EE434 Jason Adams Mike Dierickx Class D Audio Amp EE434 Jason Adams Mike Dierickx

Basic Operation The Class D Audio Amplifier uses Pulse Width Modulation to change an analog audio signal into a digital continuous-time signal. This digital signal is fed through a set of switching power MOS devices to create high efficiency, as in all switching mode power devices.

Basic Design Requirements Uses 3.5 Volts of supply and receives an input signal that swings from -5 to +5 Volts Has a left and right channel Has digital volume control Can be fit into a die of 900 um square

Design Flow – Attenuator Since the signal swing of the input is greater than the swing of our allowed supply lines, the input phase needs to be attenuating Also, the output impedance of our previous stage is unknown, but can be assumed high Therefore, an operational amplifier-based attenuator was chosen

Attenuator – Cont. It was decided that this would be the logical place to add our volume control as well, and a scheme using eight possible levels at 3 dB increments (factors of 2) was implemented This is done by changing the value of the feedback resistor in the attenuating op amp

Potentiometer Realization Only one transistor is allowed to conduct at a time, so the op amp in the circuit can see anywhere from 64 kOhms down to 500 Ohms of total resistance in its feedback loop. Note the transistors have widths of 6 um or 12 um. This is so that their conducting resistance is negligible compared to the other series resistances.

Volume Control - Decoder Rather than have the volume controlled by 8 pins, a 3 to 8 decoder is used. This decoder was made using Synopsys, the original and program-optimized code are included as items 2 and 3 in the handouts.

Final Tuning The input resistance on the op amp was selected as 200 kOhms, giving a maximum gain of 64/200, or .32. With an input signal swing of 10 Volts, this means the maximum swing after this phase is 3.2 Volts, so clipping is impossible if the input is in the specified range.

Testing The testing was done using simply a 1 Volt DC signal, so that changes in the gain would be very visible. The results are item 4 in the handout. The eight different levels at 3 dB increments are clearly identifiable.

Results

Triangle Wave Generator The triangle wave serves as the carrier in the class D audio amp. The circuit to generate this signal is item 5 of the handout. This follows a basic schematic from the Sedra and Smith textbook, consisting of a bistable multivibrator on the left and an integrator on the right

Triangle Wave Results Using the equations in the textbook, we were able to create a generator to operate at 175 kHz, which represents significant oversampling. The simulation results showed the actual frequency to be closer to 155 kHz, still highly oversampled. The results are item 6 in the handout.

Triangle Results

Modulator The modulator is simply a comparator, a very simple part of the circuitry which uses only an op amp. The output is high (+1.75 V) when the modulating triangle wave is above the incoming attenuated audio signal. This is why 3.2 V p-p for the tri wave was important

Modulator Simulation

Modulator Simulation

Final Stage – Switching Power The final stage is the power transistors that make the amplifier a switching power device, and a low-pass filter that extracts the signal from the carrier This stage was decided to be OFF-CHIP. Using the whole die area for one NMOS transistor (w = 900u with 250 multiplier) produced only about 37% power across the speaker

Final Stage – LP Filter The LP filter is off-chip in virtually every class D amplifier, since the use of inductors and the size of the capacitors makes integration nearly impossible The purpose of the LP filter is NOT to improve the audio quality, but rather to eliminate the energy in the higher harmonics to protect the speaker

Bridge Simulation

Attenuator Layout

Attenuator PLS

Triangle Generator Layout

Triangle Generator PLS

High-level Schematic

High-level PLS

Conclusions In spite of the difficulties, we both feel the project was very educational and really helped to bring together the material we had been covering in the lab.

Conclusions However, throughout the process there were a few elements that continued to annoy us: Trouble with on-chip current Computer lock-ups and loss of data Op Amp troubles

Conclusions However, there were many things that went well: Once we had a functional op amp, we found out all out designs were correct All things considered, the layout went VERY well, and was probably completed in under 25 man hours.

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