Lab 7 - ECE 420L 

Authored by Silvestre Solano,

Email: Solanos3@unlv.nevada.edu

4-10-2015

 

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.
 

 
For the push-pull amplifier seen above, the gain was found to be very low in lab. We estimate that its gain was about 1. So, professor Baker suggested we connect second NMOS to the ouput. With this suggestion, the amplifier shown below was designed.  For the push-pull amplifier, I assume that the RS1 and RS2 resistors represent the output resistance of the MP3 player and therefore, the real input (output of the MP3) is to the left of the RS1 and RS2. So, for the modified design shown below, I labeled the node that represents the input to be amplified as Vin. I also removed the unamplified speaker from the schematic.
 
The gain is calculated by using the equation shown below and substituting the approriate values.
 

 
The above equation for the gain was kindly provided by professor Baker in lab. This equation works if the we assume that Vin is much less than Vx and the output resistance of all the transistors is very big. The appropriate values for the resistors can be seen from the schematic. The values for gm1 and gm2 are determined from the Spice error log, which is shown after the schematic below. gm1 = 0.262 and gm2 = 0.240. Using these values, the gain is calculated as follows:
 

 
This is a pretty big gain.
 

 
 
 
Simulate your design. Document the results in your lab report.
 
Spice error log with gm1 gm2 and gm3.

 

Input and output waveforms

 
Gain

 
 
As seen from the above frequency response, the input is amplified from at least 100 Hz to 20 KHz as specified by the design criteria. The simulated gain is about 135 (42.6 dB). This is very different from the calculated value of 22311. This could be for a number of reasons. Most likely this is because the assumptions made to justifiy the gain were wrong. The output resistances of the transistors  are probably not small enough that they can be neglected.
 
Build and test your design.
Document the performance of the design including power dissipation, output swing, input resistance, output resistance.

 
The circuit was bult in lab and the input and output waveforms where captured and are shown below.
 

 
Unfortunately, the waveforms do not make any sense because the input from the function generator was only 1 volt amplitude, but for some reason, the oscilloscope shows the input to be a ridicolus 9.4 volts. Also, the output waveform  is about half of the input. I spent a fair amount of time trying to figure out why this was so, but I could not determine the cause. Also tried adding a second push-pull amplfier stage, but the waveforms for that also did not make much sense. However, the sound coming from the speaker was louder with the amplifier than without it. So apparently, it does its job of amplifying sound.
 
The power dissipation of this amplifier can be deduced by the power supplied by the 10 volt source. The power supplied from the 10 volt source can be determined by the picture shown below. Thus, the power dissipated is 10V*0.29A = 2.90 watts. This is quite large for the transistors used in lab because the transistors got very hot in the actual experiment.
 

 
Since the scope waveforms did not yeild accurate results, I will have to rely on the simulation for determining the output swing and input/output resistances. The output resistance is shown to be about 8 Ohms, which is what I exptected. The input resistance is about in the range of the expected 10k Ohms.

 

Output Resistance

  

Input Resistance

 

The output swings from about 420 mV and -330 mV, as shown by the simulated input and output waveforms shown in the begining of this report. 

 

And as always I will back up my stuff as shown below.

 

 

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