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ECE 202 – Experiment 4 – PreLab Homework
OPERATIONAL AMPLIFIERS
YOUR NAME_________________________
LAB MEETING TIME______________
Reference: C.W. Alexander and M.N.O Sadiku, Fundamentals of Electric Circuits
Chapter 5
VOLTAGE COMPARATOR
List any references you may have used to answer questions in this PreLab
1. Op Amp could work in two different modes, depending on the way it is connected in the circuit:
a) If there is no return loop, it is used to compare values of two input terminals (as comparator).
 Explain how v0 changes depending on the values of v1 and v2.
+VCC
v1
-
v2
+
v0
-VCC

What are max and/or min values for v1, v2 and v0?
b) If there is a return loop, it is used as an ideal Op Amp.
 Explain values of labeled voltages and currents: v1, v2, v0, i1, i2 and i0
v1
i1
v2
v0
+
i2

i0
What are the max and min values of v0?
ECE 202 – Experiment 4 – PreLab Homework
2. The circuit shown below is a voltage comparator. Note that this circuit has no feedback and is not
an amplifier; the output voltage is not proportional to the input voltage. Its output is, ideally, either
+VCC or –VEE, depending on the instantaneous value of the input vin.
a) Assuming the Op Amp has a very large open-loop gain, plot vo vs. t for Vx = 4 V and vin (t) =
2.5 sin(2πft) V with f = 200 Hz.
Reminder:
b) Using the above voltage comparator, design a comparator that will illuminate an LED (light
emitting diode) when the input voltage vin > 1 V. The current from the 4 V reference voltage
source should be no more than 1 mA and the LED current should be limited to ~10 mA by a
series resistor. (At 10 mA, the voltage drop across the LED is typically 1.5 V) Give a
complete circuit diagram of your design. (Hint: Circuit should be designed in such a way that
+15/-15 V at the OpAmp output from part 2.a will turn on/off the LED)
Design:
v0
1.5V
Step #1: Choose values of resistors so that current from the 4 V reference voltage source is no
more than 1 mA.
ECE 202 – Experiment 4 – PreLab Homework
Step #2: The LED current (i.e. output current from operational amplifier) should be limited to
~ 10 mA by a series resistor R. Given that at i =10 mA, the voltage drop across the LED is
typically 1.5 V, consider LED diode as an “ideal voltage source” (VLED=1.5 V) that consumes
energy i.e. current flows into the positive sign of “the voltage source” and compute the value
of series resistor R so the current through the diode is 10 mA. (Hint: Choose R such that 10
mA creates a (Vcc – VLED) voltage drop on R)
Give a complete circuit diagram of your design.
INVERTING AMPLIFIER
3. a) For the inverting amplifier shown below, find the closed-loop gain ACL=v0/vin assuming the
Op Amp is ideal. Show your work here:
b) Plot v0 vs. vin for -2 ≤ vin ≤ 2 V.
ECE 202 – Experiment 4 – PreLab Homework
DESIGN CHALLENGE: A DEPENDENT SOURCE MADE WITH AN OP AMP
4. You will now design a voltage-controlled voltage source with the gain 4.
Design this dependent source using a noninverting amplifier circuit whose noninverting input
receives voltage va and whose output is 4va. Choose resistor values such that the load on the
circuit’s output, when nothing else is attached, is 8 kΩ. Sketch the complete circuit below.
4va
+
-
A CIRCUIT WITH A NEGATIVE THÉVENIN RESISTANCE
5. The dependent source designed in part 4 is now incorporated into a circuit.
4vA
The only viable approach for calculating its Thévenin resistance is that employing a test voltage
and current. Using the node voltage method, derive below the test current for a test voltage of 1 V
and then the Thévenin resistance:
itest = ____________
RTh = ____________
ECE 202 – Experiment 4 – PreLab Homework
DECIBEL SCALE AND BODE PLOTS
Bode plots are a tool engineers use to describe frequency response of a circuit. They are defined for
sinusoidal inputs to a circuit. The x-axis of a Bode plot represents frequency of the sinusoidal wave
that is being fed into the circuit and the y-axis shows gain (in dB) and phase shift (in deg or rad)
between the output and input signals.
System gain / Decibel scale
The gain is often displayed in decibels (dB), which represent:
𝑉
𝑔 = 𝐺𝑎𝑖𝑛𝑑𝐵 = 20 log10 (𝑉𝑜 )
(Eq.1)
𝑖
Thus, a gain of 0 represents the output being equal to the input and a gain of 20 represents a ratio of
output and input signals of 10. All negative values represent the output being smaller than the input.
Calculate gain of a system for which:
a) vo = 10 vi,
b) vo = 104 vi,
c) vo = 0.1 vi
ga = _________
gb = _________
gc = _________
The gain may be calculated using ratio of output and input powers (not voltages):
𝑃
𝑔 = 𝐺𝑎𝑖𝑛𝑑𝐵 = 10 log10 ( 𝑃𝑜 )
𝑖
(Eq.2)
Calculate ratios of output and input powers (eq.2) and voltages (Eq.1) in a circuit whose gain is:
d) g = 3 dB
e) g = -3 dB
d) Po/Pi = _____
e) Po/Pi = _____
vo/vi = _____
vo/vi = _____
Bode plots:
Although not required, if you wish to learn more about Bode plots, you may refer to section 14.4 in
our textbook.
The following Bode plot is from the datasheet of the op-amp chip
you will be using in this lab (http://www.ti.com/lit/ds/symlink/lf412-n.pdf).
Note:
Gain is shown in red,
Phase shift in black.
What do you notice happens to the gain as the frequency
increases? What do you expect this will mean for the output of your inverter circuit as you increase
the frequency of its input?
ECE 202 – Experiment 4 – PreLab Homework
PLANNING BREAD BOARD CONNECTIONS
6. You will be connecting circuit from Part 2 of this PreLab during your lab session. Draw one
way that you may decide to connect elements:
ECE 202 – Experiment 4 – PreLab Homework
PREPARING THE RAIN SENSOR
7. During lab session, you will be connecting a very simple rain sensor. You are expected to
create your own sensor board using small piece of protoboard and wire given to you last
week. The sensor should have alternating rows of connections that act as “open” while sensor
is dry, and “short” once a water droplet falls on it (water will create a short between two rows
of contacts).
Circuit diagram and the best possible way to create a sensor are shown below.
+6 V
sensor
+6 V
A
470 Ω
B
LED
-
1 MΩ
1 MΩ
+
Sensor:
A
vout
B
LED: VLED = 1.5 – 1.8 V when “on”
In order to create a sensor, you will need to solder short pieces of wire connecting every other
row or contact and longer leads that will connect your sensor to the bread board. Leads A and
B connecting sensor to the bread board should be at least 6” long to allow placing sensor
away from the board and lab equipment.
Each team must bring following supplies to the Lab session:
- Already created sensor
- Two paper towels: one to place under the sensor and one to wipe water off the sensor.
TA will have pipettes with water. You will not be allowed to bring your water bottle to this
lab session  All rules for food and water containers in the lab still apply; course instructor
and TA have received special approval from the Lab Manager to use water during this lab.