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Context:
The audio is synthesized by the microcontroller, and then modulated through a PWM. Once demodulated, the audio could feed a power amplifier which is not shown here. The hole circuit is powered up by a single 5V source, and the amp-ops are NOT rail-to-rail, that's why the voltage dividers are needed.
Input:
- The left-most op-amp, whit its two AC sources, simulates the microcontroller.
- The PWM is 5V amplitude, 128kHz frequency, with a duty cycle that ranges from 0 to 100%.
- The modulated signal is audio, 20Hz to 20kHz.
Voltage dividers:
- The two 50k potentiometers adjust the signal amplitude and offset, so as not to saturate the next stages.
- The 5V source in the second potentiometer is the power supply.
- Right now they're adjusted not to attenuate, so if you watch the output you can see that the audio is clipped.
- With simulation running, you can play with the potentiometers to correctly center the PWM input signal.
Voltage follower:
- The middle op-amp isolates the voltage dividers from the low-pass filter.
- Without it, adjusting the potentiometers would mess the cut-out frequency and gain of the next stage.
- The amp-op is configured to output between 0.5V and 4.5V to simulate that it is not rail-to rail.
Low pass filter
- The right-most amp-op is a typical Sallen and Key configuration.
- Cut out frequency is 28kHz and low frequency gain is 1.
Load
- Represented by the right-most capacitor and resistance.
- You could connect, for example, a LM386.
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