Lab 7 - EE 420L
reedj35@unlv.nevada.edu
- Your
lab report should detail your thoughts on the design of the amplifier
including hand-calculations.
- A
good place to start is with the push-pull amplifier characterized in lab
6.
- Simulate
your design. Document the results in your lab report.
- Build
and test your design.
- Document
the performance of the design including power dissipation, output swing,
input resistance, output resistance.
For this
lab, I based my audio amplifier design off the push-pull amplifier we
experimented with in Lab 6. I first simulated the schematic shown in the prelab
(using a 25Ω resistor).
|
Figure 1: Circuit Schematic |
Figure 2: Circuit Waveforms |
I know from Lab 6 that if you
increase R1 in this circuit that you will, in turn, increase the gain. I do not
know exactly the size of the audio signal produced by an MP3
player, but I can assume
that the signal will be small. This is the reason why an amplifier is required
in order for humans to be able to hear the music. For my design
of this audio amplifier, I
will try to increase the gain of the amplifier by looking at the resistor
between the gates and drains of both the NMOS and PMOS. Let’s first
look at some hand
calculations to find the gain of the push-pull amplifier above (neglecting RS1,
Rspeaker1, and Vout1).
All of the below hand calculations
were found using: 
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DC Calculations |
AC Calculations |
Input and Output
Resistance |
|
|
|
Input Resistance:
Output Resistance:
|
Simulations:
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Figure 3: Circuit Implementation to decide
optimal value for R1 |
Figure 4: Voltage gain for each resistor
value from 10k to 100k incrementing in steps of 10k |
|
Figure 5: Simulation waveforms for
speaker power dissipation, circuit power draw, and output swing |
Figure 6: Input Resistance Measurement |
|
Figure 7: AC Analysis Speaker Power
Dissipation |
Figure 8: Output Resistance
Measurement |
After reviewing the data
given above, we decided to choose R1 to be 20kΩ.
|
DC Calculations |
AC Calculations |
Input and Output
Resistance |
|
|
|
Input Resistance:
Output Resistance:
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Simulations:
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Figure 9: Circuit Implementation
with resistor value chosen (20k) |
Figure 10: Voltage gain |
|
Figure 11: Simulation waveforms of
speaker power dissipation, circuit power draw, and output swing |
Figure 12: Input Resistance
Measurement |
|
Figure 13: AC Analysis Speaker Power
Dissipation |
Figure 14: Output Resistance Measurement |
Experimental Measurements:
|
Figure 15: Voltage gain of audio
amplifier with 20k resistor |
Figure 16: Voltage gain of music
signal with 20k resistor |
|
Figure 17: Voltage gain of audio
amplifier with 100k resistor |
Figure 18: Voltage gain of music
signal with 100k resistor |
Gain of 20k audio amp: 560
(if doubled, it would be 1.12, which is consistent with 1.006 calculated
above).
Gain of 100k audio amp: 1.18
(if doubled, it would be 2.36)
We notice that the output
swing of the output voltage is approximately half of what was simulated and calculated
above.
I believe that this is due
to an error made where R2 was not connected to the circuit correctly and was
not realized
until it was too late. This
means that instead of an output resistance of 12.5Ω, the output
resistance was 25Ω. Thus, the voltage
drop across the speaker was doubled,
and half the voltage expected was measured. Otherwise, the calculations,
simulations,
and experimental
measurements appear to agree.
|
Figure 19: Video of audio amplifier
playing music with 20k resistor (click picture) |
Figure 20: Video of audio amplifier
playing music with 100k resistor (click picture) |
As we can see
from the videos above, when the 20k resistor is used, the sound output of the
speaker is lower than when
the 100k
resistor is used. We wanted to test what the song “Bennie And The Jets” sounded
like with each resistor value.
The videos show
that we can compare the output signals and see that the output signal of the audio
amplifier with a 100k
resistor between
the gates and drains of the NMOS and PMOS is higher.