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Experiment 5 – Design of an Operational
Amplifier Using Multisim
Department of Electrical Engineering & Electronics
January 2021, Ver. 5.1
Experiment specifications
Module(s) ELEC271
Experiment code 5
Semester 2
Level 2
Lab location 3
rd floor
Work Individual
Timetabled time 7 hrs
Subject(s) of
relevance
Transistor Circuits, Differential/Operational Amplifiers
Assessment method Pre-lab test (20%)
Formal report, using the report template (80%)
Submission
deadline
As announced by the lab coordinator for your group, via Canvas.
Exp 5, Ver 5.1, January 2021
Instructions:
The Pre-Lab Questions must be answered before the lab day (deadline is 9 am).
They are available on Canvas ELEC273/224/222 Modules and worth 20%.
This is a design experiment. You are expected to do work and self-study before the
lab day. Read this script before attempting the experiment. Prepare the design
before attending the lab to save time.
Review Multisim software before attempting the experiment. Check Canvas for
Multisim resources. See online material and resources as well.
Keep a record of all schematics and results.
Refer to the hints and guidance given in the lectures of Module ELEC271
whenever needed during the lab.
Use the report template (available on Canvas) to write your formal report after
the lab.
If you have any feedback on your laboratory experience for today, please write it
down in the last page of this script.
1. Learning outcomes
At the end of this lab, you will:
be able to produce a functioning operational amplifier circuit by combining different
stages.
be aware of problems and challenges to meet a specification for designing and
characterising an operational amplifier.
2. Objectives
It is required to design an operational amplifier with the aid of Multisim software to satisfy the
following design specifications:
a) Differential input impedance greater than 100 k .
b) Voltage gain (that is, ‘open loop gain’) greater than 500,000.
c) Output impedance less than 1 k .
d) Output voltage to be approximately zero volts for zero input.
e) Frequency response down to dc (0 Hz).
f) Supply voltage 9 V.
g) Total current consumption not greater than 5 mA.
3. Introduction
This experiment requires you to design and simulate an operational amplifier circuit. A
practical integrated circuit version (e.g. a 741 type) would in addition contain a ‘push pull’
output stage and extra components to provide further temperature and supply variation
immunity.
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Exp 5, Ver 5.1, January 2021
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The experiment is intended to reinforce lecture material in module ELEC271, and reference
to the relevant lecture notes is essential. Refer also to the preparatory slides/stream in the
introductory lectures to Experiment 5.
3.1. Operational amplifier
An operational amplifier (op-amp) is a high-gain voltage amplifier with a differential input and
a single-ended output [Figure 1 (a)]. It is among the most widely used electronic devices today,
being used in a vast array of consumer, industrial, and scientific devices, with very low cost.
The op-amps had their origins in analogue computers where they were used in many linear,
non-linear and frequency-dependent circuits. Their popularity in circuit design largely stems
from the fact that characteristics of the op-amp circuits with negative feedback (such as their
gain) are set by external components with little dependence on temperature changes and
manufacturing variations in the op-amp itself. Examples of op-amp circuits are shown in Figure
1 (b) and (c).
(a) Op-amp symbol
(b) Inverting amplifier circuit, Gain= Rf/Rin. (c) Non-inverting amplifier circuit, Gain=1+(R2/R1).
Figure 1. Operational amplifier and circuit applications
The op-amp is one type of a differential amplifier. Other types of differential amplifiers include
the fully differential amplifier (similar to the op-amp, but with two outputs), the
instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar to
the instrumentation amplifier, but with added tolerance to common-mode voltages that would
Exp 5, Ver 5.1, January 2021
4
destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more
op-amps and a resistive feedback network).
3.2. The design
The op-amp you are required to design can be constructed from four basic ‘building block’
circuits:
a) An emitter follower
b) A common emitter amplifier
c) A current mirror circuit
d) A differential input stage (long-tailed pair)
The designed operational amplifier schematic will be simulated using Multisim software
library models located in the Components library. To place a component, open the ‘Select a
Component’ window from the Place/Component menu (Ctrl+W) or by clicking on a
component icon. In this experiment, two transistor types, npn and pnp, are used. You can find
these transistors in the ‘Select a Component’ window:
Q2N2222 npn transistor, from Transistors/BJT_NPN/PN2222.
Q2N2907 pnp transistor, from Transistors/BJT_PNP/PN2907.
In the above notation, ‘Transistors’ is the Group, BJT_NPN (BJT_PNP) is the Family, and
PN2222 (PN2907) is the Component name. This notation is used throughout this document.
For example, to place a 2N2222 transistor, select ‘Transistors’ from Group, BJT_NPN from
Family, and select PN2222 from the Component list. Alternatively, you can search for PN2222
in the search box below ‘Component’.
The output from your differential amplifier should be fed into the input of the common emitter
stage. It is also necessary to include an emitter follower circuit as a buffer between the two
amplifier stages. Figure 2 shows a block diagram of the op-amp.
Figure 2. Block diagram of the op-amp
The properties of these blocks should have been investigated and design procedures established
in the exercise ‘Pre-lab Test for Experiment 5’. It is essential that this is done before
commencing.
Differential
Amplifier
Emitter
Follower
Common
Emitter
Emitter
Follower
Vin+
Vin- Vout
Exp 5, Ver 5.1, January 2021
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4. Experimental work
4.1. Part I: Transistor output characteristics
Objective: To obtain a set of output characteristics for the two transistor types to be used in
this experiment.
Obtain a plot of the output characteristics of the 2N2222 transistor, i.e. IC vs. VCE (0 to 20 V in
steps of 0.1 V) for a range of IB (0 to 40 μA in steps of 4 μA).
Procedure:
Task-1: Input the circuit schematic of Figure 3 in Multisim schematic worksheet. You
can find the other components in ‘Select a Component’ window:
o Voltage source: Sources/POWER_SOURCES/DC_POWER.
o Current source: Sources/SIGNAL_CURRENT_SOURCES/DC_CURRENT.
o Ground: Sources/POWER_SOURCES/GROUND.
Double click on voltage source and current source and change their labels to VCE and
IB, respectively.
Task-2: To setup the simulation, click on the simulation setup icon (or from Simulation
menu select Analysis and simulation). In the Analysis and Simulation window, select
DC Sweep from the left panel. On the Analysis Parameters tab select VCE as the Source
for Source 1 and IB as the Source for Source 2. Set the sweep parameters into the
desired range, as indicated above. On the Output tab, select I(Q1[IC]) and add it to the
Selected Variables for Analysis panel on the right.
Task-3: Click on the run icon (green triangle) to start simulation. A response graph will
appear. Take a screenshot of the graph. Alternatively, you can copy the graph and paste in
a Word document.
Task-4: From the graph, estimate the dc current gain, (also known as hFE) at IC around
2 mA. Estimate also the ac (small signal) current gain, 0 (also known as hfe). Hint:
see your notes on how to do this. Record your results.
Figure 3. Schematic diagram of Part I.
Q1
PN2222 VCE
20V IB
4uA
Exp 5, Ver 5.1, January 2021
4.2. Part II: Achieving the specification of the operational amplifier [35 Marks]
Objective: To build the complete operational amplifier circuit and obtain the required
specification.
Start now building the complete circuit of the operational amplifier (see the given ‘tutorial’
lecture for ELEC271 on Canvas).
Procedure:
Task-1 [5 Marks]: Combine the required stages to build a complete operational
amplifier in Multisim.
Hint: Match the differential amplifier and common emitter stages with an emitter
follower stage, make the
~ 10

.
Task-2 [5 Marks]: Connect a signal source to one input and connect the other input to
ground. Obtain the transfer characteristics of your amplifier by performing a DC Sweep
from –9 V to +9 V. Identify the useful input voltage range from your plot. You will
need to narrow the sweep range to achieve an accurate useful range.
Task-3 [5 Marks]: Find the open loop gain (Aol) of the amplifier from the useful range
in the above step.
Task-4 [5 Marks]: Determine the required dc voltage offset from the transfer
characteristic. Use this value to help balance the amplifier – you can apply a small dc
offset to one of the inputs to try to centre the output close to zero volts. Hint:
Alternatively, you could use the technique mentioned in the lecture notes to obtain Aol
which would also give you the dc offset.
Task-5 [5 Marks]: Obtain a set of input/output waveforms (in useful range) from a
Transient simulation, and from which, calculate the gain of the amplifier. Verify that
the specification has been met. Record your results.
Task-6 [5 Marks]: Perform a Transfer Function simulation. Check the value of the
gain and compare it to the value from your calculations and the value you found in
Task-5. Check the values of the input and output impedance and compare them to your
calculated values.
Task-7 [5 Marks]: Place a few voltage, current, and power probes on different points
of your schematic diagram (power probes should be placed on a component). Run an
Interactive simulation and observe the voltage, current, or power showing by the
probes.
Hint: You can double click on each probe and on the Parameters tab select the
parameters that you want to be shown by the probe.
4.3. Part III: Obtaining the frequency response of the designed amplifier [10 Marks]
Objective: To obtain the gain and phase Bode plots of the designed amplifier.
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Exp 5, Ver 5.1, January 2021
Now, obtain the frequency response of the designed amplifier by determining the gain and
phase Bode plots.
Procedure:
Task-1 [5 Marks]: You can find the frequency response of the amplifier by performing
an AC Sweep simulation. In AC Sweep page in Analysis and Simulation window, on
the Output tab, add the output voltage from the left panel to the Selected variables for
analysis on the right panel (placing a voltage probe at the output makes it easier to find
the output node in the list of variables). To draw the frequency response as Bode plots,
on the Frequency parameters tab, select Sweep type: Decade and Vertical Scale:
Decibel.
You can also obtain the frequency response of the other parameters, like the input
impedance. In the AC Sweep page, on the Output tab, click on Add expression button.
In the Expression field, write an expression for the input impedance in form of V/I. For
example, if you have named your input signal source as V1, the expression for input
impedance would be V(1)/I(V(1)).
Task-2 [5 Marks]: Add a phase compensating capacitor (say 30 pF or any suitable
value) between the collector of the common emitter stage and the base of the first
emitter follower. Investigate the effect on the Bode plots. Copy the graphs and record
your findings.
4.4. Bonus Part: Response to common-mode signal
Investigate the response of your amplifier to common-mode signals.
5. General questions
a) What can you deduce about the stability of your amplifier from the Bode plots in Part
III
b) What is the purpose of the ‘Phase compensating capacitor’
6. Report writing guidelines for Experiment 5
This experiment is assessed by means of a formal report. Use the formal report template
(available on Canvas) to write your report. The idea behind this report is to document
your experience and technical findings in this experiment.
In your ‘Results’ section, include the following:
– All your findings and measurements along with a copy of all schematic diagrams
and all simulation results. Make sure to comment on each result and simulation.
– The table shown overleaf (Table I) with your own results. Make sure to write
relevant comments in the fourth column.
In your ‘Discussions and Conclusions’ section include the following:
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Exp 5, Ver 5.1, January 2021
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– Describe any problems you have experienced while carrying out both experiments
and explain your problem-solving methods that you have followed.
– Include the answers to the general questions (Section 5 above).
– Does your designed op-amp achieve the specification If not, explain the reasons.
Important: Make sure that all of your figures (schematic diagrams and simulation
results) are clear and all the numbers on the figures are readable.
Table I. Design specifications table
Parameter Specification Your value Comment on the
value obtained
Differential input impedance > 100 k
Open loop voltage gain > 500,000
Output impedance < 1 k DC output voltage ~0 V DC offset voltage None given Frequency response Down to DC (0 Hz) Total current consumption < 5 mA Bandwidth with compensation capacitor None given 7. Report marking scheme The report has 20 marks of the module. It will be marked out of 100% and then the mark will be scaled to 20%. The pre-lab test has 5 marks of the module. The report marking scheme is as follows: Results of Part II (calculations, schematics, screenshots, etc.) with explanation and comments: 35 Marks Results of Part III (calculations, schematics, screenshots, etc.) with explanation and comments: 10 Marks The design specifications table with comments: 24 Marks Discussions and Conclusions section (including answers to the general questions of Section 5): 26 Marks Overall report presentation: 5 Marks Note: High marks require very good comments and explanations. Exp 5, Ver 5.1, January 2021 9 8. Plagiarism and Collusion Plagiarism and collusion or fabrication of data is always treated seriously, and action appropriate to the circumstances is always taken. The procedure followed by the University in all cases where plagiarism, collusion or fabrication is suspected is detailed in the University’s Policy for Dealing with Plagiarism, Collusion and Fabrication of Data, Code of Practice on Assessment, Category C, available on: https://www.liverpool.ac.uk/media/livacuk/tqsd/code-of-practice-on_xfffe_assessment/appendix_L_cop_assess.pdf Follow the following guidelines to avoid any problems: a) Do your work yourself. b) Acknowledge all your sources. c) Present your results as they are. d) Restrict access to your work. References [1] S Hall, Lecture notes-Module ELEC271, 2015. Version history: Name Date Version N Sedghi January 20201 Ver. 5.1 N Sedghi July 2020 Ver. 5.0 M López-Benítez September 2019 Ver. 4.1 A Al-Ataby February 2015 Ver. 4.0 A Al-Ataby February 2014 Ver. 3.2 A Al-Ataby February 2013 Ver. 3.1 A Al-Ataby March 2012 Ver. 3.0 S Hall September 2011 Ver. 2.2 S Hall March 2011 Ver. 2.1 T Dowrick/S Hall December 2009 Ver. 2.0 T Dowrick/S Hall August 2008 Ver. 1.0 Exp 5, Ver 5.1, January 2021 Feedback: If you have any feedback on your laboratory experience for this experiment (e.g. timing, difficulty, clarity of script, demonstration…etc) and suggestions to how the experiment may be improved in the future, please write them down in the space below. This feedback is important for future versions of this script and to enhance the laboratory process, and will not be assessed. If you wish to provide this feedback anonymously, you may do so by detaching this page and submitting it to the Student Support Centre (fifth floor office). Script re-writing award If you think that this experiment could do with enhancement or changes and you have some ideas that you’d like to share, why not re-write this script yourself and you may get an award from the lab organisers with an official letter of thanks, and your name will be added to the version history list in future versions of the script. Something good for your CV. Contact one of the lab organisers for more details. 10