Download ELEC 350L Electronics I Laboratory Fall 2011

ELEC 350L
Electronics I Laboratory
Fall 2011
Lab #1: Bar Graph Voltmeter Using Op-Amps
Introduction
The operational amplifier, or “op-amp,” is one of the most important building blocks in modern
analog circuit design. Its predictable performance and unique properties make the op-amp a very
popular choice for many important applications. One such property is its ability to compare two
voltage levels and produce an output voltage that indicates which of the two is greater. In this lab
exercise you will take advantage of this capability to construct a simple voltmeter that indicates
the measured voltage using a bar graph.
Theoretical Background
The circuit symbol for the op-amp is shown below in Figure 1a. It has five key terminals. The
two labeled VPOS and VNEG are connection points for DC voltage sources (power supplies) that
activate the device and allow it to operate properly. The two voltage sources are usually equal in
magnitude and opposite in sign; for example, VPOS might be +12 V and VNEG might be −12 V.
Note that one terminal each of VPOS and VNEG is assumed to be connected to ground (i.e., to the
reference node). For example, if VPOS = +12 V, then the positive side of a 12-V source is
connected to the op-amp, and the negative side is connected to ground. Likewise, if VNEG =
−12 V, then the negative side of a 12-V source is connected to the op-amp, and the positive side
is connected to ground. The most significant constraint on the two power supplies is that VPOS
must be greater than VNEG. Usually, the two values must be separated by at least several volts.
Finally, note that significant current can flow into and out of the VPOS and VNEG nodes. In fact,
the output current of the op-amp that flows into or out of the vo node comes from one power
supply or the other, or both.
vn
vp
VPOS
−
s+
d
vo
+
vn
−
VNEG
(a)
−
v
+
vo
+
vp
−
+
_
s
d
Av =
A(vp – vn)
(b)
Figure 1. (a) Circuit symbol for operational amplifier. (b) Equivalent circuit
model applicable under conditions of linear operation. The open-loop gain A of
the voltage-controlled voltage source is very large; typical values for commonly
available op-amps are 105 to 108. The small triangle indicates the ground
(reference) node.
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The terminals labeled vn and vp are called the inverting and non-inverting inputs, respectively,
and the terminal labeled vo is called the output. Under conditions of linear operation, the function
of the op-amp can be modeled by the equivalent circuit shown in Figure 1b. Node voltages vn, vp,
and vo are measured with respect to ground. Op-amps have very complex internal circuitry, but
the purpose of that complexity is to produce a device that behaves in a manner that is modeled
accurately by the very simple representation shown in Figure 1b. The voltage sources
representing the power supplies are not included in the model because their contributions are
modeled by the voltage dependent voltage source. Nevertheless, their presence is essential.
Several important properties of op-amps become evident after a bit of study of the equivalent
circuit model. First, because there is an open circuit between the inverting (vn) and non-inverting
(vp) inputs, no current flows into either terminal. (Actually, some current does flow, but it can be
assumed to be negligible in most practical cases.) Also, the output voltage (node vo) is directly
proportional to the difference between the two input voltages if clipping is not occurring; that is,
vo  Av  Av p  v n .
where the constant A, called the open-loop gain, has a very large value (105 to 108). Although the
circuit model shown in Figure 1b does not explicitly indicate it, there is a limit on the range of
output voltages that an op-amp can produce. Specifically, the output voltage can be no greater
than VPOS and no less than VNEG, and for most real op-amps the upper and lower limits are one to
two volts tighter than those two values. For example, if the power supply voltages are ±12 V,
then vo might be able to vary between −11 V and +11 V or maybe −10.5 V and +10.5 V.
Consequently, many practical op-amps circuits employ negative feedback, which is designed to
keep the differential input voltage (v) down to very small values, typically only a few microvolts.
When the output voltage of an op-amp reaches the upper or lower limit of its permissible range,
the op-amp is said to be saturated. This property is put to use in a wide variety of practical
circuits. If the non-inverting input voltage vp is just a bit larger than vn, then the output of the opamp will saturate at VPOS. Likewise, if vp is a little less than vn, the output will saturate at VNEG.
Thus, an op-amp can be used as a comparator, a device that indicates whether one voltage is
greater than or less than another. An ideal comparator produces an output voltage at one of the
power supply values if vp > vn or vp < vn. The condition vp = vn produces an undefined output in
an ideal comparator. The basic operation of a comparator is depicted in Figure 2.
VPOS
vn
+
−
+
−
−
s+
d
vo = VPOS, if vp > vn
VNEG, if vp < vn
VNEG
vp
Figure 2. An ideal op-amp used as a comparator.
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Experimental Procedure
An application of the comparator is shown in Figure 3. The circuit is a simple voltmeter that
indicates the measured voltage, represented by Vtest, using a set of light-emitting diodes (LEDs).
The voltage to be measured is applied across the non-inverting inputs and ground. The inverting
inputs are connected to reference voltages Vref1 through Vref4, which are established by circuitry
you will design. If the measured voltage Vtest is greater than a given reference voltage, then
vp > vn for the corresponding op-map, and the output voltage of that op-amp saturates near the
value VPOS (+10 V in this case). If Vtest is less than a given reference voltage, then the
corresponding op-amp remains saturated near VNEG (−10 V).
+
s−
d
Power
Supplies:
Vtest
10 V
10 V
+ −
+ −
−10 V
+10 V
+10 V
Vref4
−
s+
d
−10 V
+10 V
Vref3
−
s+
d
−10 V
+10 V
Vref2
−
s+
d
−10 V
+10 V
Vref1
−
s+
d
−10 V
470 
LED4
>4V
470 
LED3
3-4 V
470 
LED2
2-3 V
470 
LED1
1-2 V
Figure 3. A simple bar graph voltmeter constructed from op-amps and lightemitting diodes. If no LEDs light up, then Vtest is less than 1 V (or is negative). If
LED1 lights up, then Vtest is between 0 and 1 V, and so on. Connections to the
ground node are indicated by small triangles. Wires that cross at intersections not
covered by dots are not connected together.
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LEDs allow significant current to flow through them only in one direction (indicated by the
arrow in the circuit symbol). In the circuit shown in Figure 3, if the output of a given op-amp
saturates at VPOS, then the node voltage at the top of the LED becomes positive with respect to
ground, current flows through the LED, and the LED lights up. Conversely, if the output voltage
of the op-amp is at VNEG, then the upper terminal of the LED is negative with respect to ground,
current does not flow through the LED, and it remains off. (Recall that by definition the node
voltage at the reference node is 0 V.) Thus, if Vtest rises through 1 V, 2V, 3 V, and 4 V, then
LED1 will light first, LED2 second (LED1 will remain on), and so on. The number of lit LEDs
indicates the range of voltage values in which Vtest lies. The 470- resistors limit the amount of
current that can flow through the LEDs. They prevent the LEDs from burning out.
There is one other important point to be made about Figure 3. Note that the power supplies,
represented as ideal voltage sources, are drawn separately from the main part of the circuit. The
ungrounded terminals end in small circles labeled “+10 V” and “−10 V.” This is a commonly
used method for representing DC power supply connections in circuit diagrams. All of the circles
labeled “+10 V” are connected together, and all of the circles labeled “−10 V” are connected
together. The connecting wires are not drawn because doing so would create a morass of
crossing conductors that would make the diagram difficult to read. In a similar fashion,
connections to the ground node are indicated by small triangles in the figure. That is, all of the
wires terminated in a triangle are connected together and collectively comprise the ground
(reference) node. Note that current can flow into or out of the ground node; wires ending in
triangles are therefore not open circuited. Likewise, current can flow along wires ending in
circles.
Complete the following tasks:

Design a circuit that can be added to the bar graph voltmeter circuit shown in Figure 3 to
provide the required reference voltages Vref1 through Vref 4 so that the LEDs light up under the
following conditions:
Vtest (V)
<1
1 to 2
2 to 3
3 to 4
>4
LEDs Lit
None
LED1
LED1 and LED2
LED1, LED2, and LED3
All
The new circuit should connect to the one in Figure 3 at the nodes labeled Vref1 through Vref 4.
An infinite number of solutions are possible, but you should strive for simplicity. It is okay if
the LED lighting thresholds are not exactly at the desired voltage levels. Errors of 0.1 V or so
are acceptable. The primary goal is to design an effective reference voltage circuit and
achieve a good understanding of how comparators work.
Hint #1: You should be able to accomplish this task using only one of the power supplies
(i.e., +10 V or −10 V) and a handful of resistors.
Hint #2: Negligible current flows into/out of the inverting inputs of the op-amps.
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The note-taker should clearly explain how your group developed your circuit. All
assumptions should be clearly explained; all variables should be defined; and the calculations
of all numerical resistor values should be shown. You should not force the notebook reader to
go to significant effort to understand your design process.
Obviously, this circuit is not highly accurate, but it does give a very clear visual indication of
the approximate level of the voltage you are measuring. Bar graphs similar to this one are
used as fuel gauges, audio volume indicators, and in other important applications where a
high degree of accuracy is not critical.

Assemble the circuit using the ±25-V variable bench-top power supply to provide the ±10 V
needed for the op-amps and your reference voltage circuit. Type LM 741 op-amps and red
LEDs will be provided to you. The pin-out of the LM 741 can be found in its data sheet,
which is available via a link on the laboratory web page. Use the 6-V variable power supply
to create a test voltage Vtest to measure. The ±25-V and 6-V supplies should all have a
common ground connection.

Set the 6-V supply to several test voltage levels in the 0-6 V range. Record which LEDs light
up for each voltage level in a well organized table in your notebook. You should include
voltage values just above and below the various threshold values (e.g., at 2.9 and 3.1 V).

Demonstrate your working bar graph voltmeter to the instructor or TA. You will be asked to
scan Vtest from 0 V to 5 V or so. The bar graph (LEDs) should light up correctly at all voltage
values.

Sketch a schematic diagram of your reference voltage circuit and leave it with the instructor.
This diagram should be on a loose sheet of paper and is in addition to the diagram sketched
in your notebook. (You did include a diagram in your notebook, didn’t you?)
Grading
The notebook recorder(s) in each group will be assigned grades according to the criteria outlined
in the lab policies and procedures document. Lab notebooks must be submitted by 5:15 pm on
the day of the lab session.
Each group member’s lab performance grade will be assigned according to the following
guidelines. Each group member will receive the same grade.
30%
50%
20%
Quiz on pre-lab reading
Effective reference voltage circuit design
Successful demonstration of a working circuit
© 2006-2011 David F. Kelley, Bucknell University, Lewisburg, PA 17837.
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