creases with input power, and is maximum at saturation. This means that it will have a low average efficiency for typical modulation schemes, which require the amplifier to dynamically operate over a wide range of input powers. The Doherty amplifier (DA) is a linear amplifier which, in theory, gives high efficiency over a wide range of input powers [1, 2]. It uses two amplifiers connected with quarter-wave transmission lines, as shown in Figure 1. At low power levels, only the main amplifier is on, and Z m is equal to twice the amplifier's design impedance. When the input voltage reaches a certain level, typically half of maximum input voltage, the peaking amplifier turns on, lowering Z m through active load modulation.In practice, the DA is not particularly linear, and while an efficiency improvement is achieved, it is not as high as that predicted by theory. The peaking amplifier requires a certain transfer characteristic, namely automatic turn-on, a linear slope, and identical maximum drain current as the main amplifier. There have been several approaches to realize this: class-C peaking amplifier with increased device periphery or input power compared with the main amplifier [3, 4], multiple transistors [5], or adaptive peaking amplifier bias [6]. However, the efficiency is still lower than theoretical expectations [1, 2], and this may be attributed to the less than perfect match between obtained and desired peaking amplifier transfer characteristics. It has been suggested that FET soft turn-on is the reason the peak is not present in a practical implementation [5]. This article investigates the effect of soft turn-on on the linearity and efficiency of the amplifier. THEORETICAL ANALYSISTo perform a theoretical analysis to gauge the effect of FET soft turn-on on its own, as well as to ensure tractability, a number of assumptions were made. It was assumed that the FETs had no triode-region, no charge-storage and no parasitics. In practice, the triode region and resistive parasitics will predominantly scale down the efficiency and the charge-storage and reactive parasitics will impact on matching circuit design. The FET drain current (I d ) was related to the gate voltage (V gs ) by the following drain current transfer characteristic,where x ϭ 1 for linear transfer, and x ϭ 2 for square-law transfer; I DSS is the saturation current of the transistor, and V T is the threshold voltage. These cases represent bounds of a range of different transfer characteristics with the linear case exemplifying abrupt turn-on, while the square-law case exemplifies an extreme case of soft turn-on. The analysis considered the case of the main amplifier biased in class B, and the peaking amplifier in class C. To counteract the low gain of the class C peaking amplifier [3][4][5], the peaking amplifier FET periphery was 2.55 and 5.3 times that of the main amplifier FET for the linear and square-law drain current transfer charac- teristics respectively, to give identical maximum fundamental output currents.The following values for the...
In a Doherty amplifier employing a class-B main amplifier and a class-C peaking amplifier, harmonics are normally suppressed at each amplifier. If odd harmonic currents generated by the peaking amplifier are permitted to flow into the main amplifier, the loadline of the main amplifier is shaped in such a manner as to allow a lower supply voltage to be used.Introduction: As the RF power amplifier draws a significant amount of current from the power supply, it is important to maximise its DC to RF conversion efficiency over a wide dynamic range. However, a conventional RF power amplifier only achieves maximum efficiency at maximum drive and this means low average efficiency when the envelope is modulated. The Doherty amplifier [1] alleviates this problem by using two transistors with one, the main amplifier, operating over the entire range of input powers, and the other, the peaking amplifier, only operating over the upper end (usually the top 6 dB) of the dynamic range. Over the upper end of the dynamic range, the load resistance seen by the main amplifier decreases with input level, causing it to operate at maximum efficiency in this range; this is called load modulation. Usually the main amplifier operates in class-B, and the peaking amplifier in class-C. It is often assumed that harmonic suppression occurs within these two amplifiers, and only the fundamental plays a role in the Doherty amplifier operation.In this Letter we show that odd harmonics generated in the peaking amplifier can be used by the main amplifier to further enhance efficiency. We call this harmonic modulation, and the main amplifier operates in a similar manner to class-F operation [2]; but this approach is distinct from class-F Doherty amplifiers that use separate class-F main and peaking amplifiers [3,4].
A Doherty amplifier is described in which odd harmonics from the class‐C peaking amplifier flow to the main amplifier to achieve class‐F operation, without the need for complicated space consuming harmonic control circuits. The measured efficiency exceeded 50% above the 6 dB back‐off. The measured second and third harmonic levels were below −19 dBc. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26035
This article introduces a gate bias control scheme for Doherty amplifiers, based on the measured gate‐voltage to drain‐current transfer characteristics of the FETs used, and their ideal fundamental output currents. An amplifier was realized at 800 MHz, and achieved over 35% drain efficiency from 8–14 dBm input power. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2152–2156, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24565
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