Chapter 2 Basic Concepts of Radio Frequency Power Amplifier
2.6 Classification of Power Amplifiers
An RF PA designer always has to negotiate a trade-off between power gain, linearity and efficiency. However, the importance of each criterion is different for various
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applications. For example: linearity is important for a base station, whereas efficiency is vital for handsets on battery operated amplifiers used in broadcasting. These specific performances can be achieved by choosing appropriate class of operation and architecture of RF PA. There are primarily two groups of PAs: linear PAs and switch- mode PAs.
In the typical linear PA, the transistor works as a current source controlled by the input RF signal at the gate. Linear PAs are usually suitable in applications where linearity or bandwidth is important; however, these PAs have low efficiency. In switch-mode PAs, the transistor operates as a switch. In an ideal switch-mode amplifier, the transistor is either open to allow current flowing through the device but with no voltage across it, or closed, with voltage across the device but allowing no current flowing through it. This allows the PAs to achieve 100% efficiency. However, this high efficiency is achieved at the expense of decreased linearity and bandwidth.
2.6.1 Linear Power Amplifiers
2.6.1.1 Class-A Amplifier
It can be observed in Figure 2.18 that the device is biased in the exact middle of its active region where IDSq = IMAX/2 and VGSq = (VGS,MAX - Vth)/2; and at all times of the input cycle, it operates in the active region. This biasing condition creates a Class-A amplifier.
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In [2.30], the conduction angle is defined as “the angle measured in degrees or radians over one period for which the device remains conducting”. For a class-A amplifier it is equal to 360 degree or 2π radians. The RF voltage waveform or the maximum value of the voltage swing across Rload is (VDS,q – Vknee) and the current waveform has a maximum amplitude of IMAX/2. For a class-A amplifier, IDS,total can reach to a maximum value of IMAX.
The DC power consumption is given by equation 2.22:
DSq DSq DC I V
P (2.22)
And the RF power can be calculated using:
load load
/2RF I V
P (2.23)
Therefore, the maximum RF output power, where the current and voltage swings have the maximum values, is:
2
2
2
/
, knee DSq DSq knee DSq MAX MAX RFV
V
I
V
V
I
P
(2.24)The maximum drain efficiency of a Class-A amplifier is given by:
DSq knee DSq DC MAX RF MAX V V V P P 2 , (2.25)
The maximum efficiency of a Class-A amplifier can be 50%, if only given that Vknee = 0 V. The calculated Vknee for the GaN HEMT devices studied in this thesis is the range of 3 V to 4.7 V. As the Vknee is more than 0 V, the efficiency is reduced even further in practice. Since the device is on for the whole time, the output of Class-A amplifier can totally resemble the input signal, only amplified and thus this amplifier shows very good linearity. However, even the Class-A amplifiers display some weak nonlinearity such as intermodulation distortion even at lower levels [2.28].
2.6.2 Class-B Amplifier
In a Class-B amplifier, the transistor is biased at the threshold voltage. If a sinusoidal RF input signal with a peak value of Vs is applied to the transistor biased at Vth, then the input voltage can be written as:
t VV
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The output current at the drain which is given by:
in th
m DSq
out I g V V
I (2.27)
Now when Vin < Vth, there is no current as the device does not conduct any longer. The Class-B amplifier has a conduction angle of 180 degrees or π radians. Since the device conducts for 180 degrees compared to 360 degrees in Class-A amplifiers, the DC power consumed is reduced and as a result, efficiency in a Class-B amplifier is higher than that of Class-A amplifier. As the device conducts for half cycle of the input signal, the output current resembles a half-wave rectified sinusoid signal. The drain voltage swings from VDS,q down to VKnee and up to 2(VDS,q-VKnee), so the zero-to-peak amplitude is (VDS,q - VKnee). The maximum value of current across the load (Iload) is IMAX. The DC component of the drain current in Class-B amplifier is given by IMAX/π. The maximum efficiency of a Class-B amplifier [2.30] is given by:
4
DSq knee DSq MAX load MAX V V V I I (2.30)When Vknee = 0 V and Iload = IMAX, the maximum value of drain efficiency of a Class- B amplifier is close to or 78%.
Figure 2.19: Transfer characteristic and forward transconductance of an ideal amplifier. Bias conditions for class-A, B, AB and C amplifiers are also shown.
Although Class-B amplifier is clearly more efficient then Class-A amplifier, it shows poor linearity. A Class-B amplifier could be linear if the transconductance (gm) was constant throughout the conduction angle as shown in Figure 2.19. However, unlike
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the ideal case, in real devices the forward transconductance (gm) gradually starts to decrease and falls to zero around the threshold and saturation voltages and therefore if the transistor is biased at its threshold voltage, the output suffers from distortion. Therefore, in practice the device is biased slightly above the threshold voltage for a Class-B amplifier.
2.6.3 Class-AB Amplifier
A Class-A amplifier is used when linearity is the main priority. However, it suffers from very low efficiency of maximum of 25% to 30%. On the other hand, in applications where higher efficiency is important, Class-B amplifier is suitable, but it suffers from poor non-linearity due to its bias-point near threshold voltage. Therefore, in an application where both linearity and efficiency are crucial, a Class-AB amplifier can be employed. A Class-AB amplifier is biased somewhere in between the bias points of Class-A and B operations. When the input voltage swings below the threshold voltage, the device does not conduct for that portion of the cycle. It overcomes the gain loss at low gate voltages around threshold voltage in Class-B amplifier and at the same time, since the quiescent current is lower, it has better efficiency than a Class-A amplifier. Depending on the bias point, the conduction angle of a Class-AB amplifier has a value in between 180 and 360 degrees and it has efficiency between 50% and 78%.
Figure 2.20: Output waveforms and conduction angles of Class-A, AB, B and C amplifiers.
2.6.4 Class-C Amplifier
The transistor is biased below its threshold voltage in a Class-C amplifier, and as a result, the device is active for less than half of the input RF signal and the conduction angle is between 180 and 0 degrees. The Class-C PA achieves high efficiency by
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compromising linearity. As it can be seen in Figure 2.20, the efficiency can be increased further by reducing the conduction angle. However, this results in a decline of output power toward zero [2.28]. The common trade-off is an ideal efficiency of 85% at a conduction angle of 150 degrees.