

E9. Design the circuit to act as a half-wave peak rectifier with a capacitance of your choice and

no load (i.e. disconnect R7). Based on the amplitude of the input sine wave, the gain of

input circuit, and the turn-ratio of the transformer, does the DC output voltage at VP3_OUT

match your prediction? (Hint: think about the voltage that is dropped across the diodes.)

E10. Now configure the circuit as a half-wave rectifier with no capacitance at the output and a

load resistance of ~ 5 k. Observe the sinusoidal input, Vin, on CH1 and the output of the rectifier, VP3_OUT, on CH2 of the oscilloscope. Compare the output to Fig. 9.c. Now enable

all four diodes to create a full wave rectifier and observe the change on CH2. Does this

match Fig. 12.b?

E11. Solve the following design problem. You are required to design a half-wave rectifier with

a DC output of 6 V, an output current of 0.5 mA, and a 20% output voltage ripple. What

load capacitance and input frequency can you use to achieve these specifications?

Configure your design on the Diodes board and measure the output to validate this design.

How does this compare with the results predicted by eq. (3)? Now switch to a full-wave

rectifier and measure the peak output voltage and the output voltage ripple. Which topology

makes a better AC-DC converter, half-wave or full-wave? Explain why.

Part 4 Diodes as Small Signal Resistors E12. In order to implement Amplitude Modulation (AM), we require two separate signal

sources. For the first source, configure the input circuit (OpAmp board) as a non-inverting

amplifier with a gain of 2 V/V and send a DC input from the DAC to ANALOG2. This

will establish the signal Vin on the Diodes board, which should then be connected to Part

4. For the second source, use the built-in Waveform Generator (Audio) to generate a 400

mVp-p, 440 Hz sine wave and send this audio signal to ANALOG4, which is the carrier

signal, Vcarrier, for the variable attenuator in Fig. 15 (note that you have turned your PC into

a function generator!).

E13. Based on the gain of the VCCS, set the DAC voltage to bias the diodes D14 and D15 at a

DC current of ID = 0.2 mA. Measure the peak-to-peak output voltage at VP4_OUT and

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calculate the gain of the attenuator. Does this match your expectation from eq. (5)? Double

the DC bias current and measure the effect on the gain.

E14. Now we want to set up a low-frequency sinusoidal signal to be used as the modulation

signal, Vin. Make sure the function generator is connected to BNC1 and send this signal to

the appropriate ANALOG channel on the input circuit (OpAmp board). Configure the

function generator and input circuit such that the signal Vin on the Diodes board is roughly

a 1 Hz, 8 Vp-p sine wave with a +4 V offset. In order to add the DC offset, you can either

use the built in controls on the function generator, or you can configure the input circuit as

a summing amplifier. Make sure that the Waveform Generator (Audio) signal is still being

sent to Vcarrier. Observe the voltage Vin and the AM output signal, VP4_OUT, on the

oscilloscope and listen to it using your headphones (dont forget about the volume control knob!). Change the frequency of the modulating signal between 1 Hz and 1 kHz and listen

to the result. Describe what you hear. What happens when the modulation signal and the

carrier signal are equal?

E15. Turn off the Waveform Generator (Audio) by clicking the stop button on the GUI and

instead play an audio file of your choice from your PC. Observe the voltage Vin and the

AM output signal, VP4_OUT, on the oscilloscope and listen to it using your headphones.

Change the frequency of the modulating signal and listen to the result. Describe what you

hear.