Class E oscillator with commercial components

In electronics, the power amplifier class is an alphabetic symbol applied to different types of power amplifiers. This class provides an extensive description of the characteristics and performance of amplifiers. These categories are related to the time period during which the active amplifier device passes current, expressed as part of the period of the signal waveform applied to the input. In this paper, we will talk about different type power amplifier and mainly focus on Class E amplifier, including it's structure and theory. 

1.1 Class B Amplifier

Class B amplifier improves the full power efficiency of the ordinary Class A configurations. This power amplifier is one of the most commonly amplifier, which often be used as error amplifier, Current-to-voltage converter, and Bromley amplifier. Figure 1.1 is the basic configuration of Class B Amplifier. While the gate voltage (Vsin) is positive, M1 will be turned on and VDD start charge the inductive load (R Load). While the gate voltage become negative, M2 will be turned on and the inductive load would flow the voltage to ground through M2.

Figure 1.1: Class B Amplifier Configuration

Figure 1.2 is the Class B waveform, which highest power efficiency is only 78.5 percent. In this design, crossover distortion may happen when the phase is changed. If the PMOS and NMOS is mismatch, the power consumption may increase and imbalance. Therefore, this type of amplifier is not fit for our requirement. 

Figure 1.2: Class B waveform

1.2 Class C Amplifier

Due to the low efficiency of Class B Amplifier, Class C Amplifier is designed to increase the high efficiency. Class C Amplifier is often used in high frequency sine wave oscillators and radio frequency amplifier.

Figure 1.3: Class C Amplifier configuration

Figure 1.3 is the configuration of Class C Amplifier, which has strong noise. Therefore, LC harmonic circuit is used to eliminate the noise. In the configuration, M1 works as a switch, while M1 is turned on, the capacitor is charged by the voltage of DC power source, in the same time, the current will flow to the inductor. When the M1 is turned off, the inductor and capacitor will exchange energy and established an oscillation of the resonant frequency. Figure 1.4 is the waveform of Class C Amplifier; the conduction angle of Class C Amplifier is less then Class B Amplifier.

Figure 1.4: The waveform of Class C Amplifier

1.3 Class D Amplifier

Class D Amplifier uses high speed switching of transistors to generate a square wave output voltage and high-power efficiency, which is greater than 90 percent in current design.

Figure 1. 5: The configuration of Class D Amplifier

 Figure 1.5 is the configuration of Class D Amplifier, by turning on or off for each transistor to amplify the analog signal and minimize the energy loss. Due to the original signal is converted into digital signal first, this configuration is also called digital amplifier. Some losses may occur while the transistors mismatch each other. If both transistors are turned on at the same time, the efficiency reduction and potentially damage may happen.

1.4 Class E Amplifier

Class E Amplifier is a highly efficiency oscillator which often be used in radio frequency. Class E Amplifier was reported in 1975. Like Class D Amplifier, NMOS is operated as a switch. The function of class E Amplifier is producing higher output voltage than voltage supply and zero voltage switching. Zero voltage switching means to save the power consumption and increase the performance. Unlike Class D Amplifier, the parasitic capacitance can be resonated with inductor with high efficiency. Figure 1.6 is the configuration of Class E Amplifier.

Figure 1. 6: Configuration of Class E Oscillator

Figure 1.7 is the waveform of Class E Amplifier. In this waveform, the drain voltage works will in the triode region. This effect is very positive and can increase the efficiency. While the M1 is turned on, DC voltage supply will charge L₁ and flow to the ground without through R_LOAD. While the M1 is turned off, DC voltage supply and L₁ make the current through R_LOAD. Therefore, the power consumption is very small and also have high efficiency.

Figure 1. 7: Class E Amplifier Waveform

1.5 Conclusion

In this paper, we describe different types of amplifier. Each type of amplifier have their advantage and disadvantage. For example, Class AB amplifier can be used in the wireless communication system. The reason is it can provide "0" and "1" status clearly. On the other hand, Class D Amplifier often be used in the wireless charging system. Wireless Charging system doesn't need digital signal. Instead, high performance power transmission is the critical issue in this section. Class E Amplifier has following advantages, zero-voltage switching, low power consumption, and can provide high output voltage make this device become a very popular configuration right now. 

In the next section, we will explain the whole structure, theory and working process of Class E amplifier, and combine this structure with Hartley Oscillator to make an Class E Oscillator. 

Thank you, 

Shang-kai, Wei.