Lab 7 - EE 420L 

Authored by Shada Sharif,

sharifs@unlv.nevada.edu   

10 April 2015

Pre-lab work:

• This lab will again utilize the ZVN3306A and ZVP3306A MOSFETs so review lab 6.

• This lab was about designing an audio amplifier that has a frequency range of 100Hz~20kHz using the ZVN3306A and ZVP3306A transistors, as well as two capacitors one is a 10 uF and the other had to be a 100 uF.
• Using a 10 V VDD to power the transistor, and an output of 8 ohm speaker. 

• Design process along with hand calculations.
• Simulations using LTspice of the circuit.
• results from testing design.
• comments on the performance of the design.

• The experiment started with designing the circuit needed using spice, so before starting to test anything experimentally LTspice was used to check how the circuit would work.
• The picture below shows the circuit that was first used, the bottom branch that does not have any Mosfets shows how a load (10k ohms) connected to the speaker directly would look like.
• In the simulation, one can see that connecting this huge load directly to the speaker the output is super low due to the impedence mismatching and the power dissipation output is so small that it is almost zero. 

• Looking at the picture on the left this shows the output which is comparably small and not amplified, as well as the current. As for the picture on the right, the current and output voltage across the 8 ohms resistor is shown in comparison with the 1V input sine wave, here one can see how it is almost zero thus if the experiment was conducted that way, one cannot hear the speaker since it is not amplified. 
• After these results, and seeing how the output is super small, a push-pull amplifier was used due to its high gain characteristics. From Lab 6, we know that the push-pull amplifier's gain can get to the kilo range so that would be perfect in this case. 

• So the gain of the push pull amplifier would be -(10k)(18m+11m)= -290V/V.

• Above the output with the help of the push pull amplifier is seen to be amplified and not ~ zero as before. The output voltage peak now is ~540mV since the input that is after the 10k ohms resistor is ~221mV and with the gain ~290V/V the output (vout2) is ~640V.
• After these simulations the circuit was constructed on the breadboard to test the results, while conducting the experiment the transistors started to heat up dramatically that the power supply had to be turned off. From this we noticed that the heating could be a result due to the huge output impedence of the push-pull amplifier that is directly connected to a very small impedence in comparison (the speaker).
• So thinking of a way to have a high input impedence and a low output impedence the source follower would be the best choice here.

• Results from the simulations above show that adding the source follower did not affect the gain and it only helped in reducing the output impedence to a lower value so this circuit was tested again on the bread board to see how well it will perform.

• This was the results of the circuit, the scope picture above shows that the output signal from the speaker has noise and it is not perfectly clean, we tried adding more decoupling capacitors in order to decrease the noise but the noise added can be also from impedence mismatching. 
• The output swing shown is 1.32V peak to peak so the range is -650mV~600mV. The results are not exactly correct since the noise can increase the output swing.

• The input impedence of our design can be easily seen from the schematic to be 10k ohms as well as shown in the picture above. The 10k ohms input impedence is a typical input for a audio amplifier.
• As for the output impedence, it is just the resistance of the speaker which was written on the speaker to be 8 ohms. 

• For calculating what the power dissipation is one needs to know the supply voltage and the current through each stage of the circuit. So to have the total currents of all the stages we used the current shown from the power supply, basically it is the total current drawn to the circuit. So knowing that power is current multiplied by voltage the total power dissipation of our circuit is 10V*0.282A=2.82W.

• As a conclusion, the circuitry designed is not perfectly good which can be seen from the scope images and the power dissipation of the circuit. This shows how much impedence matching is important in designing. Impedence matching allows the amplifier and the speaker to have the best performance in the circuit so that is why it is important to think of the input and output impedence in circuitry. When there is improper matching problems can occur like noise, distortion, and high power use. This was conformed because during testing even after adding the source follower and lowing output impedence, after using the circuit for a while transistors still got hot, and the scope picture shows noise introduced to the wave.
  

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