EE 420L Engineering Electronics II - Lab 7 Design of an Audio Amplifier

 

Authored by David Flores

Email: flored6@unlv.nevada.edu

Due: April 3, 2019

 

Lab Description

For this lab we were to design an audio amplifier that has a frequency range of about 100Hz-20kHz with an input is an audio signal from an MP3 the Output of the design in an 8ohm resistor which is the speaker.

 

Pre-lab

 

Lab Instructions

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).

 Ensure that your html lab report includes your name, the date, and your email address at the beginning of the report (the top of the webpage).
When finished backup your work.

 

Below is a comparison between driving a speaker without (red, Vout1) and with (black, Vout2) an audio amplifier. The source resistance is 10k meaning that the source can supply 1 V (blue, Vs) at 100 uA maximum. The simulation files used to generate this figure are found in lab7_sims.zip. 

 

 

Lab Report:

For this lab we made a push pull audio amplifier. We changed some of the components to better our output with an input of an aux cord from the computer, so we could output out of the speaker. We know from previous labs that if increase R1 in the push pull circuit we increase the gain this is needed so that the audio signal produced by the MP3 can be heard by a human. We also designed it so that we could have a range of frequency from 100Hz to 20kHz. Here are Hand Calculations  from the modified circuit above.

 

Hand Calculations:

With the hand calculations we can see that to increase the voltage gain which increases volume we need to increase the resistor between Vin and Vout. This will also increase the power consumption. To increase the power consumption more we need to decrease the Rs Resistor this limits the Current through the Push Pull. So we increased R1 to 100k and decreased R2 to 2k. We used an output resistance of 8 ohms because that is how much the speaker was we could have used a different speaker with 25 ohms resistance but we would need more current through that resistor which means a bigger voltage drop. Here are our LTSpice from the circuit above except modified as stated previously.

 

LTSpice Results for Audio Amplifier

The red waveform is our sinusoidal at 1V Pk-Pk which is representing what our input audio signal would be. The blue waveform is our Vout1 which is our output with no amplification. Our turquoise waveform is Vout2 which is our amplified output.

As we can see from our results in comparison to the previous circuit we did get a bigger amplification. There are some draw backs to this design though, the outputted signal can sometimes become a bit noisy and it shows that the waveform is not as smooth on LTSpice.

 

Oscilloscope Results of above circuit at 1kHz

Here we simulated the above circuit we got our yellow signal which is our Vin = 1V Pk-Pk at 1kHz. The blue signal is our Amplified signal Vout2 = 248mV. The purple signal is our unamplified signal Vout1 = 2.4mV (VDD = 5V).

 

 

Frequency Range From 100Hz-200kHz.

For these results we used Vin as 1V Pk-Pk and at 5V VDD.

 

Oscilloscope Results at 100Hz                                                                                    Oscilloscope Results at 20kHz     

                       

Here at 100Hz we got a gain of 272mV and the unamplified signal gave us 1.6mV. At 20kHz we got a gain of 232mV and the unamplified signal gave us 1.6mV.

Power Dissipation

Here is the power dissipation from LTSpice ideally we would want to have a larger power dissipation but for the push pull design the best option was to lower it.

 

 

Final Results:

https://www.youtube.com/watch?v=xbiRdW42jdU

 

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