Different Classes Implementation of Power Amplifier

Generally speaking, PAs can be separated to two different categories;

one is in its amplifying mode and the other is acting as a switch. The amplifying mode device normally acts as a current source and works as linear class of power amplifier. The switching mode category is referred to as the nonlinear power amplifier. And sometimes, we can use the dc bias condition to classify the linear or nonlinear mode of PAs. In general, RF power amplifiers can be specified as class A, B, C, D, E and F that different kinds of power amplifier classes have different transistors conduct cycle. [7]

The generation of significant power for power amplifiers depends on the standards of wireless communication. Each application has its own requirements for operating frequency, bandwidth, power, efficiency, linearity and cost. Besides, power amplifier is not only a simple linear amplifier which operate in small signal, on the other hand, power amplifier operate in large signal and has different properties comparing to traditional operating amplifier.

The phrase “linear” is just a concept to describe how output signal close to the input signal, as stated earlier, this just identifies the group of PAs in which the device is intended to operate in its amplifying region.

For amplification of amplitude-modulated signals, the quiescent current can be varied in proportion to the instantaneous signal envelope. Since the devices are meant to operate in their amplifying region, it should be apparent that there exist some relationship between the magnitude of

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input and output, regardless how linear that is. And the relationship between linearity and efficiency are mutual trade off [8] [9]. The linearity is direct proportional to conduct angle which means that the longer the conduction cycle the better linearity a PA has. However, the average power is consumed even when no signal is applied that will lead to lower efficiency.

Figure 2-1 Basic Class-A implementation

The above description describes the group of PAs known as Class-A PAs and the device conducts current for entire input sinusoid cycle. The amplifying device is biased in such a way that it always remains in its amplification region, even under maximum input signal conditions. The bias voltage is set to keep the device still operate in saturation region even when the swing of input signal reaches the maximum the gate

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voltage should over threshold voltage. The output voltage swings around its bias point; in general is the supply voltage, VDD. On the ideal situation, the maximum amplitude of the output swing is just V_DD which can help us determine the peak efficiency of the Class-A configuration. If we consider that

I

And the peak efficiency is given by

As we can see that the peak efficiency of class-A PA is 50%. As a result, class-A PAs are used in applications which require low power, high linearity or high gain.

If the bias condition is going to change and make PAs not always operate in saturation region which introduce the idea of class-B PAs.

Class-B PAs sometimes also called push-pull output stage. In standard implementations, two amplification devices are used and used differential input to maintain the original waveform. Each of the devices has only half sinusoidal period that is to say the conducting angel is only half of the class-A PAs. The efficiency of this implementation is greater than that of the class-A implementation and the theoretical peak efficiency of the class-B PAs is given by

78%

4

η = π =

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Figure 2-2 Basic Class-B implementation

Although the efficiency will enhance by using class-B implementation the linearity will degrade at the same time. And sometime if the gain through the devices is not exactly the same, the output will not be smooth sinusoid. Therefore, the issue of mismatch between two devices occurs no matter the parameter of threshold voltage or mobility difference. Crossover distortion is the existences of dead-zone during the devices turn on by turns leading to output signal distortion which also decreasing the linearity.

In order to gain better linearity we should obtain the balance between class-A and class-B configurations and this is the reason of class-AB generation. The power amplifier is now based such that it is on for more than half the cycle. In this case, the problem of dead-zone is avoided because there is a portion when both devices in a push-pull implementation are on [6] [7]. In narrowband RF implementations, class-B and class-AB PAs can also be implemented using a single device adding an RF filter at the output to extract the fundamental frequency

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component of the output waveform. Normally, a single device would induce a problem that the device would be off for part of the input cycle and generate a chopped and thus become extreme distortion of output waveform while the input power is large. However, through the use of narrowband RF filters, the component of the output waveform at the fundamental frequency can be extracted and the amount of distortion can be reduced.

The group of “Nonlinear” PAs is also known by a more descriptive name: waveform shaping-mode or switch mode PAs. For RF PAs, the two classes of switched mode PAs which have received the most attention are class-D and class-E PAs.

Figure 2-3: Class-D implementation

The class-D architecture is similar to what is used in a bridge DC-DC converter [10]. In the style of DC-DC converter, the devices acting as switches change the polarity of the input voltage onto the load

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and the resulting output is averaged to create an output voltage that is some fraction of the input voltage, depending on the duty cycle of the switching. If the implementation of the switch is assumed to be ideal, then no on-resistance and the output voltage will be zero exactly when the switch is closed the ideal maximum efficiency of the power stage can be 100%, as no power will be consumed in the transistor.

The class-E PA, is also used the idea of soft switching in order to further reduce power consumption by the device in the switched-mode PA. This class of power amplifier has also been recently implemented in a CMOS implementation [11] [7] [10]. Basically, the class-E power amplifier tries to force the voltage on the output node becoming zero voltage at the instant that the switch is closed, so there is ideally no time at the transition when both the output voltage and current are non-zero.

Not only is that but also there is no CV² energy loss from the output capacitance discharging as the switch is turned on. In order to account for timing errors in the switching instants, the slope of the output voltage waveform should also be zero at the instant of that the switch closes.

Because of any timing error when the switch closes, the power consumed attribute to any overlap in the output current and voltage waveforms will be minimal; since the slope of the output voltage at the correct instant is zero, the value of the output voltage at instants close to the output voltage will be very small to improve the drain efficiency. So, an ideal class-E power amplifier consists of a single supply voltage Vdd, an RF choke inductor Ldc, a switch with a parallel capacitor Cp, a resonant circuit Lo-Co, and a load RL, or intrinsic impedance 50-Ω.

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Figure 2-4: Class-E implementation

And like the class-D PA, the theoretical efficiency of the class-E power amplifier is 100%, again, practical considerations, especially in CMOS limitations, have limited the efficiency to about 50%[11], although GaAs implementation have reached close to 60%

efficiency[10].

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