Operational Amplifiers Questions

EEE 202 – Circuits ILab 6 Operational Amplifiers (configurations, simulation, implementation)
Lab 6 Operational Amplifiers
Reference Resource (optional):
Practical Electronics for Inventors, Paul Scherz and Simon Monk, 2016, ISBN 978-1-25-958754-2
Lab Overview:
Examine and understand use of Operational Amplifiers to design a simple amplifier. Also understand how to
make a resistor ladder voltage reference for operation of a general-purpose amplifier as a single-ended
amplifier. Understand the design process of simulation verification, in this case using LTSpice, and then
physical validation on a prototype board.
Inventory List:
The following is a list of available inventory for this Lab
Part
Details
Resistors
Assorted from 5 and 10% as needed
Waveform generator
AD 2
Oscilloscope
AD 2
Breadboard
830 Tie points
Op-Amp
Microchip MCP6022 or Analog Devices OP484
Alligator clips
Connectors
Assorted jumper wire
Red, black, white, green, blue, orange
Digital Multimeter
MS8217, DT-830B, DT838, or similar
Software
LTSpice
You will need to refer to Lab 1 for LTSPICE and Measurement, Lab 2 for resistor networks, and Lab 3 for
voltage dividers.
Linear Technology has produced a video that shows you how to perform AC analysis in LTSPICE. The link for
this video is http://www.linear.com/solutions/4581.
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Background Information:
A) Waveform Generators and Oscilloscopes in your Analog Discovery 2 Kit (Mandatory):
Check the following two videos:
1- Digilent’s explanation video: https://youtu.be/HUAy0J3XqaU
2- ASU’s explanation video (also posted in the lab folder): https://youtu.be/rtsABzoV-WI
B) Waveform Generators and Oscilloscopes in a Practical Lab (Optional):
A waveform generator (WFG) supplies a variety of time-varying voltages at its output terminals. There
are two terminals on the WFG – “Sync” and “Output” as seen in the figure below.
Sync Terminal
Output Terminal
For EEE 202, only the Output Terminal is used – you will never use the Sync Terminal.
1. Turn on the WFG using the Power button.
– By default, the WFG will deliver a 100mV peak-to-peak sine wave with a frequency of 1kHz
2. Press the “Freq” softkey to see the frequency
– To change the frequency, press the “Freq” softkey and enter the magnitude of the desired
frequency using the numeric keypad and select the desired units
3. Press the “Ampl” softkey to see the peak-to-peak voltage
– To set the peak-to-peak voltage, press the “Ampl” softkey and enter the desired amplitude
using the numeric keypad and select the desired units
4. The rotating knob can also be used to adjust the amplitude and frequency values, rather than the
numeric keypad
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Oscilloscopes are used for measuring output waveforms and unlike a multimeter, they provide a visual
image of the waveform. The accuracy of oscilloscopes can vary greatly from model to model. Some
scopes have higher resolution, some are very sensitive to input noise, and some have built in statistical
measurements that can jump around when in use. In industry, you would have to select the
appropriate oscilloscope to meet your specific needs. The scopes used in the EEE 202 lab (or similar)
are shown below.
Scale Knob
CH 2 Scale Dial
Channel 2 Input
Channel 1 input
For EEE 202, only Channels 1 and 2 will be used – you will never use the AUX Channel (Trigger).
The specifications for each Channel can be found on the oscilloscope noted above the inputs. The
resolution of an oscilloscope is the ability to tell two points apart. Looking at the oscilloscope, you can
find the minimum voltage and time that can be displayed.
Connect the WFG output to the input of Channel 1 on the scope. Use the adapters (one on the WFG
and one on the scope) – the red terminal is positive and the black terminal is negative. The black
terminal ALWAYS connects to the ground side of the circuit. Push the “Autoset” button to reset the
scope and push the “Menu off” button to get rid of the menus. Each division is one FULL box. The
horizontal axis is time and the display will tell you how much time is represented by 1 division of the
screen (bottom of the display, just right of center). The vertical axis is voltage and the volts per
division is displayed bottom left corner for each channel.
To manipulate the display of the signal on the screen, use the Scale Knob for the horizontal axis and
the Scale Dial associated with the desired Channel. Try adjusting both scales and observe what
happens – as you change the scales, does the information on the oscilloscope still match the output of
the waveform generator?
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The waveform generator is computer controlled – the chip inside makes an assumption about the
devices connected to it. The WFG assumes the oscilloscope has an input impedance of 50 Ohms and
creates a model with that assumption, when in reality, it has a much larger input impedance. This
erroneous model used by the WFG causes a discrepancy between the WFG display and the
oscilloscope result.
To match the oscilloscope with the waveform generator, do the following on the oscilloscope:
1. Press the “Utility” button
2. Navigate the menu to set the output termination
3. Press the “Output Setup” menu key
4. Press the “Load” menu key to choose “High Z”
5. Press the “Done” menu key (notice “High Z Load” in the upper right corner of the display)
For all EEE 202 labs, we will use the High Z Load setting.
A few notes for taking measurements on the Oscilloscope:
1. Always use high resolution mode: “Aquire”  Mode  Hi-Res
2. Use the scope to display various statistics (peak-to-peak voltage, frequency, period, etc.) using
the “Measure” button
**** End of Optional Part *****
Data Sheet Part 0
1) Note the maximum voltage of the Waveform Generator.
2) Adjust the WFG to output a 1V (peak to peak) sine wave with a frequency of 2 kHz.
3) Record the Oscilloscope input impedances of Channel 1 and Channel 2.
4) Record the maximum input voltages that can be applied to Channel 1 and Channel 2.
5) Record the voltage resolution of the oscilloscope.
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Lab Instructions:
For the Final Design Project, Sparky Solar has asked you to measure the voltage and current of their solar
panel systems. To effectively measure current, without impacting the load circuit that the Sparky Solar is
providing power to, an op-amp will be used. Op-amps are very effective tools for measurement as they can
boost voltages or currents to higher magnitudes (to make them more readable), they can reduce high voltages
or currents to lower magnitudes (to protect measurement equipment), and they can change the phase of the
signal, all without impacting the load that is being supplied.
Sparky Systems wants to build an amplifier design prior to the final design project to verify their assumptions.
Using the lab instructions below, design an inverting amplifier, so that the Sparky representatives are
confident in their design decisions (and so you can get some experience before the Final Design Project).
Sparky Systems has given you some specifications for your amplifier design:
1) The input AC signal has voltages ranging from 2 to 3 Volts
2) The output voltage desired from the amplifier is greater than 0 and less than 4 Volts.
3) The op-amp circuit output should be 2x the amplitude of the input.
4) The op-amp circuit output should be 180 degrees out of phase with the input (in other words, v o is
“inverted” compared to vi).
5) You will have a single DC supply of 4 Volts to run your amplifier.
Step 1: Build a voltage reference
Virtual ground voltage reference
To operate an op-amp, you will need a high-side DC supply (V dd), a low-side DC supply (Vss), and a DC voltage
reference (VRef). In this design, we only want to use one 4V power supply for the op-amp. We can set V dd =4V
and set Vss to ground (0V). This means, we must create a “virtual ground” for VRef. If Vdd was 4V and Vss was
-4V, we could make our reference point in the middle at 0V, but that would require two distinct power
supplies (a 4V supply and a -4V supply). Instead, we’ll design a virtual voltage reference so that we can use a
single 4V power supply. Remember this information from Lab 2 concerning Ohm’s Law and power and power
dissipated by a resistor. Feel free to refer to Lab 2, and the lecture materials for a review of these concepts.
You will need to make a resistor divider to create the voltage reference (V Ref), which you will use as a “virtual
ground” for the op-amp. Remember these are the considerations in making a resistor ladder:
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Normally you build a resistor divider without RC, but it
has been included to emphasize the assumptions of
the resistor divider, which is that the current IA = IB,
that is IC ~ 0, or at least can be considered insignificant
compared to IB. IC is the current that this voltage
reference VRef supplies to the op-amp pin it is
connected to. We will see the impact of this in our opamp design configurations.
Vdd =4V
Vss = 0V
𝑹𝑩 𝑹𝑪
𝑽𝑹𝒆 𝒇
𝑹 𝑩 + 𝑹𝑪
=
𝑹𝑩 𝑹𝑪
𝑽𝑺
+ 𝑹𝑨
𝑹 𝑩 + 𝑹𝑪
Data Sheet Part 1 (You can use the 34401A or Portable Multimeters for this part of the lab)
1) Assuming IC ~ 0A for Vdd = 4 V what should be the relationship between RA and RB to set VRef = 2 V?
On your breadboard, build the circuit without Rc and using large values for RA and RB (around 10 k). With
a multimeter, measure the value of VRef and record the value along with your values of RA and RB in the
data sheet below. (Note: the multimeter draws current out of the circuit during measurement, but it is
assumed to be an insignificant amount so that it doesn’t impact the measurement significantly.)
2) Assume RC = 1 M and RB = 100 k in your circuit and calculate RA so that VRef = 2 V. On your breadboard
build the circuit with these values. If you cannot find a resistor with the value R A in your inventory, use a
combination of series and parallel making sure your V Ref is close to 2 V. Measure IA, IB, and IC in this case
and record that along with your value of RA in the Datasheet below (refer to Lab 1 if you don’t remember
how to measure current – hint…break the circuit!).
3) Assume RC = 10 k and RB = 1 k in your circuit and find RA so that VRef = 2 V. On your breadboard build
the circuit with these values, and make sure VRef is close to 2 V. Measure IA, IB, and IC in this case and
record that along with your value of RA in the Datasheet below.
4) Assume RA = RB and we want VRef to be at 2  0.05V (that is somewhere between 1.95V and 2.05V) where
RC is 1 M. What are RA and RB? On your breadboard build the circuit with the values you have chosen,
then measure VRef with a multimeter. Get as close as you can to VRef and record RA, RB, and your measured
VRef in the datasheet below.
5) Assume RA = RB and we want VRef to be at 2 0.05V where RC is 10 k. What are RA and RB? On your
breadboard build the circuit with the values you have chosen, then measure V Ref with a multimeter. Get as
close as you can to VRef, and record RA, RB, and your measured VRef in the datasheet below.
For an ideal op-amp, we assume that the input resistance is infinitely large (i.e. RC is very big). For actual opamps in the lab, there will be a small current going into the input pins (RC will not equal infinity) – think about
how that relates to what you saw in parts 4 and 5 above.
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Operational Amplifier (Op-Amp)
In the lecture, you learned about the calculations and applications of basic ideal op-amp circuit configurations
with resistive feedback. Op-amp circuits are one of the most useful circuits across industries because they are
fairly simple to design, low cost to prototype, and can accomplish a number of functions including
amplification, buffering, mathematical operations, measurement, and more. All of the calculations and
analysis done on op-amps in this course have assumed ideal op-amps. Let’s start to simulate and build some
physical prototypes to better understand how real op-amps work.
Data Sheet Part 2
The figure below shows an inverting configuration op-amp. Note that it requires a V+ and V- supply. For the
MCP6022 and OP484 a DC supply voltage of 4V will work. This type of supply and operation with a positive
and negative supply is referred to as dual-ended. This means input v i and output vo can have voltages that are
positive or negative with values approaching (but never up to) the supply voltages you are using.
The design you will build will use a single battery supply, which does not have positive and negative voltages
available. This is shown in the figure below. In this case v i and vo can have voltages between 0 and close to the
supply voltage, Vs. This is referred to as a single-ended design.
In LTspice build the circuit shown below using a DC power supply Vs = 4V using the op-amp, resistors R1, R2,
RA, and RB. In LTSpice use Op07 (aka OP-7) for the Op-Amp model to simulate the physical Op-Amp.
Given what we know about the input impedance of the op-amp, which scenario from Part 1 should we use
for RA and RB and how does the value of R1 impact this?
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This is an inverting configuration, single-ended amplifier circuit:
 R2
vI (you derived this in the lecture), but we need to
R1
R
modify this equation slightly for our single ended design:  vO  VR EF   2  vI  VREF  .
R1
The inverting configuration equation is equal to vO 
To achieve the amplification required, what ratio of R2/R1 do we need? If you wish you can assume a value of
R1 = 10kohm.
What value will you use for R1 and R2?
Using LTspice, simulate your design. Use an input sine wave with a peak-to-peak amplitude of 1V and an offset
of 2V with a frequency of 1kHz. Your input sinusoidal waveform should be a sinusoid that goes from 1.5 to 2.5
volts and has a period of 1ms (this means that the waveform repeats after 1 millisecond or 1/1000 th of a
second)
In the run control set the simulation type to transient simulation. Simulate for 0.01 sec with samples ever
0.1ms. Note the supply voltage is 4 V (DC). This sets the label VCC to 4 VDC. The resistor values are hidden by
gray boxes, so you will have to fill in those boxes and then simulate.
When the simulation runs place a probe at the output of the Op-Amp and the positive terminal input source
V1.
Data Sheet Part 2
 Record your calculations and values for R1, R2, RA, RB in the datasheet.
 Capture an image of your LTSpice circuit and include it with the datasheet.
 Capture the results of your simulation and include it with the datasheet.
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Data Sheet Part 3: Physical Validation
You will be using a rail-to-rail op-amp, the Microchip MCP6022, which will operate correctly with a supply
voltage of about ±2V to ±6V or the Analog Devices OP484, which will operate correctly with a supply voltage of
about ±3V to ±18V. A “rail-to-rail” op-amp means that the op-amp is designed to work very closely to the high
and low supplies. An op-amp that is not designed to be rail-to-rail, will start to lose functionality as the inputs
and outputs get too close the supplies (Vdd and Vss). For this lab you will use an 8-pin or 14-pin through hole
dual in line package (DIP). This is commonly used for prototyping with a breadboard. Surface mount devices
have a smaller, more compact package and are more common on modern circuit boards. Most modern opamps are available in either package.
An Op-Amp is called a “rail-to-rail” opamp if it can output voltage values that
can reach the two “rails” of its power
supply (i.e. the 0 and the +4V for your
design). The following figure shows the
output of a rail-to-rail op-amp and that
of a non-rail-to-rail one with a supply
voltage of 10V. The non-ideality in the
latter results in a 0.5V “clipping”, and
thus distortion, to the signal.
The pin-outs for the op-amps ICs (integrated circuits) can be found below:
MCP6022 Pin-Out
OP484 Pin-Out
As seen above, each IC contains more than one op-amp on the chip, so you could build multiple circuits with
multiple op-amps using a single chip. You can determine the location of the pins on each physical chip using
the semi-circle cut-out at the top (which is also on the physical chip). These pin-outs act as a map for you to
build the circuit. Note: The high-side and low-side DC supplies of the op-amps are shared on a single IC. For the
MCP6022, VDD is the high-side supply, VSS is the low-side supply. For the OP484, V+ is the high-side supply, Vis the low-side supply.
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Use your prototype board (breadboard), op-amp and resistors and build the single-ended inverting op-amp
circuit you simulated in LTspice.
Capture an image of your prototype board and include the image with the Datasheet.
Use the DC power supply for the 4V supply. Use the waveform generator to create the input signal with 2 V DC
offset and 1V amplitude at 1kHz.
View this on an oscilloscope, connecting Channel 1 to the output of the waveform generator and Channel 2 to
the output of the Op-Amp. Use Channel 1 as a trigger (or reference) and get a stable figure showing the input
and output relationship plotted against time. This result should be similar to what you expected in your design
and to your simulation results. If it is not, remember that the objective of physical validation is to circle back to
your design calculations, simulation or breadboard work and correct mistakes or misunderstandings. It is very
typical to find that the physical results don’t match the ideal results exactly.
Data Sheet Part 3
 Capture an image of your physical prototype board and include the image with the datasheet
 Capture the oscilloscope screen image entirely and include it with the datasheet.
 Is your Op-Amp a “rail-to-rail” Op-Amp? If not, what are the upper and lower clipping voltages?
 Answer the discussion questions about the lab
Fill in your data sheet and print the required LTspice schematics, images, and waveforms. Labs can be done as
a team, but each member must complete and submit their own data sheet.
Lab CLOS:
– Understand how to make a single-ended inverting op-amp design
– Use LTSpice to do a simulation verification of your design
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–
Use a waveform generator and oscilloscope
Use a breadboard to do a physical validation of an op-amp design
Understand how to design with inverting configurations of an op-amp
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