E.
Determine 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.
a.
Using 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 (one) Hz.
c.
Complete the table below.
f (Hz)
(
)
in pp
V
(
)
out
pp
V
Gain
1
100mV
1.685
168.5
10
100mV
1.641
164.1
100
100mV
1.684
168.4
1k
100mV
1.673
167.3
10k
100mV
1.036
103.6
100k
100mV
1.573
157.3
500k
100mV
1.563
156.3
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.
e.
Continue 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.
F.
Connect 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.
a.
Prototype the advance amplifier circuit for this step.
Set the gain to
20.
b.
Connect 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
1
R
to the
highest
value.
c.
Write down your observations.
Record the information below.
d.
Apply power and record what you hear.
Slowly decrease
1
R
and
record any changes.
Continue to decrease
1
R
.
Use an oscilloscope
to observe the output signal.
What is happening to the output
waveform?
The waveform should expand out and sound gets lower.
e.
Change the input frequency to 1 k Hz and 10 kHz and record the
change.
RMS stays the same, frequency increases.
f.
Change the frequency generator to a square wave and repeat the
steps above.
Record the results.
RMS is the same. Peak to peak decreased.
g.
Remove the frequency generator and connect a dynamic
microphone to the input.
Test the circuit and record the results.
Vary
1
R
for maximum output.
Can you get maximum gain from the
circuit without oscillation?
Part B - More LM386
1.
The 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
