Download Lab-07-Operational-Amplifiers

Lab 7
Operational Amplifiers
Objectives
1.
2.
3.
4.
5.
Learning practical issues in the use of amplifiers
Constructing a variable voltage supply
Measuring amplifier gain
Observing the effects of negative feedback
Characterizing amplifier saturation
Key Prerequisites
 Chapter 5: Operational Amplifiers
Required Resources
 Circuit Kits
 MATLAB or FreeMat
The operational amplifier is a workhorse in the analog circuit domain. So named because
they can perform mathematical operations on analog signals, “op-amps” were
incorporated into the earliest analog computers, back in the age of vacuum tubes.
Modern op-amps employ numerous transistors to achieve the performance characteristics
of a very nearly “ideal” amplifier.
Fig 1. Three views of an op-amp a) circuit schematic, b) circuit symbol, c) IC chip
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Lab 7: Operational Amplifiers
Discussion and Procedure
In 1934, a Bell Telephone engineer named Henry Black discovered that an amplifier
designed with a huge amount of gain—thousands of times more than would ever be
needed--could be set to whatever gain one desired by feeding back a portion of the output
voltage to the input. This feedback circuit would reduce the overall gain of the amplifier
based on the amount of resistance in the circuit.
From Op-Amp History, by Walter Jung of Analog Devices
In this lab we will explore some basic properties of amplifiers and negative feedback by
looking at op-amps from several different angles – simulation, breadboarding, and
applications.
Part 1. Circuit Simulation of an Operational Amplifier [Optional]
This part is optional and may be skipped if time is limited. A link to a video
demonstration of these simulations is provided in the datasheet for this lab.
Step 1) EveryCircuit Simulation
Run the simulation of the Inverting Op-Amp in EveryCircuit, shown in Figure 2 below.
In this circuit, which we will build in today’s lab, we are using a dual +/- 12V power
supply to run the op-amp, and a variable voltage supply using a potentiometer (variable
resistor, or “pot”) to provide an input voltage to the amplifier that we can adjust by
moving the “wiper” (center tap of the “pot”).
What we should observe is the characteristic inverting amplifier gain (-Rf/R1) relating
the ratio of output voltage to input voltage. For the following circuit, with a feedback
resistor of 10 k, and an R1 of 3.3 k, the gain will be about 3.03. This is reflected in
the relationship between the input voltage, shown as -1.4V, and the output voltage,
shown as 4.67 V.
If we adjust the pot so that the input voltage approaches -5V, we will notice that the
output voltage climbs higher toward +12V and reaches a point where it can grow no
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Lab 7: Operational Amplifiers
further. This limit to the output swing of an amplifier is known as saturation, and is
determined by the voltage supplies used to power the amplifier, since the output voltage
cannot go beyond these limits.
Reversing the input voltage so that it heads toward +5V, we can observe the opposite
behavior on the output voltage, as it drops to -12V and goes no further.
Fig 2. EveryCircuit simulation of an Inverting Op-Amp (see link above)
Step 2) Circuits.IO Simulation
You can get a more realistic view of an op-amp by running the simulation of the
Inverting Op-Amp in circuits.io, shown in Figure 3 below.
This circuit shows the practical aspects of building an op-amp circuit. We are using an
LM741 Op-amp Integrated Circuit (IC), placed in the breadboard across the central
channel. In this simulation, we can view the input and output voltages on a voltmeter, and
observe the changes that take place when we rotate the knob on the potentiometer.
Do you notice any differences from the EveryCircuit simulation? Does the gain appear to
satisfy the inverting amplifier formula?
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Lab 7: Operational Amplifiers
Fig 3. Circuits.io simulation of an Inverting Op-Amp (see link above)
Part 2. Construction and Testing of an Inverting Op-amp
Step 3) Construction of a Dual +/- 12V Power Supply
Before we build the op-amp circuit, we will need to wire up our two 12V power supplies
in such a way that we can draw +12V and -12V from the top and bottom “rails” of the
breadboard. This balanced power supply arrangement is required to run the LM741 opamp chip.
The following schematic in Figure 4 shows how the barrel jacks are wired in order to
bring +12V and -12V to the breadboard. For the top barrel jack, we simply wire it in the
way we always have, with the jack’s “-” terminal connected to the blue line, and the
jack’s “+” terminal connected to the red line. However, for the bottom jack, we are
switching things around. By connecting the jack’s “+” terminal to the blue line (our
reference node) and the “-” terminal to the red line, we can push the “-” terminal to -12V,
provided we also connect the blue line of the top rail to the blue line of the bottom rail—
using the green wire shown in the breadboard diagram.
In order to keep this straight, by convention the blue line on the breadboard will always
be used for ground, and the red line will always be used for power (+12V on the top rail,
and -12V on the bottom rail).
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Lab 7: Operational Amplifiers
Fig 4. Circuit diagram and breadboard diagram for the dual +/- 12V supply wiring. Use your voltmeter to
verify that the red line on the top rail has +12V relative to the blue (reference node) line, and the red
line on the bottom rail has -12V relative to the blue line.
Verification: Before continuing, plug in both power supplies and use a voltmeter to
confirm that you are able to read approximately +12V at the red line on the top of the
breadboard, and -12V at the red line on the bottom of the breadboard, with the COM
(black) probe of the voltmeter attached to either the reference node (either top or bottom
blue line).
Enter these values into Table 1 of your datasheet. If you don’t measure the expected
values shown on the datasheet, STOP and fix your circuit or get help before proceeding.
Before proceeding, we will also add two LEDs to our breadboard to verify our power
rails are properly connected at all times, as shown in Figure 5. This will help protect the
LM741 chip, which could be damaged if we accidentally disconnect one of the supplies.
Note that the anode of an LED is indicated by a long wire. In the breadboard below, the
LEDs are mounted with the anode (long wire) oriented toward the bottom of the page. A
1 k resistor is added in series to prevent too much current from passing through the
LEDs.
Fig 5. Breadboard diagram showing dual +/- 12V supply plus additional monitoring LED circuits. The long
wire of the LED is the anode and must be connected to the high potential (+) side where indicated. Use
1 k resistors to protect the LEDs from excessive current.
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Lab 7: Operational Amplifiers
Power up the breadboard and verify that both of your LEDs are now illuminated. Be alert
for any discrepancies between your wiring and the diagram above. For instance, if you
have to plug the anode (+) side of the LED to where the (-) side is indicated in order for
the light to come on, you may have incorrectly wired one of your 12V supplies.
Step 4) Construction of a Variable Voltage Supply
In order to do a voltage sweep of the input to the op-amp, we are going to need a variable
voltage supply. We will be constructing this out of a 5 k potentiometer.
Your kit will have 1 or the other of these 5K potentiometer. You may find it helpful to straighten out the
wires of the potentiometer on the right with the pliers before inserting into the breadboard.
The potentiometer is a variable resistor that has 3 wires. The center wire sweeps up and
down the 5k resistor, and once wired into your breadboard, will provide an adjustable
“voltage divider” that can be set to any value in the range from -12V to +12V by rotating
the screw in the center of the potentiometer. The schematic for the potentiometer, and the
breadboard wiring diagram are shown below.
Schematic diagram showing potentiometer wired to provide a
variable voltage supply in the range +12V to -12V.
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Lab 7: Operational Amplifiers
+12 to -12V variable
Breadboard diagram showing potentiometer wired to provide a
variable voltage supply in the range +12V to -12V. The pins of the
potentiometer are indicated by dots on the top surface.
When you finish wiring this part of the circuit, measure the voltage at the wire labelled
“+12 to -12V variable” above relative to the reference node, while turning the shaft on
the potentiometer with the screwdriver in your kit. Enter the minimum and maximum
voltage you observe in your datasheet under Step 4. If the range does not go from -12V to
+12V (give or take a few tenths of a volt based on your actual supply voltages) you have
an error somewhere in your circuit and will need to debug.
If everything works, set the voltage at the output of the potentiometer to 0.5V before
proceeding.
Step 5) Construction of the Inverting Op-amp Circuit
First, gather up the resistors you will use 3.3 k, 1 k,, and 10 k, and measure them
using your DMM. Enter the values in Table 2 under Step 5 of your datasheet.
You may now wire the inverting op-amp circuit below. Place the LM741 on your
breadboard so it straddles the central channel. Make sure to press the pins in firmly, the
black housing should be almost if not fully in contact with the breadboard (Note: to
remove the LM741, it’s helpful to insert the screwdriver slightly on either side and rock
the pins loose from the breadboard.
Wire up the components as shown in Figure 4 below. The Offset Null and NC pins will
be left unconnected. Make sure you identify the dimple in the top left corner of the
chip. Some chips have a U-shaped notch instead. These are provided to help you
identify the correct orientation of the chip. Incorrect placement (such as backwards) can
damage your chip. If you need help on wiring your circuit, click here.
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Lab 7: Operational Amplifiers
Dimple is by Pin 1
Fig 4. Inverting Op-Amp Schematic and LM741 Pinout Diagram
Before powering on the circuit, have a voltmeter ready, so you can quickly determine if
there is an error in the circuit, and power off before any damage occurs. Turn on power
and set Vin to about 0.5V and measure Vout. It should be around -1.6V. If not, debug.
When your circuit is working properly, step the input voltage (potentiometer) through a
sweep from -5V to +5V in several steps. Record Vin and Vout along the way in Table 3
under step 5. Also record the voltages IN- and IN+ (we refer to these as Vn and Vp in
lecture), which, due to negative feedback, the output voltage is attempting to maintain
equal.
You should find that IN- is very close to IN+ or 0V as long as the input voltage is
between -3V and +3V. Beyond that range, the output voltage cannot reach the value
needed to maintain equality, and the feedback loop is essentially broken. When this
occurs we say the amplifier is in saturation, meaning the output voltage is no longer in its
linear range.
You can get a visual understanding of saturation by plotting the Vout/Vin curve (Vin on
the x-axis) in FreeMat. Provide a title and axis labels, including units, and copy the image
into your datasheet.
Finally, answer the remaining questions under step 5 in your datasheet.
Part 3. Construction and Testing of a Non-Inverting Op-amp
Step 6) We are now going to modify the circuit you’ve been working on to turn it into a
Non-Inverting Op-amp, and repeate the DC Sweep we did in Step 5.
To change from an Inverting to a Non-Inverting amplifier, all you have to do is:
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Lab 7: Operational Amplifiers
a) Remove the ground wire from the V+ (or IN+) terminal
b) Connect the variable voltage source (potentiometer) to the V+ (or IN+) terminal.
c) Ground the left side of R1.
These changes are illustrated in the schematic below.
Fig 5. Non-Inverting Op-Amp Schematic and LM741 Pinout Diagram
Part 4. Audio Experiments
Step 7) Create a Volume Control circuit.
Your kit comes with two components that can let you play audio through your circuit
(Figure 6). The first is the 3.5mm Audio Jack to screw terminal adapter that allows you to
bring audio from a media player through jumper wires to your breadboard. The second is
a 100 Ohm speaker that could be inserted into your breadboard, but since the pins are so
thick it is best connected using alligator clips. Using a blue R502 potentiometer to vary
the input resistance (see figure 7, construct the circuit and plug in your mobile audio
device with the 3.5 mm audio cable provided in your kit. Rotating the position of the
potentiometer will change R1 and affect the gain of the circuit (increase or decrease).
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Lab 7: Operational Amplifiers
Fig 6a) 3.5mm audio jack to screw terminal adapter. Fig 6b) 100  speaker. You
can play only one channel (L or R) through the amplifier at a time.
Fig 7. Audio volume control amplifier circuit. One of the interesting effects you can
listen for is the onset of saturation (a noticeable audio distortion) that takes place
when the output voltage is “clipped” by the nonlinear limits of the amplifier.
Step 8) [Optional] Create an Audio Mixer Circuit. Those of you with a little extra time
or creativity may want to scrounge or improvise another audio jack and switch the opamp to an inverting summing amplifier, shown in figure 7. Doing so allows you to create,
without too much trouble, an inverting summing amplifier, that will let you combine
(mix) two independent audio signals onto the same line.
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Lab 7: Operational Amplifiers
Fig 8. An inverting-summing op-amp used as an audio mixer to combine two
different audio channels onto the same line. Each has its own volume control.
Please insert any photos or links to audio/video clips you may have created into your
datasheet to document the activities in Steps 7 and 8.
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