operational amplifiers: remote laboratory circuit

Operational Amplifiers:

Remote Laboratory Circuit Experiment: Non-Inverting & Inverting

Amplifiers


Important Announcement:

This is combining the two operational amplifier labs since it will be easy to switch the simulation

between the inverting and non-inverting amplifier. This gives us two weeks to perform the labs

and write the reports. I would like everyone to relax, look at this as a learning experience. At

one time, I was a Field Support Engineer for in oil exploration and would go to area that required

an engineer’s assistance. Sometimes these sites were in remote areas such as the jungles of

South America or the Great Plains of South Dakota and I was working remotely. Usually I

worked with support over the phone and computer connections at 9600 bits per second. Mostly

the work just took extra time but we still got everything done. We are in much better shape here.

We can have the phone, Video Call Conferencing and can run programs all at the same time.

Have patience and we will all get what we need accomplished. One day you will be telling

stories about how you had to go to school remotely, instead of my stories about walking five

miles to school, barefoot in the snow and uphill in both directions. And that was in the Summer!

Equipment Needed:

Computer with Operating System Version to be stated in report. (? Windows 10?)

Multisim Simulation Software to be stated in report. (Multisim 14.1)

Reference Material:

Operational amplifiers are the main building block of analog systems. They can provide gain, buffering, filtering, mixing and multiple mathematic functions.

https://en.wikipedia.org/wiki/Operational_amplifier

When reviewing this Wikipedia entry, the things to take away are:

1. The electronic symbol – see Figure 1.

2. The electrical equivalent circuit – see Figure 2.

3. There are two different types of input terminals

a. A Positive or Non-inverting or + (plus) terminal

b. A Negative or Inverting or – (minus) terminal

https://en.wikipedia.org/wiki/Operational_amplifier



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Figure 1: Op-Amp Circuit Symbol Figure 2: Op-Amp Equivalent Circuit

4. An Ideal Op-Amp has the following characteristics:

a. Infinite Open-loop Gain where Gain = A = Vout/Vin & Vin = V+ minus V-, the

voltage difference between the two input terminals.

b. Infinite Input Impedance, that is Rin (Figure 2) is infinite [Input current = 0]

c. Zero Output Impedance, Rout = 0 (Figure 2), => Vout = G*Vin without regard

to the output current needed.

5. These infinite gain amplifiers are used in circuits with Negative Feedback.(see below)

FEEDBACK:

There are two types of feedback – Positive Feedback and Negative Feedback.

In psychology, positive feedback encourages a continuation of the same behavior and negative

feedback discourages the continuation of the same behavior.

Similarly in electronic circuits, positive feedback causes the continuation or enhancement of a

response to a stimuli and negative feedback acts to stop the response to a stimulation.

This can be illustrated by two bar pendulums, each placed in a stable but opposite position.

See the Figure 3 below.



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Figure 3: Two Pendulums in stable states.

Each Pendulum is in a stable position in that if no outside force disturbes the pendulums they

will stay where the are indefinitely – Pendulum A straight down and Pendulum B straight up

(and balanced). The feedback force on each pendulum is gravity pulling down. Now if there is

the slightest force that displaces Pendulum B such as a gravitational wave that monentrially

causes the right half of the pendulum to be heavier than the left and the top of Pendulum B

moves almost inperceivably to the right. The positive feedback force of gravity will continue

and amplify this movement until the pendulum will not return to its former stable state. The

force of gravity is a positive feedback force here because it acts to increase any displacement

movement.

Now if Pendulum A is in its stable state of straight down and there is a momentary small

displacement force that moves the pendulum out of its stable state, the negative feeback force of

gravity will eventually bring it back to its stable position. Here gravity is negative feeback

because it counter acts the force that tries to displace the pendulum.

NEGATIVE FEEDBACK:

Before viewing the Khan Academy videos consider the circuit in Figure 4, below:

Figure 4: Voltage Follower, Gain = 1



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The Op-Amp terminals Vs+ and Vs- are the power supply terminals and provide the power for

the op-amp to function.

The Ideal Op-Amp’s output can be infinite times any voltage difference between V+ and V-.

If V+ is more positive than V- then the output goes up and if V- is more positive than V+ then

the output goes down. V+ is the non-inverting input terminal and V- is the inverting input

terminal.

This ideal op-amp is now connected in a circuit as shown in Figure 4 with a feedback connection

from the ouput to the inverting terminal – a negative feedback connection.

When a positive voltage, Vin, is placed on the V+ terminal, there is a voltge difference between

V+ and V- and V+ is more positive, the Vout starts going up.

Vout is connected directly to V- so V- = Vout and V- starts going up.

When V- becomes more positive than V+, then Vout starts going down so V- starts going down.

As V- drops below V+, V+ is more positive and Vout goes up so V- goes up.

This V- is more positive than V+, then V- is less positive than V+, oscillation continue but with

the difference being less and less each time.

This continues until Vout = V- = V+ so there is no difference between V+ and V- and Vout = V-

= V+. After the transitent if there is one, the circuit settles into a stable state.

This circuit has a gain of one (A=1) and is called a Voltage Follower or an Op-Amp Buffer.

This is a demonstration of Negative Feeback, the type of feedback that stablizes a circuit and

prevents oscillations and prevents the output from causing itself to increase or decrease without

bounds.

In this case Vout = V- and all the output voltage was ‘fedback’ to the input.

What would happen if only part of the output was fedback?

Say the output voltage had a resistor voltage divider circuit connected that divided the voltage in

half ( the voltage value between the two resistors was one-half the output voltage Vout).

Connect this one-half Vout value voltage to the V- terminal, See Figure 4.

Figure 4: Non-Inverting Amplifier Circuit

What does the value of Vout need to be for V- to equal V+ and stablize the amplifier circuit?

Now watch the “Operational Amplifier” videos at Khan Academy. There are seven videos,

Please watch all seven.



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1. What is an Operational Amplifier

2. Non-inverting op-amp

3. Feedback

4. Inverting op-amp

5. Virtual ground

6. Virtual ground – examples

7. Summing op-amp

Note any issues or questions you have and email me for help or I will be available on my office

phone (505) 277-4409 by appointment during class and lab time. The TA will also be available.

His email is [email protected]. He can host a Zoom meeting as well.

https://www.khanacademy.org/science/electrical-engineering/ee-amplifiers

or

https://www.khanacademy.org/science/electrical-engineering/ee-amplifiers/ee-opamp/v/ee-

opamp-intro

After watching the videos, we will build and measure some Op-Amp Circuits in simulation using

Multisim.

If you haven’t already downloaded Multisim: “Download Mutisim Trial Instructions” word file

on the class website provides instructions on how.

The latest version of this procedure should be online at the class website.

Beginning of the Operational Amplifier (Op-Amp) Lab Procedure:

You should be familiar with the concepts and characteristic of Op-Amps now. If not, read the

above sections and view the videos. Note any questions and misunderstandings you may have

and email me or call on the phone during class and lab time (10:00 AM and 5:00 PM of Friday).

Email to make an appointment for other times.

You can still work with a partner if you can coordinate your work over the phone or with Skype

or other conferencing software. You can work alone if you like. If you work with a partner then

turn in one lab report as always. If you work alone, you will turn in a lab report for yourself.

In this lab we will construct a Non-Inverting Amplifier and an Inverting Amplifier using an Op-

Amp. We will make measurements of the gain and phase shift at a set of test frequencies.

The gain will be set using the last digit of your student ID. If the last number is 1 or 0 then use

your partners ID number, if possible. If your partner is also 1 or 0 or you work alone and your

ID last digit is 1 or 0 then set the gain to 4.

mailto:[email protected]

https://www.khanacademy.org/science/electrical-engineering/ee-amplifiers

https://www.khanacademy.org/science/electrical-engineering/ee-amplifiers/ee-opamp/v/ee-opamp-intro

https://www.khanacademy.org/science/electrical-engineering/ee-amplifiers/ee-opamp/v/ee-opamp-intro



Amplifiers

Non-Inverting Op-Amp based amplifier.

Calculate the value of the resistors needed for the desired gain assuming Ri is 1 kOhm:

For example, we want a gain of 5.25 (Your gain will be a whole number between 2 and 9)

Since the gain of the Non-Inverting is calculated using the formula Gain = Vout/Vin = 1 + Rf/Ri,

see Figure P1. 5.25 = 1 + Rf/Ri => 4.25 = Rf/Ri and if Ri is preselected as 1kOhm then Rf =

4.25 kOhms.

Figure P1: Non-Inverting Amplifier based on an Op-Amp

Start Mulitsim.

Go to the toolbar and select View>>Toolbar>>Virtual

Check the Virtual so this toolbar will be displayed.

See Figure P2: Toolbar selection.



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Figure P2: Toolbar selection

This will bring up the Virtual Toolbar shown in Figure P3: Virtual Component Toolbar.

Figure P3: Virtual Component Toolbar

Bring up the Basic Component Selection Menu by clicking the Resistor Icon ( third from the

left). See Figure P4.



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Figure P4: Basic Component Selection Menu

From this Menu select two resistors and place them on the working grid and double click each

one. In the pop up Menu set the Label and your calculated value for each resistor – Rf andRi.

For Rf label see Figure P5.

Figure P5: Set label for Rf

For Rf value see Figure P6.



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Figure P6: Set Rf value [NOTE your value will be different!]

Repeat this for Ri with Ri equal to 1 kOhm.

Now we will place the power sources and the ground. To get the Power Source menu,

Click the Power source Icon on the Virtual toolbar (third from the right) see Figure P7.

Figure P7: Power Source Menu

We will need two DC Power sources and a ground. Again see figure P7.

Place these on the circuit grid.

Double click each source and set the voltage to 15 V.

Now we need to get the 741 Op-Amp.



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Click the [Place] command on the main tool bar and the select the ‘Component’ from the drop

down menu that appears. See Figure P8.

Figure P8: Click Place and select Component from Pop Up menu.

This will bring up the Pop-Up Menu, “Select a Component”

Now Change the “Group” to “Analog”.

Then select the “OPAMP” entry.

The 741 should appear under the Component.

Select the 741 and click OK and place the 741 on the circuit grid.

See Figure P9.

Figure P9: Selecting 741 Op-Amp

Close the “Select a Component” menu.

From the instrument toolbar on the right side of the Multisim window

Select the Agilent Function Generator and place on the circuit grid. See P10

Then select the Agilent Oscilloscope and place on the circuit grid. See P11.



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Figure P10 Agilent Function Generator Figure P11: Agilent Oscilloscope

Now connect everything as shown in Figure P12.

Figure P12: Non-Inverting Amplifier Circuit using an Op Amp



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Now double clicking on the Function Generator circuit symbol/Icon and the Oscilloscope

symbol/Icon. This will bring up these instruments’ Front Control Panels.

See Figure P13.

Figure P13: Non-Inverting Amplifier Circuit with Function generator drive and oscilloscope

measurements.

Instrument Setup:

You will need to turn on the power for both instruments by clicking on each of their [POWER]

buttons on the front panels.

Then set the the function generator to Sine Wave by clicking the “sine button” top row left.

Click the [Ampl] button (second row, second from left) and set the value to 1 Vpp by viturally

turning the large knob (Right Top) with the mouse over the dark spot on the button and holding

down the mouse left button. Moving the mouse clock wise will increase the value and moving

counter-clockwise will decrease the value.

Measurement Steps:

STEP 1; Now click the frequency button [Freq] and set the frequency to 100 Hz the same way

the amplitude was adjusted by rotating the knob with the mouse.

STEP 2; Next click the Run button (Green, right-pointing arrow on the Multisim toolbar.

STEP 3; Adjust the oscillosope by clicking the [1] and [2] buttons in the “Analog” or Vertical

Section so they are both green.

STEP 4; Next press the [Auto Scal] button , just above the Analog label.



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STEP 5; Then click the [Single] button in the Run Control section on the oscilloscope, upper

right. This should provide a frozen image similar to the screen in Figure P13. If the waveform

lines are too thin, they can be thicken for improved visibility by using the [INTENSITY] button

in the lower left of the oscilloscope.

STEP 6; Now press the [Quick Meas] button and click the Peak-Peak button that appeared at the

bottom of the oscilloscope Disply screeen. See Figure P14

Figure P14: soft buttons at bottom of Display

STEP 7; Next click the [Source] button (showing the source to be 1) and select 2.

Again click the [Peak-Peak] button, then the [Frequency] button. This will put the input voltage

(1) and the output voltage (2) peak to peak values as well as the current test frequencies at the

bottom of the Oscilloscope Display just above the soft key labels. See Figure P14A.

Figure P14A: Scope Display with Peak to Peak voltages and test frequency.

STEP 8; Click the [Single] button again to ensure the measurements displayed are good.

Now you can use the Snipping Tool and capture the oscilloscope screen.

This give a record of the frequency, the input voltage, and the output voltage.

Next we want to get the phase shift.

STEP 9; Now click the [Cursor] button next to the [Quck Meas] button in the Measure Section

just below the Horizontal section. See Figure P15.

Figure P15: Measurement Section.



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At the bottom of the Display the cursor controls soft buttons have appeared. Figure P16.

Figure P16: Cursor Control soft buttons

STEP 11; Select the X cursors and the X1 cursor and using the knob with the circular arrow

above it – See Figure P15.

STEP 12; Adjust the X1 curser so that it is on the center of the peak voltage of Channel 1

waveform (top waveform), then select curser X2 and adjust it until it is on the center of the peak

of Channel 2 waveform (lower waveform) that is under and just right of the upper waveform

peak. The cursors have been enhanced in the image and will not be colored or as thick on the

actual oscillosope image. This is for the Non-inverting amplifier. See Figure P17A.

Figure P17A: Measuring time delay on Non-Inverting Amplifier

On the Inverting amplifier, the second cursor should go to first trough’s lowest point after the

input peak. The cursors have been enhanced in the image and will not be colored or as thick on

the actual oscillosope image. See P17B.



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Figure P17B: X Cursors position to measure time delay on input to output for an Inverting

Amplifier.

STEP 13; Click the [Single] button (Upper Left of instrument).

STEP 14; Using the Snipper tool and capture an image of the Display screen.

For the non-inverting amplifier:. This image has the time difference (dX = XXX ms) at the

bottom. See Figure P17B. This is the td for calculating the phase shift in degrees with the

formulas: t1 = T/360 (T is the period, T = 1/Freq) and phase shift in degrees = td/t1. See

figure P17A

This is the same as was done in the last lab with the low pass filter.

For the inverting amplifier the above measure the time difference from the input peak to the

nearest output trough yields the number of degrees from the 180 degrees of phase shift

introduced by the “invert” of the inverting amplifier. See Figure P17B.

See Note 1 at the end of the procedure for a more indept explanation.

To simulate with the next test frequency, stop the simulation running, change the Function

Generator Frequency to the next test frequency and redo steps 1 to 14. You can try not stopping

the simulation but if the memory fills, then there could be incorrect results.



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The test frequencies are: 100 Hz, 300 Hz, 1 kHz, 3 kHz, 10 kHz, 30 kHz, 100 kHz, 300 kHz, and

1 MHz.

There should be two measurements, a gain = Vout/Vin and a phase shift in degrees = td/t1,

for every test frequency.

After all the measurements have been made for the non-inverting amplifier, the measurements

for the inverting amplifier will be made the same way. But notice that the non-inverting

amplifier had 180 degree phase shift at 100 Hz and near through many of the lower test

frequencies.

The Inverting Amplifier will have 180 degree phase shift at 100 Hz and near through many of

the test frequencies. This is the meaning of the minus sign in the gain formula for the inverting

amplifier, Gain = A = -Rf/Ri.

To change the non-inverting amplifier to an inverting amplifier, do the

following steps.

Delete the wire connecting the Non-Inverting terminal to the function generator.

Delete the wire connecting the Ri resistor to ground.

Leave the Channel 1 of the oscilloscope connected to the output of the function generator.

Connect the Non-Inverting terminal of the op-amp to the ground.

Connect the unconnected end of Ri to the Function Generator Output. See Figure P17.

Figure P18: Inverting Amplifier Simulation Circuit.



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Now repeat the measurements as they were done for the Non-inverting Amplifier.

In the Theoretical Analysis section state the desired gain (from your last digit of your ID) and

calculate the value of the Rf given that Ri = 1 kHz. This is the expected value for all frequencies

given that resistors impedance does not have a frequency dependent part.

The expected phase shift for all frequency is 0 for the non-inverting amplifier and 180 degrees

for the non-inverting amplifier, again since there is no freqency dependent part in the gain

formula.

After having all the measurements. Construct two Verification Results Tables for each

amplifier.

Two for the Non-inverting Amplifier and Two for the Inverting Amplifier.

The first table will present the gain at different frequencies and the second will present the phase

shift at different frequencies.

For the gain table the rows will be the test frequencies and the columns will be the expected gain

(which is the same for all frequencies), the measured gain, difference in gain, the percentage

error in gain, and comments if the error is large (>10%).

For the phase shift table, the rows will again be the test frequencies and the columns will be the

expected phase shift (all expected is 0 for non-inverting amp and 180 for inverting amp, since

Resistors have a zero phase shift at all fequencies), the measured phase shift, the diffierence, the

percentage error, and comments if the phase shift is large (>10%)

Put the gain vs frequency into a spreadsheet where the gain is one column and the frquency is the

next column. Use the spreadsheet plot function, plot the gain verses the log of the frequency,

where the gain is the Y axis and log of frequency is the X axis. Put this plot diagram in the

report. Do this for both amplifier circuits.

The reason the gain reduces and the phase shift increases is because the op-amp response is

bandwidth limited. It is not infinitely fast. The highest speed that an op amp output can change

is called the slew-rate. Once the input signal is changing faster than the op amp output can

change, the output amplitude reduces because the input signal has peaked and reversed direction

heading back to zero before the output could get to its peak.

Another indication of this bandwidth limitation, is how the sine wave input is changing value

faster than the output can change, so the output just changes as fast as it can. This produces a

straight line ramping up or down as fast as the output can change. The sine wave waveform on

the input become a triangle waveform on the output, see Figures :17A & P18B.

If you have questions please email me or the TA.

You have two weeks to perform this experiment and write and submit the report.



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The due date is April 9th

at 11:59 PM. Please don’t wait until the last minutes to do the report.

You will learn more if you do the report when you are not so tired and have time to think about

what is happening in circuit and why it is happening.

The next and last lab will be on the 555 Timer as an Astable Multivibrator (oscillator) and as a

Monostable Multivibrator (single shot.)

NOTES:

Note 1: Obviously, we could measure from the positive peak of the input to the first positive

peak of the output and calculate the complete phase shift.

Why measure from the peak of the input to the trough of the output when determining the phase

shift of the Inverting Amplifier and using this as the difference from the 180 degree phase shift?

In this case the number of degrees measured is the amount of degrees to add or subtract from 180

to get the phase shift of the circuit at the test frequency.

The reason is to improve the accuracy of our measurement.

It is similar to the way accuracy is improved in a Digital Multimeters by using the correct range

setting so most of the scale is used when making the measurement.

When measuring the phase shift of an inverting amplifier, if we measured the phase shift from

input positive peak to output positive peak, we would need the time base adjusted to display at

least one half of the waveform period. Let’s say the phase shift is 180.2 degrees on a 1kHz signal

and for some reason the 0.2 degrees is important. Since the Period is 1 mS, the screen would

need to display a little more than 500 uS (microseconds). Now a period of 1 ms means the time

for 1 degree is 1ms/360 = 2.778 uS (microseconds). There are 500 uS across the display and 10

major units across the display so each major unit is 50 uS. There are 5 divisions in a major unit

and you usually read to ½ a unit, so we can read 5 uS, may be 2.5 uS. We are looking for a

change of 0.2 degrees that has a time difference of .5556 uS but we can only really see

differences of maybe 1 uS. The shift off the 180 degrees is about as small as or a little smaller

than can be seen on the Display.

We need to make the Display to have less time per unit but then both peaks are not visible.

Now, if we measure from the positive peak of the input to the nearest negative peak of the output

there should be only about .5556 uS difference and we can adjust the horizontal scale to

prominently display this difference where both the peak and trough are visible. We know that if

the output negative peak is to the right of the input peak then we add 0.2 degrees to 180 and if it

is to the left then we subtract 0.2 degrees from 180. Our reading and measurement is much more

accurate by measuring the difference from 180 degrees on the inverting amplifier. We are using

more of the Display

operational amplifiers: remote laboratory circuit

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