The effect of FET soft turn‐on on a Doherty amplifier

Abstract

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...

A novel Doherty power amplifier with self‐adaptive biasing network for efficiency improvement

“…Soft turn-on effect, which opposes to the second assumption above, is not easily solved and its influence is analyzed in Ref. 6. Although adaptive biasing schemes can help mitigate this detrimental effect, classical methods are complicated, bulky in size and power-wasting [7,8].…”

“…[4] has reported the circular disc monopole antenna with coplanar waveguide (CPW) feed. Recently, various UWB fractal antennas have also been reported for UWB applications [5][6][7][8]. In Ref.…”

“…5, crown square microstrip antenna is proposed to reduce the size. The frequency notched UWB microstrip slot antenna with a fractal tuning stub is proposed to achieve frequency notched function [6,7]. Ding et al [8] and Kumar and Sawant [9] have proposed a new UWB fractal antenna by adopting the fractal concept on the CPWfeed.…”

Optimum design of highly linear and efficient GaN HEMT Doherty amplifier considering the soft turn‐on effects

“…Therefore, power amplifiers (PAs) in repeaters and base stations should be operated at a large back-off output power (BOP) from the saturation power (P sat ) to achieve proper linearity, but this result in poor efficiency. Therefore, Doherty power amplifiers (DPAs) have been widely investigated and developed as the prime candidate for high efficiency at a large BOP since the conventional DPA shows the efficiency peaking at a 6-dB BOP [1][2][3][4][5][6][7][8]. However, in the practical implementation, it is difficult that the DPA shows the efficiency peaking at a 6-dB BOP due to the soft turn-on characteristic of the transistor [8].…”

“…Therefore, Doherty power amplifiers (DPAs) have been widely investigated and developed as the prime candidate for high efficiency at a large BOP since the conventional DPA shows the efficiency peaking at a 6-dB BOP [1][2][3][4][5][6][7][8]. However, in the practical implementation, it is difficult that the DPA shows the efficiency peaking at a 6-dB BOP due to the soft turn-on characteristic of the transistor [8]. To compensate for this problem, various techniques such as uneven power drive, N-way structure, multistage structure, and so on have been suggested [4 -7].…”