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How does a switch mode amplifier... amplify?

Discussion in 'Audio' started by michan, Feb 27, 2018.

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  1. michan


    Feb 21, 2015
    I have had experience with continuous amplifiers that make use of a transistors transconductance to achieve some level of gain, but I am unsure how switch mode amplifiers such as class D or E amplifiers would amplify (voltage, current, or power)? I have tried looking into this online but I have only sources describe how the circuits operate rather than the actual mechanism that creates the gain and creates the amplified signal. I know PWM can be used in these switching amlifiers but that wouldn't produce amplification, just modulation as the name implies. Any help is appreciated, thanks!
  2. davenn

    davenn Moderator

    Sep 5, 2009
    the net has lots of useful answers, no point me repeating all the text

    Class D amps ....

    go do some reading :)

    Class E amps ....

    dont appear to be overly common, and little info online

    here's one of the few links I cold find
    Last edited: Feb 27, 2018
    michan likes this.
  3. michan


    Feb 21, 2015
    They seem to be really useful for how the circuit operates and the waveform and things but I haven't seen any of the links give a straight answer as to how the signal is amplified. One link mentioned pwm but I don't understand how pwm creates any kind of gain, I would have thought the maximum voltage or current would just be the same value as the peaks of the pulses (voltage or current).
  4. dave9


    Mar 5, 2017
    It doesn't directly PWM the input signal. It is like an opamp in that it uses a feedback loop with (either a fixed or adjustable depending on the amp IC) a voltage divider to create a lower apparent output signal (at that point in the circuit), which the PWM regulator uses to modulate the power rails to produce the desired real output signal level which matches the input level after the resistive divider drops the signal by a multiple equal to the target gain # - only going back as feedback to the opamp-like portion of the circuit, while the gain result goes to the load.

    Thus the maximum output voltage is the power supply voltage minus the drop across the switching transistor, and maybe a little more if there's an inductor or other resistance on the outputs, or less if the gain isn't set high enough to need that much voltage.
    michan likes this.
  5. BobK


    Jan 5, 2010
    Do you know how a switch mode power supply works? If so, a class D amplifier is nothing more than a switch mode power supply whose output voltage follows an audio signal.
    The key is switch mode.

    A transistor used as a switch amplifies a digital (on or off) signal. Using a MOSFET (which any current class D amplifier does,) you can get a super high power gain in a digital amplifier with a single stage. For example, say I have a 5V signal that can supply only 1mA of current . That is 5 mW. If I apply that signal to a logic level MOSFET which switches 50V to a 10A load, that is 500W for a power gain of 100,000.

    Once the audio signal has been processed into a PWM signal, that is how a class D amplifier produces gain, and only one stage is needed. Of course there is probably an analog amplifier to get the signal up to the level needed to produce the PWM.

    michan likes this.
  6. michan


    Feb 21, 2015
    Thankyou, that helps a lot! I didn't realise it was using the same sort of gain mechanism as smps.
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