Lab Four – Audio
Systems III with Lab
Part A: Audio Amps
Given an audio amplifier application that uses an
integrated amplifier, examine the overall amplifier’s performance and verify
the operation by using suitable input and output circuits.
of the LM386 Audio Amplifier takes place during week 4. The LM386 is a versatile special purpose
op-amp used to amplifier audio frequencies.
The device is contained in a single 8-pin dip package and has a gain of
up to 200 and a power output of 1-watt.
A. Use an
Internet search utility, such as Google, find an audio amplifier such as a
LM386 download the specification sheet and determine its general
characteristics such as its general description, the package type and pin
a. Use the
Internet to search for a LM386 data sheet.
Try.national.com">www.national.com or.datasheetarchive.com">www.datasheetarchive.com. Print the datasheet.
b. From the
data sheet find the following information and record.
its general characteristics?
B. Use the
specification sheet to determine its electrical characteristics and operating
the electrical values of the LM386.
information from the data sheet or other sources and an application such as
Multisim, Visio, or a CAD program draw the schematic and verify the component
values of an audio amplifier application using the device.
the Typical Applications section or other source find an amplifier with a gain
equal to 20.
b. Use an
application such as Multisim, Visio, or a CAD program to create the circuit
schematic. Label the schematic, LM386
c. Label the
pin numbers on the LM386 and give each component a name and value.
briefly how the gain of the LM386 can be changed.
the audio amplifier on your breadboard with various gains (20 and 200). Using a frequency generator as an input,
verify the amplification of the audio amplifier. Use 10mV peak as the input
signal and keeping all the component connecting wired as short as possible.
Also keep all the components close together on the breadboard.
measurements purposes, prototype an amplifier similar to the one below.
Remember keep all component connecting wires as short as possible, and keep all
components as close to the audio amplifier as possible. The 10mV peak input is
set between the 1k resistor and C6.
the table below.
R C5 .gif"> .gif"> Expected Measured
-- -- _____ _____ _____ _____
.gif">.gif"> _____ _____ _____ _____
-- .gif"> _____ _____ _____ _____
with pin 1 an 8 open and the frequency generator set to 10 mVp at 1 kHz, use
the oscilloscope to measure the voltage values.
Calculate the gain of the amplifier, .gif">. Record the results
on the sheet.
this procedure until you have determined the three gain values.
there any problems with the different gain values? If so, explain the symptoms and how to
correct the problem.
and plot the audio amplifier’s frequency response. Determine midband gain, 3-dB bandwidth,
critical frequencies and roll-off rate. Compare the results to the data sheet.
the information above, set the gain of the amplifier to 20.
b. Set the
output of the frequency generator to a sine wave with amplitude of 100 mV at 1
the table below.
f (Hz) .gif"> .gif"> Gain
1 _____ _____ _____
10 _____ _____ _____
100 _____ _____ _____
1k _____ _____ _____
10k _____ _____ _____
100k _____ _____ _____
500k _____ _____ _____
d. With the
frequency set to 1 Hz, use the oscilloscope to measure the above values and
calculate the gain. Note that very low
or high frequencies may be difficult to measure.
changing the input frequency by a power of 10 (10, 100, etc.) until you reach 500kHz.
f. Use a
piece of graph paper to plot the results.
A work sheet can be found in Doc Sharing, week 4 -- “graph_paper.doc”.
an input circuit such as a frequency generator and a microphone and a load such
as a speaker verify the operation and troubleshoot any problems.
the advance amplifier circuit for this step.
Set the gain to 20.
a frequency generator set to a sine wave at 400 mV peak and 100 Hz to the input
of the circuit. Connect an 8-ohm speaker
to the output of the circuit. Set .gif">to it highestvalue.
down your observations. Record the
power and record what you hear. Slowly
decrease .gif">and record any
changes. Continue to decrease.gif">. Use an oscilloscope
to observe the output signal. What is
happening to the output waveform?
the input frequency to 1 k Hz and 10 kHz and record the change.
the frequency generator to a square wave and repeat the steps above. Record the results.
the frequency generator and connect a dynamic microphone to the input. Test the circuit and record the results. Vary .gif">for maximum
output. Can you get maximum gain from
the circuit without oscillation?
Part B - More LM386
LM386 may be used to amplify voice inputs.
Dynamic or Condenser (Electret) microphone are two common
input devices. Dynamic microphones can
be connected directly to the input of the LM386 amplifier and uses resistance
variation within the microphone to convert voice fluctuations into small
electrical signals. The condenser type
microphone use capacitance to detect these changes and requires a bias voltage
applied to the microphone.
.gif" alt="Mic advanced schematic">
specifications of a typical condenser microphone are shown below.
to 10V DC
to 8 KHz
the information above, create a bias circuit for the microphone. Draw the schematic and label each component.
bandwidth is defined as the:
½ power points
3 db down from the max db level
f (Hz) .gif"> .gif"> Gain Gain(Db)
1 56mV 320mV 5.71 15.1
10 64mV 320mV 5 14
100 112mV 1.28V 11.42 21.15
1k 112mV 1.28V 11.42 21.15
10k 160mV 1.20V 7.5 17.5
100k 3 9.54
If the above data
were taken the bandwidth would be at:
Since these are
voltage gains .707*Vmax
11.42 *.707 = 8.1
After plotting the
graph we need to go below 100 hz and read the frequency that the gain is 8.1.
This is f1
Then go above 1k
until the gain is 8.1 and read that frequency. This is f2
Bandwidth = f2-f1
If it is in Db then
the bandwidth would be at the 21.15DB-3Db or 18.15Db
This should be the
is semi log paper, note the bottom horizontal scale is frequency, and always
gets 10 times larger on the next decade. There are no zeros on the graph. 10^0 = 1 so that is the first number, 10^1 =
10 so we jump from counting 1’s to counting 10’s, for example
1,2,3,4,5,6,7,8,9,10, 20,30 etc.
vertical scale is gain in decibels and uses a linear scale.
point plotted is a frequency of 10 hertz and a gain of 7 db..jpg">