Application of Charge Pump Technology (Non Inductive Boost) in Class G Amplifiers

    Due to the use of 10 V amplifiers instead of 5 V amplifiers commonly found in portable devices, maintaining high efficiency becomes the key to extending battery life for MAX9730 and MAX9788. Although Class D technology has high efficiency and is often used in portable devices, it does not meet the driving requirements of ceramic speakers. The design challenge is very clear - ceramic speakers require other driving technologies. Therefore, Class G amplifiers, an unknown amplifier technology, have been introduced to the market due to their perfect adaptability.

    The working principle of a G-class amplifier is similar to that of an AB class amplifier, except that the G-class amplifier has multiple supply voltages rather than a fixed voltage. As the amplitude of the input signal changes, the G-class architecture automatically selects the appropriate power supply, thereby minimizing the voltage drop of the output transistor and greatly improving efficiency. Class G amplifiers are usually powered by two positive power sources, with high power sources used to output higher levels and low power sources used to output lower levels.

     The G-class amplifiers MAX9730 and MAX9788 utilize G-class technology in a unique way by generating negative pressure from the charge pump instead of high and low positive power sources. When the amplifier generates a smaller output signal, the amplifier is powered by battery voltage and ground. In this mode, the device operates similarly to the common 5 V Class AB amplifier (Figure 1a). When the output signal exceeds the power supply voltage, the amplifier selects battery voltage and negative charge pump output power supply (Figure 1b). As a result, the output signal of Class G amplifiers is much higher than that of traditional amplifiers.

    MAX9730 and MAX9788 can ensure that switching between the two power sources does not generate audio noise. When the output signal reaches the limit allowed by VCC and GND power supply, the negative power supply is automatically connected to the output stage. In this way, the output signal will not experience a clipping on the negative swing side and will still be clamped at its positive swing. To correct this, the amplifier adds an additional correction signal to the negative polarity output, as shown in Figure 2. When observing the positive and negative outputs separately, the positive half cycle waveform is significantly truncated, while the negative half cycle exhibits significant distortion. Although these signals exhibit severe distortion, they are strictly controlled and fully utilize the advantages of this architecture. The actual output signal applied to the load is not distorted.