EXPERIMENT 7B:
MICROPHONES, FILTERS, OSCILLOSCOPES, AND AMPLIFIERS
Objectives
Discover swept (multiple)
frequency generation and display.
Discover filter response to
swept (multiple) frequencies.
Predict, build, and measure
amplifier circuit gains.
Equipment and
Supplies
BK Precision 1651 A Triple
5-Pin Connector
Protoboard
Resistors (100 W, 1 kW, and 10 kW)
Capacitors (0.01mF and 0.1mF)
Inductors (10 mH)
Operational Amplifier
Global Specialties 2001A
Function Generator
Tektronix TDS210
Oscilloscope
DAS (Acquire Waveforms and
Graph.vi)
Additional
Reference Material
A traditional introductory
electrical engineering principles and applications text like: Hambley, A.R., Electrical
Engineering Principles and Applications, 1997, Prentice Hill, Chapter 6
“Frequency Response, Bode Plots, and Resonance” and Chapter 8 “Amplifiers:
Specifications and External Characteristics”.
Or, Rizzoni, G., Principles and Applications of Electrical
Engineering, 3rd Ed., McGraw Hill, Chapter 6 “Frequency Response
and System Concepts,” and Chapter 9, “Transistor Fundamentals.”
Function Generator
Description and Specifications at: http://www.globalspecialties.com/2001a.html
Hint: go to the MEL Web Site
and click on these links
Amplifiers
Many of the signals from
sensors used in industrial monitoring, manufacturing, and robotics have small
amplitudes. The amplitude of the signal
from most audio devices is too low to be heard if it is directly input to
speakers. Signal inputs and outputs from
the microphone could have very low amplitudes.
Therefore, an intermediate signal-conditioning component, an amplifier,
will probably improve the product. You
have used amplifiers in previous experiments, particularly for the strain-gage
bridge output voltage. In this part of
experiment 7, you will construct your own amplifier and active filter using
operational amplifiers, resistors, and capacitors. Op-amps are very commonly used in a variety
of instruments and can be simply configured to produce a variety of responses.
They are a cost-effective way of building an amplifier into a product. Figure 7-2 illustrates an amplifier circuit.
An idealized operational amplifier has an infinite gain and infinite input
impedance. Consequently, 1) the input terminals of the operational amplifier
will always be at the same voltage, and 2) current flow through the input
terminals of the amplifier is zero.
Write the equation for the
output voltage of the amplifier shown in Figure 7-2, as a function of input
voltage and the circuit resistor values. Calculate the voltage gain of the
amplifier.
Figure 7-2. Amplifier Figure
7-3. Op-amp pin outs
Use the components available
to wire the amplifier. The connector
color code is as follows: Yellow: 12 V,
Blue: –12 V, Black: Ground, Red: 5 V.
Refer to Figure 7-3 for Op-Amp pin outs.
Measure the gain as a
function of input frequency using the signal generator and oscilloscope. Does the Gain “roll off” at higher
frequencies? What is the 3 dB roll-off
point?
In most applications we do
not build a simple amplifier per Figure 7-2.
We also need to discriminate between the desired signal and noise. Therefore we add a filter to reduce the noise
in active filter circuits like Figure 7-4.
Figures 7-4a
and b. Op amp circuits with capacitance
Predict (calculate) the gain
as a function of frequency for the Figure 7-4 circuits. Make Bode Plots with spreadsheet software.
What type of filtering do the circuits accomplish, if any?
Wire the 7-2 and 7-4
circuits and measure and plot a set of points above and below the half power
frequency to compare with the predicted plots.
How many points on each side of the half –power frequency are necessary
to completely evaluate the filters?
Compare with your predictions.
Are the differences between measured and predicted values caused only by
component tolerances or are other factors also responsible for the differences?
Add low and high frequency
asymptotes to each plot. Identify the
half-power frequencies.
Experiment 7B
Preparation Results
DATE___________ NAME___________________________________
GROUP MEMBER
NAMES__________________________________________________________
LABORATORY BENCH NUMBER_____________
1. When capturing a scan from 160 to 16000 Hz in 1 s
to evaluate the frequency response of a band pass filter, what data acquisition
sampling rate is necessary to acquire 10 samplesper cycle? Show your calculations clearly and
completely. Define any variables
used. (5 pts)
2. Explain why the gap between the two 5-hole columns
of holes on a protoboard is useful for wiring to an IC (like an Op-Amp). Refer to the Wiring a Circuit with a
Prototype Circuit Board Reference Material.
Include a sketch in your explanation (15 pts).
3. Predict the gain as a function of frequency for
the figure 7-4 circuits. Make Bode plots
from your prediction with EXCEL spreadsheet software and attach the plots (30
pts).
4. What type of filtering do you predict from circuit
7-4a? _________________
7-4b?______________________
(10 pts)
5. Attach copies of the procedures that you plan to
use in Experiment 7b. (20 pts)
6. Attach copies of the circuit and wiring diagrams
that you plan to use in Experiment 7b. (20 pts)
Experiment 7B
Report
DATE___________ NAME___________________________________
GROUP MEMBER
NAMES__________________________________________________________
LABORATORY BENCH NUMBER_____________
* identifies
questions that require the same answer for the entire group. All other questions require individual
answers.
1. Write the equation for the output voltage of the
amplifier shown in Figure 7-2, as a function of input voltage and the circuit
resistor values. (2 pts)
2. Calculate the Voltage Gain of the Figure 7-2
amplifier. (2 pts)
3. *Complete the tables
below to show the differences between calculated and measured values for the
Figure 7-2 and 7-4 circuits: (6 pts)
7-2
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Predicted Vout/Vin |
Measured Vout/Vin |
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7-4a
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Measured Vout/Vin |
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7-4b
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Frequency |
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Measured Vout/Vin |
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4. *Attach graphs of the calculated gain as a
function of frequency for the Figure 7-2 & 7-4 circuits in the form of Bode
Plots. Add low and high frequency asymptotes to each plot. Identify the half-power frequencies. Format
the graphs properly. (8 pts)
5. *Measure and plot at least six points from the
Figure 7-2 and 7-4 circuits to compare with the predicted plots. Add these points to graphs from question 4.
(4 pts)
6. Explain how well the measured and calculated
values compare. Are some lower, some
higher? By what percent? Why are they different? (5 pts)
7. Does the Gain of the Figure 7-2 amplifier “roll
off” at higher frequencies? If so, what is the 3 dB roll-off point? (2 pts)
8. What type of filtering do the Figure 7-4 circuits
accomplish, if any? (2 pts)
7-4a______________________________
7-4b______________________________
9. Calculate how much of the differences between
calculated and measured values are due to component tolerances. (Resistor
tolerance = ±5 %) (4 pts)