**2.3 Switched-mode power amplifiers**

In subsection 2.2, average efficiency enhancement techniques with the goal of maintaining high efficiency over a wide range of input power have been described. Considering peak efficiency at peak output power, switched-mode power amplifiers can achieve higher efficiency than widespread class AB power amplifiers. In case of switched-mode, the power transistor operates as a switch so that output voltage and current of the device (drain of FETs and HEMTs or collector for BJTs and HBTs) do not have high values at the same time. For the "off" state, the current is near to zero and the voltage is high and vice versa for the "on" state resulting in theoretical efficiency of 100%. In the following, switched-mode class E, F and D will be briefly described.

## **Class E**

The first class E amplifier has been proposed by Sokal in 1975 (Sokal, 1975). Thereafter, other variations of class E amplifiers have been constantly presented with higher operating frequency where not only class E operation is ensured, but also, practical issues such as small circuit size and simple matching have been taken into account. A good example of such progress in class E amplifier design was represented by the class E amplifier with parallel circuit proposed by Grebennikov (Gebrennikov, 2002). Class E offers high efficiency by avoiding simultaneous existence of high drain voltage and high drain current and thus, avoiding power dissipation of the power transistor. Control of the output current and voltage waveforms at drain or collector node of the device is achieved using an output load network. Theoretically, as the transistor turns on, the voltage drops to zero and the current starts to flow so that the output capacitance is gradually charged. As soon as the control voltage of the switch is lower than the switching voltage threshold, the transistor is turned off and the current drops to zero while the output voltage of the device starts to increase. The ideal class E voltage and current waveforms are depicted in Fig. 5. Variations of class E amplifiers are reported to offer high power as 1 kW for switching applications at low frequency, whereas for RF applications, operating frequency of 10 GHz was already presented (Weiss, 1999). Class E is a promising switched-mode amplifier concept due to its simple architecture and flexibility compared to other switched-mode classes. Combination of a class E amplifier with average efficiency enhancement techniques e.g. EER or Doherty has been reported in the literature (Diet et al, 2004 and Kim et al 2010).

#### **Class F**

162 Wireless Communications and Networks – Recent Advances

with new peak to average power ratio value where high average efficiency is desirable. This can be simply achieved by modifying the input power division ratio between main and auxiliary amplifier. If necessary, three amplifiers can also be used in order to maintain high

In subsection 2.2, average efficiency enhancement techniques with the goal of maintaining high efficiency over a wide range of input power have been described. Considering peak efficiency at peak output power, switched-mode power amplifiers can achieve higher efficiency than widespread class AB power amplifiers. In case of switched-mode, the power transistor operates as a switch so that output voltage and current of the device (drain of FETs and HEMTs or collector for BJTs and HBTs) do not have high values at the same time. For the "off" state, the current is near to zero and the voltage is high and vice versa for the "on" state resulting in theoretical efficiency of 100%. In the following, switched-mode class

The first class E amplifier has been proposed by Sokal in 1975 (Sokal, 1975). Thereafter, other variations of class E amplifiers have been constantly presented with higher operating frequency where not only class E operation is ensured, but also, practical issues such as small circuit size and simple matching have been taken into account. A good example of such progress in class E amplifier design was represented by the class E amplifier with parallel circuit proposed by Grebennikov (Gebrennikov, 2002). Class E offers high efficiency by avoiding simultaneous existence of high drain voltage and high drain current and thus, avoiding power dissipation of the power transistor. Control of the output current and voltage waveforms at drain or collector node of the device is achieved using an output load network. Theoretically, as the transistor turns on, the voltage drops to zero and the current starts to flow so that the output capacitance is gradually charged. As soon as the control voltage of the switch is lower than the switching voltage threshold,

average efficiency over a high dynamic range.

Fig. 4. Block diagram of a Doherty amplifier.

**2.3 Switched-mode power amplifiers** 

E, F and D will be briefly described.

**Class E** 

High efficiency of class F amplifiers is achieved by shaping the wave forms of output current and voltage of the power transistor which operates as a switch. Compared to class E, where load network is required to ensure the ideal switching condition (on state with high current, zero voltage and off state with high voltage and zero current), load network of class F has additional function which attempts to shape the output voltage and current waveforms at the device's drain or collector node. For conventional class F, odd harmonic peaking of the device's output voltage is realized by providing high impedance (open circuit condition) at the odd harmonic frequencies. As a result, the voltage waveform approximates a square wave. For the drain current, even harmonics are provided in addition to the fundamental by offering the device a short circuit condition at even harmonic frequencies. As a result, the current waveform approximates a half wave signal. Ideal current and voltage waveforms of a class F amplifier are shown in Fig. 5. Another alternative variation of class F is the inverse class F where the current waveform approximates a square wave, whereas the voltage waveform approximates a half wave signal. Efficiency of class F amplifiers can be increased by offering appropriate termination (open or short) at higher harmonics. However, this occurs at the cost of circuit's complexity. Similar to class E, class F and inverse class F amplifiers can be combined with Doherty technique to obtain high average efficiency for wireless communication signals with high peak to average power ratio. By using class F or inverse class F in a Doherty transmitter, peak efficiency is increased compared to the variation with class B main amplifier (Goto et al, 2004).

#### **Class D**

Unlike other switched-mode amplifier classes, class D uses at least two transistors as switches. In case of current mode class D (CMCD), the transistor's output current has a form of a square wave whereas the voltage mode class D (VMCD) shows a square output voltage of the transistor (see Fig. 5.). For both CMCD and VMCD, a tank filter is required to obtain the sinusoidal signal at the load. For CMCD, additional BALUN is also required, whereas for VMCD, two supply voltage sources are needed (see Fig. 6.). When one of the switches is turned on, the other one is turned off, so that high current and high voltage cannot exist at the same time. Theoretically, 100% efficiency can be achieved. In practice, the efficiency is compromised by limited switching speed and device's output capacitance. Due to these reasons, frequency of operation is limited for class D amplifiers. Experimental, state-of-the-art RF class D power amplifiers can operate at frequencies in the region near to 1 GHz (Aflaki et al, 2010).

Fig. 5. Ideal current and voltage waveforms of class E, F and D switched-mode amplifiers (Raab et al., 2002). Broken lines represent the device's output current and the solid lines represent the device's output voltage.

Fig. 6. Configurations of voltage mode class D (VMCD) and current mode class D (CMCD) amplifiers.

#### **2.4 Linearization techniques**

In general, a trade-off exists between efficiency and linearity of power amplifiers. For conventional transconducdance amplifier classes e.g. class A, AB and B, it is obvious that high efficiency classes are nonlinear. In subsection 2.2, average efficiency enhancement techniques aiming to keep the efficiency high over a wide dynamic range have been discussed. Even though efficiency is the main goal of such techniques, linearity was also taken into consideration so that none of such techniques would have severe impact to linearity. However, when the desired efficiency profile is achieved, linearity might not comply with wireless communication standards leading to unacceptable error vector magnitude and bit error rates. In such a case, linearity improvement techniques can be utilized to eliminate the excessive nonlinearity of the amplifier. Widespread linearization techniques are reviewed below.
