Lab 7 - EE 420L
Prelab work:
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).
Experiment 1: Design of Audio Amplifier
The reason why we need to use an audio amplifier to connect an audio signal from a MP3 to a speaker is beucase of the mismatch of impedances, if connected directly, all of the audio signal will be lost in the source resistance. Think of a voltage divider with a large resistance connected in series to a small resistance to ground. A majority of the voltage from the input will be dropped across the large resistor and very little will be dropped across the small resistor based on the voltage divider formula. From the figure below we can see that the voltage drop across the speaker from a 1 volt signal input is close to 0 which would mean that no sound would come out of the speaker.
Our design of our audio amplifier started with just a basic push pull amplifier from lab 6 and loading the 8 ohm speaker to it. The gain of the push pull amplifier without a load is calculated by Vout/Vin = Rf*(gmp+gmn). We can estimate the gain to be around 1000 range. The problem with this was that there was still a significant mismatch in impedances from the output of the amplifier to the speaker causing a large decrease in gain when the load is added. However, this was better than connecting them directly. (see figure below)
To provide a smaller output resistance of our amplifier to better match the speaker, we deceided to add a second stage source follower becuase the output impedance of a source follower will be significantly smaller while the gain is close to 1. (See schematic and waveforms below)
2 stage audio
amplifier
2 stage audio amplifier waveforms
The output of this 2 stage amplifier is better than when there was only 1 stage but this is still not a great amplifier beucase the audio signal was not only not amplified but it was reduced. The reason for this is again beucase of impedances not matching, the output of a source follower is significantly smaller than the push pull but since the load is very small (8 ohms) the output resistance is not small enough to ideally drive the speaker. Ideally we want to connect a small impdeance to a large impdeance so that the voltage divider effect as described above allows for all the voltage to drop across the large impdeance. Therefore ideally we would want the output resistance of the amplfifier to be significantly smaller than 8 ohms.
The input and output resistances from simulation are shown below. Note that the input impedance is around 10K and the output resistance changes with respect to the frequency but ranges from 3-8 ohms.
Input resistance Output resistance
Here is the oscilloscope input and output waveforms. The yellow is the input and the blue is the output.
The power consumption can be calculated by looking at the power supply voltage and the current drawn from the power supply. From the figure below, we can see that the power supply voltage is 10V and the current is 282 mA. Therefore P=IV and the power consumption of the curcuit is 10*282mA which is 2.8 Watts. This is not really a good design becuase it is drawing a lot of current leading to a large power consumption. In addition, the transistors cannot handle this amount of current. When experimenting with this curcuit, we noticed that the transistors were heating up.
