Semiconductors and Amplifiers Lab

8/2/2019 Semiconductors and Amplifiers Lab

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Semiconductors and Amplifiers Laboratory MM1EM1

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25 January 2011

Department of Mechanical, Materials and Manufacturing Engineering

Electromechanical Systems 1 MM1EM1

Semiconductors and Amplifiers Laboratory

Important notes:You should read the instruction carefully. Your work will beinspected. You should also perform some research on the subject of thelaboratory, which will allow you to gain more benefit from the experiment.

ObjectivesTo introduce the basic concepts of semiconductors and operationalamplifiersTo understand the functions of diodes and transistors, both in terms oftheir characteristics and in terms of their applications for rectification andamplification

Important note:

There is a lot to get through and time is extremely critical in thislaboratory you have almost one hour to complete each part of thelaboratory, and no more. At the end of each hour, you might be movedto the next part, whether you have finished the previous one or not.It is therefore very important that you read the laboratory sheet inadvance, and ask in the exercise class or class if you do not understandanything.If you find you are making no progress on a particular part of thelaboratory, seek help from one of the free demonstrators.You are encouraged to perform the plotting and calculation aspects of theexercise within the laboratory session after finishing all the experiments,

but remember that your priority is to complete the experimentsthemselves. If you have not completed the plotting and calculationaspects, finish them off later.A group number will be given to you by lab demonstrator during the lab.Do not forget to write down that group number on your cover sheet.


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Part 1: Diodes

The semiconductor diode

Figure 1a: symbol of a semiconductor diode.Figure 1b: dark stripe marking the cathode of the device. Restriction: currentflows in one direction only.

Fig 1 Fig 2Forward bias: fig 2a, Va > Vb, current flows in direction of arrow.Reverse bias: fig 2b, Va < Vb, negligible current flow.

Diode I-V curve: Fig 3 shows a silicon diode I-V curve. At reverse bias, nocurrent should flow. In reality, the I-V curve departs from theory due to theexistence of leakage current but in this experiment we will not consider reversebias and assume that the diode behaves as a perfect insulator under reversebias.

Fig 3

An analogy from mechanical engineeringThe diode serves as the electrical equivalent of a one-way valve in a fluid flowsystem - just as a valve allows fluid to flow in one direction not the other, thediode allows current to flow in one direction only. The analogy can be extendedfurther:

Imagine that the moving part of the valve is held onto its seat by a forcecaused by gravity or by a spring, to keep it closed. A small forward

pressure is needed to overcome this force and allow forward flow. This isanalogous to the 0.7V forward bias required to cause current to flowUnder reverse pressure, no significant flow can occur. But in practice theseal is unlikely to be perfect, so a very small leakage flow occurs,increasing with reverse pressure. This is analogous to the current flowingunder reverse bias, and can similarly be ignored for practical purposes.

Fig. 4


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Experiment 1.1: Diode I-V Curve (Forward bias)

(Additional equipments needed: Two multimeters)

1. Construct the circuit shown in Fig 5a; Fig 5b is the schematic. (In allexperiments make sure about your wiring, setting and connections

before turning ON the system. If you have any doubts ask for help

from demonstrator)

Fig. 5

2. Switch ON the Module Power Supply.3. Adjust the 0-2V DC knob (the upper knob of the two small knobs on circuit

board) to give roughly 0.1V across the diode.4. Record the current (I) and the actual voltage in Table 1, when the readings

on multimeters are stable and not fluctuating.5. Repeat steps 3 & 4 using the voltages given in Table 1. You may need to

change the sensitivities of the multimeters.6. Plot a graph of current vsvoltage, with title, labels and units, on the grid

provided.

Nominal Actual Current

Voltage Voltage

(mV) (mV)

100

200

300400

500

550

600

650

700

750

800

850

Table 1 Diode I-V curve


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Rectification

Rectifier: diodes are commonly used in rectifying

circuits to convert an alternating signal to anunidirectional signal.

Half wave rectifier: with reference to fig 6, current canflow in the clockwise direction but not anti-clockwise.Fig 7b shows the response of the circuit for a sinusoidalinput signal (fig 7a).

Fig 7

Experiment 1.2: Half wave rectifier

(Additional equipments needed: Oscilloscope, one multimeter and signal generator)

The initial settings for the oscilloscope need to be as follows:Channel 1, DC coupled, 2V/divChannel 2, DC coupled, 2V/divTimebase 5 ms/div

Fig .6

Question 1:At what voltage does the current start to flow significantly (specifically, atwhat voltage does it exceed 5 mA)?


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1. Construct the circuit in Fig 8.(connect the earth connoctors of both probesto 2.2)

2. Move oscilloscope traces to centre of screen.3. Connect the signal generator to its input socket on the base unit and switch

it ON.4. Connect a multimeter between 2.1 and 2.2. Set it to the AC voltage range

(e.g. 20V AC range).5. Set the frequency of the signal generator to mains frequency (50Hz), and

adjust its output to 5V RMS (14.14V peak to peak).6. Set the DC offset knob on the signal generator to zero by pulling that

knob out and turn it unti-clockwise to zero.7. Align the oscilloscope trace markers for CH1 and CH2 (start of the CH1

wave on the left of the screen) to the centre line.8. Sketch the oscilloscope traces on the grid provided. Mark clearly the peak

voltages, the dc levels and the periods of the two traces.

Note the voltagesettings for CH1 andCH2 (V/div) and thetimebase setting(s/div)

CH1 /div

CH2 /div

timebase /div

Question 2:Identify the two main differences between the AC and rectified signals(hint: one of them is a result of the diode forward voltage).


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Experiment 1.3: Smoothing circuit and reservoir capacitor


Fig 9

To obtain a more uniform voltage from the half wave rectifier, a smoothingcircuit is needed. There are many circuits that can fulfill this function, a simpleone is obtained by connecting a capacitor in parallel with R3 (as shown in Fig 9)

1. Leave the circuit from the previous experiment unchanged except foradditionally shorting 2.6 & 2.9, and 2.8 & 2.11 as shown in Fig. 9.

2. Same as last experiment set the signal generator to 50Hz and 5VRMS3. Sketch the oscilloscope traces on the grid provided. Mark clearly the peak

voltages, the dc levels and the periods of the two traces.

Voltage settings forCH1 and CH2 (V/div)and the timebasesetting (s/div)

CH1 /div

CH2 /div

timebase /div


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Part 2: Transistors and amplification

Introduction to Transistors

In this experiment you will use an NPN bipolar junction transistor. It has threeconnections: collector, base and emitter, see Figure 10.

.Fig 10

The main function of a transistor is amplification, particularly amplification ofsmall currents and of small variations in voltage. The laboratory will illustratehow it operates as a current amplifier and voltage amplifier, and will illustrate afrequently-encountered amplification circuit.

Transistor current gain

Figure 11 shows a simple transistor (NPN)circuit, known as the common-emitter. Wewill investigate the relationship between thecurrents and the voltages. For practicalpurposes, it can be assumed that collectorcurrent Ic is proportional to the base currentIb with the constant of proportionality beingknown as (beta) or hFE:

bcII Equation 1

IMPORTANT:Throughout the laboratory, make sure that all voltage supplies areswitched off or set to minimum before setting up the experiment. Whilstyou are performing measurements or observing output waveforms, you may

need to change the sensitivities of the measuringinstruments. Always record the settings of theinstruments and include them in the report.

Experiment 2.1: Collector output

characteristics

(Additional equipments needed: Three multimeters)

In Figure 11, the collector current Ic is affected byboth Vc and I b. To find their relationships, one

parameter (Vc) will be kept constant at 1Vthroughout the experiment while the other two

Vc

R

Ie

Vb

Ic

Ib

Fig. 11

Fig. 12


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are measured.1. Ensure that the power supply knobs are set to zero.2. Construct the circuit shown in Fig. 12.

Set two of the multimeters to act as ammeters:Set one to the DC 200 A setting and connect between points 7.2and 7.3.(base current)

Set the other to the DC 20 mA setting and connect between points7.6 and 7.10.(collector current)

Set the third multimeter to act as a voltmeter on the DC 20V setting,connected between points 7.11 and 7.12. Get a demonstrator to checkyour wiring.

3. Switch on the unit and set the Variable DC control knob (which provides thecollector voltage) to 1V (i.e. set Vc=1V).

4. Adjust the 0 2V DC knob (base voltage) to give a base current of 10 A (i.e.set Ib=10 A) as closely as you can and record the actual value. You may findit easer to use a screwdriver or trimming tool to adjust the voltage supplies

as the circuit is very sensitive.5. Record the collector current in Table 2.6. Repeat steps 3 & 4 with base current set at 20, 30, 40 and 50 A.Note: You may notice a tendency for the collector current to creep up slowly atthe highest settings. This is caused by the dissipation of power as heat in thetransistor, which produces an increase in . Never try higher current andREDUCE the variable controls to zero when the readings are taken toprevent overheating and damaging the transistor.

7. On the grid provided, plot the transistor current gain characteristic Ic vs Ibfor Vc=1V

Base Current(target value)

Base Current(actual value)

Collector currentfor Vc = 1V

10 A

20 A

30 A

40 A

50 A

Table 2


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CollectorCurrent(mA)

20

18

16

14

12

10

8

6

4

2

0 10 20 30 40 50

Base current ( A)

The Common Emitter Amplifier

In this section, we will investigate some of theimportant aspects of a common emitter amplifier,perhaps the most frequently-encountered amplifiercircuit. Add a load resistor RL to the circuit in Fig11 to give Fig 13, and consider the sequencebelow:

Fix the collector voltage Vc at a certain value.Change Vb by a small amount.This will change the base current Ib.The collector current Ic will change (look at the results obtained from thelast section!).This will cause the voltage across the load resistor to change.Conclusion: a change in the base voltage will result in a change in theoutput voltage.From Table 2 above, it can be seen that the range of the base current is inA, whereas the collector current is in mA. Therefore, a small change in the

input leads to a large change in the output, and hence amplification!

Fig.13

VS

R

Ie

Vb

Ib

RL

Question 3: Rerrange equation 1 to show the current gain in terms of thecollector current Ic and base current Ib.Hence calculate the current gain (or hFE) of the transistor (using the lastrow of your table of values).


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Experiment 2.2: Voltage amplification: AC(Additional equipments needed: Oscilloscope, two multimeters, signal generator andsignal generator split connecting wire)

Fig. 14A very common application of the circuit you have just tested is as an amplifier,for instance within radio sets and other audio applications. It is instructive todemonstrate its use for this purpose.

Experiment 2.2 Part 1

1. Once again, reduce both power supplies to zero setting and switch off.2. Construct the circuit shown in Fig 14. The multimeter settings should be as

follows:First multimeter: 20V DC range between points 7.11 (+ve supply)and 7.12 (earth), measuring supply voltage VSSecond multimeter: 20V DC range between points 7.6 (collector)and 7.12 (earth), measuring collector voltage VC

3. Get a demonstrator to check your wiring, switch on and adjust the supplyvoltage to be 9V, using the Variable DC control knob.

4. Adjust Vb (0 2V DC knob) until the collector voltage Vc reachesapproximately 4.5V (the adjustment is very sensitive so any value between4 and 5 V is acceptable). This is said to bias the transistor so that itscollector has a potential which is halfway between the two limiting outputvalues (0V and 9V), maximizing the possible range of variations of V c(around 4.5 V with respect to its quiescent (neutral) position).

Experiment 2.2 Part 2

5. Switch off the main switch on the experiment base unit withoutresetting the voltage controls.


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6. Leave the circuit set up but additionally make the following connections:Use the appropriate wiring connector and connect the black plug from thesignal generator to point 7.4, circuit #7Connect the red plug from the signal generator to point 2.11, circuit #2Connect channel 1 of the oscilloscope also to point 2.11, circuit #2

Connect channel 2 of the oscilloscope to point 7.6, circuit #7Connect point 2.9, circuit #2 to point 7.1, circuit #7

(Note that the earths of the various parts of the circuit board, and of theoscilloscope, are connected together internally).

The demonstrators can help you to set the signal generator and oscilloscope.

The initial settings for the oscilloscope need to be as follows:Channel 1, AC coupled, 10 mV/divChannel 2, AC coupled, 1V/div

Timebase 0.2 ms/divSet the trigger to operate from Channel 2 as this is the stronger signal.

The initial settings for the signal generator need to be as follows:Frequency 1kHz, sine wave, symmetrical waveform.Set the attenuation at -40dB by choosing -20dB on the signalgenerator front panel (shift+8) by pulling out the amplitude knob(AMPL) for an additional reduction of -20dB.Amplitude knob set so that when oscilloscope is connected, anamplitude of around 40 mV peak-peak is measured on Channel 1.

The circuit should now be as follows (Fig. 15):

Fig. 15

This looks complicated, but in fact you are simply connecting a signalgenerator (simulating, for example, an audio signal from a microphone, or

other mid-frequency, low-amplitude signal) to the base of the transistor, andusing a capacitor (borrowed from another experiment) to isolate the DC


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potential of the transistor base from the signal generator. Note that a muchmore conventional layout of the circuit diagram for this experiment would beas shown in Fig. 16 (the voltmeters and oscilloscope are omitted):

Fig. 16.

7. Get a demonstrator to check your wiring, then switch on the base unit again.8. The amplifier should now be functioning correctly, and could be used in this

form as one stage of an audio amplifier. Write down the amplitudes of thesignals in Channel 1 and Channel 2 of the oscilloscope, and sketch thewaveforms. Note the voltage sensitivity (V/div) and timebase ( s/div) foreach channel.

Channel 1 actual value amplitude (V) Channel 2 actual amplitude

Voltage settings for

CH1 and CH2 (V/div)and the timebasesetting (s/div)

CH1 /div

CH2 /div

timebase /div

9. A distorted signal results if the signal input is too large or the amplifiervoltage levels are incorrectly set (incorrect biasing).

Question 4Calculate the gain of the amplifier as:

1)(ChannelinputsignalACofAmplitude

2)(ChanneloutputsignalACofAmplitudeamplifierofgainvoltageAC


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+Vcc

-Vcc

vo

vn

v

Part 3: Operational Amplifiers

The operational amplifier, or op-amp, is probably the most versatile componentavailable to an electronic engineer. It provides high gain and predictableperformance. Depending on the circuit configurations, many different functionscan be achieved. You will examine two, the inverting and non-inverting

amplifiers.

The symbol for the op-amp, which has two inputs and one output, is shown inFig 17. It is a differential amplifierresponding to the difference in the voltagesapplied to the two inputs. Normally an op-amp is used with external feedbacknetworks, which determine the function of the circuit. The characteristics of anideal op-amp are:

Voltage gain A =Output voltage vo = 0 when vp= vnBandwidth =

Input impedance Zi =Output impedance Zo = 0

Fig 17

Obviously these theoretical characteristics and cannot be achieved in practice.For example, the A741, introduced in 1968, is still available and is one of themost commonly used general purpose op-amps. It has an input resistance of

0.3M , an open loop gain of 25000, and a gain bandwidth of 250kHz. Somemore recent devices, have values of 10M , 300000 and 10MHz respectively, andare much closer to the ideal values. For many non-critical applications, ideal

characteristics are often assumed.

You will be using the Digiac 3000 Op Amp Board, a development board with theresistors, capacitors and op-amps built in, and you will only be required to insertthe wires.

In addition to this development board you also have a signal generator, digitalmultimeter and oscilloscope. MAKE SURE THAT THE ADJUSTMENTS OF THE

OSCILLOSCOPE ARE SET AT CALIBRATED.

Fig.18Digiac 3000 Board


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Experiment 3.1: Inverting amplifier


Fig 19

1. Ensure the power is OFF and that there is no link between points 2.11 and2.17. Use the multimeter to set (adjust) VR1 to 10k by setting themultimeter to a resistance setting and connecting it across VR1 (betweenpoints 2.17 and 2.19), then adjusting the VR1 knob until the meter reads10k.

2. Disconnect the multimeter and connect the circuit and scope leads asshown in Fig 19

,

ensuring that YOU REINSERT THE LINK BETWEEN2.11 AND 2.17 AND the probes are set to 1. The circuit you have set upis the one shown in Fig. 20.

3. Note the scope settings.4. Turn the DC supply switch on the board ON.5. Set the signal generator to 1 kHz and make sure the AMPL knob is not

pulled out.6. Use the oscilloscope (channel 1) to set Vin to 4V peak-to-peak (p-p).

Measure the output voltage, Vo on channel2. Record your results in theTable 4 and sketch the waveforms in grid provided.

7. Calculate the gain G = Vo/Vin and compare with theoretical gain G = -Rf/ Rin8. Switch off power and use multimeter to reset feedback resistor,VR1

to

40k.9. Repeat steps (3) and (4).10. Switch off power supply.

Fig 20


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Table 4

Channel 1setting

CH 1 probesetting

Channel 2setting

CH 2 probesetting

Timebasesetting

1 1

Rf=VR1 Vin Vo Is outputsignalinverted?

Measuredgain,G = Vo/Vin

Theoreticalgain,G = -Rf/Rin

Voltage settings forCH1 and CH2 (V/div)and timebase setting(s/div)

CH1 /div

CH2 /div

timebase /div


CH1 /div

CH2 /divtimebase /div


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Experiment 3.2: Operational amplifiers in practice; non-inverting

amplifier


Operational amplifiers are not ideal in practice due to unavoidable mismatches ofcomponents within the op-amp. This manifests itself in a number of forms,including the input offset voltage. With an ideal op-amp, the output voltage iszero when the voltage between the two inputs is zero. In practice, there isusually an offset voltage. This can be nulled using the external null inputs. Asshown in fig 21, a potentiometer is connected to pins 1 and 8 of the op-amp,and a -ve voltage (usually Vcc) is applied to the centre tap of the potentiometer.By adjusting the tap, the input offset voltage can be made zero.

Fig 21 Fig 22

To save time, the demonstrators will help to set the offset null adjustment ofthe experiment so that the output gives zero volts when there is no potentialdifference between the inputs. This maximises the range over which theoutput can vary between the positive and negative supply voltages, andserves roughly the same purpose as setting the bias of the transistor inexperiment 2. Therefore, do not adjust the offset null control

yourselves after it set(Fig. 22) as it should already be correctly set.

1. Connect the circuit and scope leads as shown in fig 23.2. Ensure that the probes are set to x13. Note scope settings and connect the signal generator to the board and turn

it on.4. Turn on DC supply board.5. Use the oscilloscope (channel 1) to set Vin to 2V peak-to-peak (p-p) with

time base of 0.2 ms/div. Measure the output voltage, Vo on channel2.Record your results in Table 5 and sketch the waveforms in grid provided.

6. Calculate the gain G = Vo/Vin and compare with theoretical gainG = (R9 + R10 )/ R107. Switch off power supply


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Fig 23 Fig 24

Table 5

Channel 1setting

CH 1 probesetting

Channel 2setting

CH 2 probesetting

Timebasesetting

1 1

Vin Vo Is outputsignalinverted?

Measuredgain,G=Vo/Vin

Theoreticalgain,G=(R9 +R10 )

R10


CH1 /div

CH2 /div

timebase /div


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Questions to be answeredafter the laboratory is completed:

Answer the following questions in a few sentences, in plain English (and in yourown words) on a separate sheet of paper (you may word-process the answers ifyou wish, though a neatly written solution is perfectly acceptable):

5. Explain the purpose of a diode.6. Explain the purpose of rectification7. Explain the process of smoothing when applied to a rectifier circuit.8. Describe how a transistor operates in terms of collector current and base

currentEND

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Semiconductors and Amplifiers Lab