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ENGR
-
201 Circuits Lab

DC Op Amp Circuits Fall 2001

Name:________________________




References:

Irwin,
Basic Engineering Circuit Analvsis

National Semiconductor,
Linear Databook, Vol. I


Purpose:
The goal of this laboratory exercise is to gain hands
-
on
experience with practical
operational amplifier (op amp) circuits that have dc input signals. Using dc measurement
skills learned earlier, students compare the characteristics of integrated
-
circuit (IC) op amps
with those of the ideal op amp.


Background:

Because of its versatility and ease of use, the op amp is one of the most
widely used electronic devices. Using op amps a designer can easily construct circuits with a
wide variety of functions, such as inverting and non
-
inverting amplifiers, comparators
, mixers,
regulators, integrators, differentiators, and filters. The op amp is an important electronic
device and deserves special consideration by anyone studying electrical engineering.


Typical op amp connections are depicted in Figure 1. The device ha
s two input terminals, the
inverting
(
-
)

and non
-
inverting
(+)
inputs, and a single output terminal. Since the op amp is
an
active

device, external power is required to drive internal components. Typically, two
power supply connections,
V
++

and
V
--
are r
equired. All input and output voltages are
measured with respect to a single reference (ground); however, there is no reference
connection to the op amp itself. Generally, the reference voltage is the common (or ground)
of the dc power supplies used to p
ower the circuit, as shown in Figure 1.


Figure 1


Op Amp Connections




ENGR201 Lab
-

DC Op Amp Circuits, Fall 2001


Page:
2

Integrated circuit op amps are available in many different packaging configurations. One
popular package is the eight
-
pin dual in
-
line package (8
-
pin DIP). The pin designations f
or
the LM741 and the LF411 op amps packaged in an 8
-
pin DIP are shown in Figure 2. Only
seven of the eight pins are used, and the pin designations are shown in the Table 1.


Figure 2
-

Pin Definitions of the LM741 and LF411 Op Amp





Table1 1
-

Pin Designations of the LM741 and LF411 Op Amps (8
-
Pin DIP)

Pin #

Pin Function

1

Offset Null, used to insure zero output when input is
zero

2

Inverting input

3

Non
-
inverting input

4

V
--
, the negative power supply connection

5

Of
fset Null (used in conjunction with pin #I)

6

Output, measured with respect to common

7

V
++
, the negative power supply connection

8

Not Used (No Connection)



The dc power supply connections are sometimes called the
bias

connections or the positive
an
d negative
rails
. The output voltage of an op amp circuit is limited by the values of V
++

and
V
--
. In fact, for the typical op amp, the maximum output voltage will be about 1
-
volt less than
the dc supply levels. For example, if the supply voltages are


15V, the output cannot
exceed about

14V. When an op amp output reaches the maximum or minimum value
established by the dc supply connections, it is said to be
saturated
. Typical integrated circuit
op amps use dc supply voltages in the range

5V to

15V
. Some op amps do not require
dual (positive and negative) supply values but can work with a single, positive supply. In
case a single supply is used, the negative bias pin is generally connected to common.


-


+

1

2

3

4

8

7

6

5

V
++

V
--

ENGR201 Lab
-

DC Op Amp Circuits, Fall 2001


Page:
3

The circuit model of the ideal op amp is shown

in Figure 3 and consists of three components:
an input resistance (R
i
), an output resistance (R
O
), and a voltage
-
controlled voltage source
with gain A
VOC

(
or just A
OC
)
. For the ideal op amp, R
i

and A
OC

approach infinity, while R
O

approaches zero. For the

practical op amp, typical values for these parameters fall within the
following ranges:



R
i
: 10M


to 10
6
M



R
O
: 10


to 100



A
OC
: 10
5

to 10
6

V/V


In order for a practical op amp to behave like an ideal op amp, external resistors used to
construct op amp
circuits should have values much less than R
i

and much larger than R
O
.
Practical op amp circuits, therefore, typically use resistors in the range from a few
hundred ohms to a few mega
-
ohms (M



Using resistors in this range generally will allow
a design
er to treat the op amp as ideal and provide good approximations for predicting the
behavior of the circuit.


Figure 3
-

Circuit Model of an Op Amp




The Inverting Amplifier:
An op amp can be used to construct an inverting amplifier by
making the connecti
ons shown in Figure 4. Select R1 and R2 such that R1
>

47K


and
R2/Rl


10. The picture attached to the end of this lab shows a typical breadboard
construction of an inverting amplifier circuit.


Figure 4


The Inverting Amplifier



Figure 4 is a schema
tic diagram that includes the dc
-
supply connections; however, this
detail is often not provided with an op amp circuit diagram. Remember, an op amp circuit will
not work without connecting both dc supplies. For this lab exercise, use V
++
= +12V and V
--

=

10k


ENGR201 Lab
-

DC Op Amp Circuits, Fall 2001


Page:
4

-
12V. When an op amp (or any circuit) does not work as expected, the dc supply
connections are a good place to start looking for the problem. Measure the dc supply
voltages at the power supply outputs

and

at the pins of the op amp.


To generate the inpu
t voltages specified in Table 2, connect a 10k


potentiometer to the dc
supply voltages as shown in the diagram. For each input, adjust the potentiometer until the
indicated voltage is obtained. Measure the corresponding output and record the value in th
e
table. Compare the measured value of the output voltage with the calculated value and
compute the percent difference, D, where


D = 100% * (Calculated
-

Measured) / Calculated


Explain any significant differences between the calculated and measured valu
es. Also
measure and record the voltage across the input terminals of the op amp, V
id
.




Table 2
-

Inverting Amplifier Measurements


V
1


V
i
d


V
O

Measured

V
O

Calculate
d

%

Difference


Comments

-
1.2






-
0.8






-
0.4






0






0.4






0.8






1
.2









ENGR201 Lab
-

DC Op Amp Circuits, Fall 2001


Page:
5

The Non
-
inverting Amplifier:

Figure 5 shows the circuit diagram for a non
-
inverting
amplifier. Note that only two changes need to be made to the inverting amplifier circuit to
convert it into a non
-
inverting amplifier. Remove power from the
circuit in Figure 4 and
construct the non
-
inverting amplifier shown in Figure 5 using the same resistor values and dc
supply levels. Record the measurements indicated in Table 3 for each input value shown.
Explain any significant differences between meas
ured and calculated values.




Figure 5
-

The Non
-
inverting Amplifier



Table 3


Non
-
inverting Amplifier Measurements


V
1


V
i
d


V
O

Measured

V
O

Calculate
d

%

Difference


Comments

-
1.2






-
0.8






-
0.4






0






0.4






0.8






1.2








R1

V
++

V
--

+

Vi
d

-

+

V
2

-

+

V
O

-

10k


ENGR201 Lab
-

DC Op Amp Circuits, Fall 2001


Page:
6


The Difference Amplifier:
Using the circuit shown in Figure 6, an op amp can be used to
generate a signal that is proportional to the difference of two different input signals. This
circuit, called a difference amplifier, is useful in instrumentatio
n applications because it tends
to cancel out any voltage common to both inputs. Often, this common voltage is "noise",
such as interference from ac power lines or radio
-
frequency (RF) interference. The
difference amplifier will reject most of the common
-
mode noise and amplify the signal of
interest. For this reason, most instrumentation amplifiers use a differential (difference)
amplifier input stage to minimize signal corruption due to noise.


Connect the circuit shown in Figure 6 using the same resist
or values for R1 and R2 and in
the previous two circuits. Use a 10k

-
pot and the +5V output of the power supply to set V1
at a constant +0.5V. Use the same voltage divider circuit used with the inverting and non
-
inverting amplifier circuits to vary V2 be
tween
-
1.0V and +1.0V. Take the measurements
indicated in Table 4 and record all measured and calculated values. Explain any significant
differences between the measured and calculated values.


Figure 6
-

The Difference Amplifier



Table 4
-

Differ
ence Amplifier Measurements


V
2


V
a



V
b



V
ID


V
O

Measured

V
O

Calculate
d

%

Difference


Comments

-
1.0








-
0.8








-
0.4








0








0.4








0.8








1.0












+

V
O

-

+

V
b

-

V
a


ENGR201 Lab
-

DC Op Amp Circuits, Fall 2001


Page:
7


Questions and Problems


1. Use nodal analysis to derive the voltage
gain equations for the inverting, non
-
inverting,
and difference amplifiers.


2. How much resistance does the input signal source see in the inverting amplifier ? In the
non
-
inverting amplifier ?


3. What is the voltage gain of the following amplifier c
ircuit? What characteristic of this
amplifier makes it particularly useful ?






4.

In a practical op amp circuit, what limits the range of the output voltage?


5. When the output voltage of an op amp reaches its upper or lower limit, what can be said

about the voltage across the op amp’s inputs? Examine your measured data (V
ID
) to answer
this question.


6. In the circuit below, determine the power delivered by source V
IN

and the power delivered
to the 100


load. Where is the power supplied to 100


load coming from?




+

V
O

-

+

V
IN

-

+

V
IN

= 25mv

-

ENGR201 Lab
-

DC Op Amp Circuits, Fall 2001


Page:
8

Inverting Amplifier Circuit