Lab 7 - EE 420L: Engineering Electronics II

 

James Mellott

mellott@unlv.nevada.edu
02/20/2017  


Design of an audio amplifier 

This lab will again utilize the ZVN3306A and ZVP3306A MOSFETs.

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).

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.

Experiment:

The first part of the experiment I started simulating the push pull amplifier to determine the value of the resistors I needed to use with the push-pull topology to meet the requirements of this lab.  Below in figure 1 is the basic push pull topology followed by the gain calculations.

Figure 1

As can be seen the push-pull topolgy is very simple to implement and easy to calculate gain.  I will show below in figure 2 the topology I decided to use in spice with simulations results.

Figure 2

Figure 2 demonstrates the amplifier meets all the requirements of the lab.  The amplifier operates over a large frequency range demonstrated here at 100Hz to 100kHz.  The outputs at 100Hz and 20kHz are nice sine waves, no DC signals.  DC signals are very bad for speakers,  A DC signal will hold the speaker in a “in” or “out” position.  This hold will damage the speakers by over heating the voice coil and eventually burning the wire.  This is known as a blown speaker.  The purpose of the capacitors in this circuit are intended to remove all DC voltages before the signal is sent to the speaker. 

After finding a suitable simulation I built the amplifier seen in figure 2 on a bread board and the output at 100Hz and 20kHz can be seen below if figure 3.

Figure 3

The results demonstrate that the amplifier works as intended.  The output resistance was calcuted as 6.49 Ohms and can be seen below in figure 4.  The input resistance is set by R2 from the schematic in figure 2.  My circuit used a 4k Ohm input resistance.  Manipulating the input resistance and R1(reference) the gain of the amplifier can be adjusted.  My circuit used 2.3 watts across the frequency range.

Figure 4

Conclusion

Lab 7 introduced me to a real world application of an amplifier, below is a demonstration of my design using the bread board and a speaker with my iPhone as the source.

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