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Switching Audio Power Amplifier without gain in the negative feedback ?

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Elik Cohen
Elik Cohen on 23 Jul 2020
Answered: Joel Van Sickel on 4 Aug 2020
Hello audio amp experts,
I'm reviewing the "Switching Audio Power Amplifier" circuit example available through the Simscape product line. I'm new to amplifier design (and negative feedback control) so please bear with me.
The example shows an amplifier with the output signal being used as negative feedback to the modulator input (see Vo in 1st figure below). If we look inside the "Feedback Controller" (2nd figure below) we see that the "meas" output is fed directly into a subtract block without any feedback gain applied. Now, if we inspect the Vin/Vout waveforms we see that the output voltage "meas" tracks the input voltage "ref" (see 3rd figure below). My question is -- where is the amplification ? How come the output voltage is identical in amplitude and bias to the input voltage ?

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Answers (1)

Joel Van Sickel
Joel Van Sickel on 4 Aug 2020
Hello Erik,
as the goal of this particular feedback loops is to control output voltage, and the reference signal is also in volts, there is no need to scale either signal before comparing them (they are both volts) so they can be subtracted to find the error between the desired voltage and the actual voltage. This then is passed though the feedback compensator block which does apply a gain to these signals and implements a closed loop control that drives the output voltage to match the commanded reference voltage (it does this by calculating the appropriate duty cycle). In this case, it is not Vin and Vout that are the same, but Vref and Vout. Vin is a constant 30V dc source.
Regards,
Joel

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