Lab 7 - EE 420L
E-mail: mcdonc4@unlv.nevada.edu
0. Pre-lab
1. Hand Calculations
2. Simulations
3. Built and Tested Design
Design an audio
amplifier (frequency range from roughly 100 Hz to 20 kHz) assuming
that you can use as many resistors, ZVN3306A transistors, and ZVP3306A
transistors as you need along with only one 10 uF
capacitor and one 100 uF capacitor. Assume that the
supply voltage is 10 V, the input is an audio signal from an MP3 player (and so
your amplifier should have at least a few kiloohms input resistance), and the
output of your design is connected to an 8-ohm speaker (so, ideally, the output
resistance of your amplifier is less than 1 ohm).
Part 1: Hand Calculations
Your lab report should detail your thoughts on the
design of the amplifier including hand-calculations.
We were recommended to
take a look at the design of a push-pull amplifier
below:
Figure 1:Push-pull amplifier topology with corresponding voltage input and output.
This topology is great
at sinking and sourcing current, however it has a large output resistance. To
combat this, a source-follower topology can be added since that topology has an
inherent low output resistance. Added to the push-pull amplifier, our schematic
should look like this:
However, upon actually implementing the circuit onto a breadboard we found
that our circuit was burning any resistor that was placed at R4. This may have
been the result of the circuit allowing an excessive amount of current to flow
through R4. So instead we utilized the push-pull topology above and adjusted a
few values to fit our design:
This design produces a
-17.675 gain where R2 is the primary parameter that will determine the gain of
this circuit. However, to keep the output resistance low we will be keeping R2
at 25ohms to match the speaker resistance.
Calculations:
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Constants:
DC Analysis:
AC Analysis:
Gain:
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Part 2: Simulations
Simulate your design. Document the results in your lab
report. Document the performance of the design including power dissipation,
output swing, input resistance, output
resistance.
Adjusting R2
To confirm our
calculations that the gain can be adjusted with R2 we ran several simulations
while changing R2. We were able to confirm that the gain could be increased by
increasing R2. We chose 25ohms as our final design choice as it seemed like a
reasonable gain to display on our oscilloscope.
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25ohm resistor |
Waveform |
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50ohm resistor |
Waveform |
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200ohm resistor |
Waveform |
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Adjusting R1
We also chose to
adjust R1 to see the effects it has upon the circuit. We were able to evaluate
that R1, when increased, can also increase the gain. We will test both of these circuits and evaluate their effects on a
signal input.
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Schematic: 20k resistor |
Waveform: |
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Schematic: 100k resistor |
Waveform: |
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Power consumption simulation:
We have found the
power consumed by these circuits to be 2.64W
Resistance simulations:
In both design
choices, we found the output resistance to be close to 13.5ohms, which is a
good number as we want output resistance to be as low as possible. The input
resistances in both the 20k and 100k design were 12.5k and 22.5k respectively.
A high input resistance is more desirable.
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Input Resistance with 20k resistor |
Input Resistance with 100k resistor |
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Output Resistance with 20k resistor |
Output Resistance with 100k resistor |
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Part 3: Built and Tested Design
Build and test your design.
We chose to test both
the 20k and 100k designs to show the change in output between the two. We found
that the 100k model increased the gain as the volume was much louder than the
20k model.
Here is our circuit
implemented onto the breadboard:
This circuit was tested by inputting a sine wave at 1v and 1kHz
frequency to determine the gain
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20k |
100k |
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Function
generator signal of 1V at 1kHz |
Gain: -3.5 |
Gain: -7.125 |
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Music
signal |
Gain:-3 |
Gain:-5.14 |
We recorded a couple videos to show the difference between the two
circuits as well showing the working operation of the amplifier.
20k video:
https://youtu.be/W8bxP2sbsKY
100k video:
https://youtu.be/y_le2sIDWOM
Power Dissipated:
We found that our circuit only dissipated 140W of power. This is
more efficient than we previously simulated, but it is still within the
ballpark of our theoretical results.
