ATENEO DE NAGA UNIVERSITY
College of Engineering
Department of Electronics and Computer Engineering
QECA312 GE21
CIRCUITS 1
Basic OP Amp Circuits
Laboratory Report No. 11
Remoquillo, Marian Nicole N.
Reyes, Roi Andrei J.
Date Lab Performed: October 30, 2019
Date Report Submitted: November 6, 2019
The inverting op amp with negative feedback is one of four basic negative-feedback circuits.
These four basic circuits have different effects on input and output impedances.
I. OBJECTIVES
1. To test a voltage-to-current converter.
2. To test a current-to-voltage converter.
3. To test a current amplifier
II. THEORY
An Operational Amplifier, or OP AMP for short, is an electronic active device most commonly
used as signal amplifier. It is also used to perform mathematical operations such as addition, subtraction,
multiplication, division, differentiation and integration. OP Amp is used for analog design. It is popular in
practical circuits because it is versatile, inexpensive, easy to use, and fun to work with. In this experiment,
we were to build a voltage-to-current converter, a current-to-voltage converter, and a current amplifier using
OP Amp.
III. MATERIALS AND EQUIPMENT
1. Power supplies: Two 15-V
2. Equipment: 2 VOMS or digital multimeters
3. Resistors: 2W --- 1k Ω (2), 10k Ω (2)
4. Potentiometer: 1k Ω (or nearest available value)
5. Op amp: 741C
IV. METHODS/PROCEDURES
Part1: Voltage-to-current Converter
1. Connect the circuit of Figure 1
2. Adjust the potentiometer to get an input voltage of 1V.
3. Read the output current and record the value in table 1
4. Repeat steps 2 and 3 for the remaining input voltages listed in Table 1.
Figure 1: Electronic Voltmeter
Part 2: Current-to-Voltage Converter
5. Connect the circuit of Figure 2.
6. Adjust the potentiometer to get an input current of 1mA.
7. Read the output voltage and record the value in Table 2.
8. Repeat steps 6 and 7 for the other input currents shown in Table 2.
Figure 2: Electronic Ammeter
Part 3: Current Amplifier
9. Connect the Circuit of Figure 3.
10. Adjust the potentiometer to get an input current of 0.1mA.
11. Record the output current in Table 3.
12. Repeat steps 10 and 11 for the remaining input currents of Table 3.
Figure 3: Measuring current gain
V. DATA GATHERING
TABLE 1: Voltage-to-current Converter
Vin , V Iout, mA
1 1.051
2 2.045
3 3.063
4 4.047
6 6.052
8 8.084
10 10.071
TABLE 2: Current-to-voltage Converter
Iin, mA Vout , V
1 -0.924
2 -0.969
3 -1.013
4 -1.051
6 -1.117
8 -1.182
8.46 (Max. Current) -1.310
TABLE 3: Current Amplifier
Iin, mA Iout, mA
0.1 -13.41
0.2 -13.47
0.3 -13.51
0.4 -13.61
0.5 (Max Current) -13.74
VI. SAMPLE COMPUTATIONS
VII. ANSWERS TO QUESTIONS
1. Ideally, the currents in table 1 should be 1,2,3,4,6,8, and 10mA. Give three reasons why the
measured currents were different.
a) Inaccurate adjustment of the potentiometer
b) Tolerance of 1k resistor
c) Internal resistance of the multimeter used for measurement
2. After removing the potentiometer and voltmeter at the input of figure 1, the circuit that remains
can be used as an electronic voltmeter. Why is this true?
- Op amps are actually used as electronic voltmeters. By removing the input voltage and
potentiometer, no current would flow to the input signals of the op amp, therefore, no current
would be converted. Hence, only the voltage that passes through the signals will be read, which is
the function of an electronic voltmeter.
3. Suppose we want 1V to produce 2mA of current in figure 1. What change can we make?
-We can change the 1k Ω resistor which is connected to the ground on one end and the other end
is connected to #2 pin of LM741 and the negative of the ammeter. The replaced value of the
resistor should be decreased at 500 Ω to have 2 mA in the output.
4. In figure 2, what is the ideal value of output voltage when the input current is 5mA?
-Based from the experiment, the ideal value of output voltage is 5V when the input current is 5mA.
5. After removing the potentiometer, 1k Ω resistor, and ammeter from the input of figure 2, the
converter acts like an electronic ammeter. Why is this true?
- This is true, since when the potentiometer is removed, 1k Ω resistor, and ammeter from the
input will allow small currents to flow.
6. What change can we make in figure 2 to get an output voltage of 2V when the input current is
1mA?
-By adjusting the value of the resistors.
7. What is the theoretical current gain in figure3?
-the theoretical current gain in fig3 is 1mA with 11mA input current amplified by 10mA
8. What is the current gain in Table 3 for an input current of 4mA?
- The current gain is -34.025
1. VIII. ANALYSIS
Marian Nicole N. Remoquillo
In this activity, concept of basic Operational Amplifiers was explained. However, there were some
slight problems that were encountered during the experiment. Some of the data that were gathered are
neither accurate nor precise, and some are very far from the ideal values that should be the result. These
problems were caused by a confusion with the configuration of the circuit. There were some problems with
the polarity of the input voltage. Also, there were problems in adjusting the value of resistance in the
potentiometer.
Despite the mentioned problem, the concept was understood after simulating the whole circuit in
Multisim. From the data gathered, several conclusions were made. From the first set of data, a direct
proportionality was observed in the current and voltage measured. The output current is dependent on the
input voltage. In the second set, another direct proportionality is present on the data. The output voltage is
affected by the input current. In the last set of data, the input current results in a negative output current.
Not only was the current amplified, the sign also changed to negative. This observations data does not
complement the data gathered from the simulation. Based on the simulation, there should be no direct
proportionality in the input current and output current. The output current is not affected by the input current
and for me, the simulation was more reliable.
In overall, op amps are electronic units and are used as voltage or current controlled sources. They
amplify, total, differentiate, integrate signals. They practical to use in circuit designs for they are cheap,
inexpensive, and easy to use.
Roi Andrei J. Reyes
In this activity, I experimented the use of OP Amp in the different circuit such as the voltage-to-
current converter, current-to-voltage converter and current amplifier. Unlike the past activities, this is the
first time I had another electronic device, other than resistors and potentiometers. OP Amp is widely used
to amplify input signals into either a bigger signals or smaller signals, depending on where the input is
pinned on either #2, noninverting input, or #3, inverting input.
However, in this experiment, I faced difficulties for the duration of our laboratory hours. First, since
this was the first time we had such complex device to our circuit, we had difficulty connecting the circuit
provided in the laboratory manual. Second, there are confusions about the schematic diagrams since the
pins for noninverting input and inverting input are interchanged. Lastly, I had difficulty in measuring the
required values and the values that were being asked to find, and also I had difficulty to determine whether
the values we measured are really the values that the activity asked for.
In the last reason why I had difficulty during the experiment, since I did not know whether the
values measured were really the values in the circuit and, at the same time, I did not know how to compute
the values to check the true values due to that I do not know how to compute values with the presence of
the potentiometer, I tried in Multisim to check. After copying the schematic diagram in the software
program, I discovered that the values shown in the program is far different to what had being measured
during the experiment.
In conclusion, we think that the laboratory procedures and the schematic diagram should be
properly corrected. I am confused with the results due to that the measured values do not reflect to what the
software program shows.
