EE 420L Engineering Electronics II - Lab 7

Eric Monahan

monahan@unv.nevada.edu

4/6/16

 

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 involved contemplating a design based upon the push-pull amplifier included in the Laboratory 7 instructions. The choice is logical due to the high gain that results from the basic push-pull topology illustrated in Laboratory 6, Experiment 4. The basic topology and gain are displayed below as a review.  

 

  

 

The schematic presented in the Laboratory 7 instructions is displayed below. The resistor R1 is proportional to the gain of the circuit. Laboratory 6 experimentation led to experimental values of gmn equal to 19.0mA/V and gmp equal to 13.3mA/V. This presents a constant in our gain. Sweeping R1 allows analysis of the circuit operation for different gains.  Ideally, this allows a design that will maximize our gain and minimize our power consumption. The speaker used in the experiment is only 2W and the transistor has a power dissipation around 600mW, so the power cannot be too high.

 

           

 

The waveform displayed above to the right shows the plots for all the different values of R1 from 10kΩ to 100kΩ. Top to bottom, these include the current in the circuit centered at 220mA with a 70mA swing, the average power centered at 2.2W with a 0.7W swing, the power through the speaker that does not receive amplification and the power through the speaker receiving amplification. Note the difference in power in speaker 2 approximately 3 times the power in speaker one. Looking at the waveforms, it is easy to see there is a point where the increasing resistance starts to adversely impact the circuit with clipping. This will be dealt with after the next design consideration.

 

The issue with this basic design is that despite the ability to set the gain by simply changing the value of R1, the amplifier needs to have an output impedance lower than the speaker impedance of 8Ω. The goal is a high input impedance to allow amplification of the voltage and a low output impedance to drive the current up. To lower output impedance, a source-follower was added to the output of the initial push-pull stage. The source-follower has a unity gain and low-output impedance, thus the benefit of the push-pull gain is preserved with the additional benefit of a smaller output impedance. The circuit below is the new circuit with the source follower added and an additional 8Ω resistor added to further reduce output resistance. The waveform to the right is the initial gain with R1 again being swept. Clearly, using too large of a resistor has a negative impact on the gain. To determine a gain that works, the steps were singled out in the simulation until a suitable resistance was determined.

 

             

 

 

The top waveform below displays the output versus the input at R1 = 30kΩ. There is clearly clipping occurring at the output. The remaining three waveforms are, top to bottom, the current, power dissipation and output versus input with R1 = 20kΩ. Notice the difference in power and current at the output. Adding the source-follower stage to the initial push-pull stage resulted in a 200mA increase with a 220mA swing. The power is now centered at 4.2W with a 2.2W swing. This led to the decision to use a 20kΩ resistor for R1. This will provide a theoretical gain of  Av = 20kΩ*(19mA/v+13.3mA/V) = 3230V/V.

           

 

As predicted, the source follower also resulted in a reduction in output resistance as seen in the waveform on the bottom. The output impedance is approximately 2.67Ω, a result of R2 in parallel with the 8Ω speaker load. This is simply (4*8)/12 or approximately 2.67Ω as displayed. 

  

 

The result of the simulation is an amplifier that provides enough power and current to drive the MP3 player. 

 

 

Build and test your design. Document the performance of the design including power dissipation, output swing, input resistance, output resistance.

 

This is the initial circuit with a 200kΩ potentiometer as R1. The initial plan was to sweep the pot until the best sound and gain was heard and ultimately treat the pot as a volume control. However, given my limited experience with amplifier design this may not have been the best approach. This resulted in more confusion than the effort was worth and was quickly replaced by a 20kΩ resistor. The inclusion in the report is simply to show the initial approach and verify there is more failure than success in the laboratory.

 

 

Building and testing the design presented a challenge. The initial design resulted in some amplification and a high pitched sound coming from the speaker with a sinusoidal input of 1VPP@1kHz. However, the amplification was small and the current was only 150mA. Varying the frequency from 100Hz to 20kHz did not result in any sound at all. To attempt to increase the gain in the push-pull stage, a second PMOS and NMOS were added in parallel to the initial devices in the schematic above.  A second NMOS was also added into the source follower to drive the current up. The simulations below show the results. Note the large increase in current I(Vdd). The power also increased considerably.

 

 

 

            

 

When this stage was added the current in the power supply jumped up to 338mA and the sound was distorted. The waveform to the right below displays Channel 4 with a voltage gain of approximately 18. Testing the frequency at 100Hz and 20kHz resulted in some noise, but not a clear sound as with 1kHz. The transistors were also noticeably warmer.

 

             

 

 

To lower the current gain, the second parallel NMOS was removed from the source follower stage and the circuit was simulated again. The results are displayed below. Note the current and power decreased. The maximum power through the speaker is approximately 420mW as displayed in the waveform to the right. The power dissipation for the NMOS is rated at approximately 625mW at room temperature, but this transistor is definitely hotter than room temperature. Theoretically, the power is within the margins, but practically the power is too high.

 

  

 

 

After removing the parallel NMOS from the source follower, the current dropped down to 299mA. This gives us an average power of approximately 2.99W. This is a little higher than desired, but for the laboratory experiment, this is satisfactory. The resulting waveform below shows a voltage gain of approximately 8V/V where Av = 3.96V/496mV. The sound coming from the speaker at 1kHz was clear and high pitched, superior to the distorted sound prior to removing the second parallel NMOS. The sound at 100Hz was cleaner, but still had crackling and some distortion. At 20kHz there was a consistent static sound.

 

            

Conclusion

 

The experiment conducted in laboratory seven provided the opportunity to design an audio amplifier and gain practical experience with the trade-offs involved in analog design. Specifically, learning how to utilize different topologies to take advantage of the inherent characteristics of each design requires study and experimentation. This laboratory offered an excellent chance to learn about manipulating input and output impedances to achieve a desired effect. Learning to create unique designs using different amplifier topologies in varying stages and comprehending how each decision impacts the overall circuit is a skill acquired via practical application.

 

 

 

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