RF power amplifier design for adaptive feedforward

The rapid development of modern wireless communication increasingly towards increasing the information capacity, improve the spectral efficiency and channel the direction of improving linearity. On the one hand, people working in the B class of widely used state of the power transistors to increase the transmission of microwave power and efficiency; on the other hand, passive devices and active devices in the introduction of multi-carrier configuration technology adoption, etc., will result in output intermodulation distortion signal. Therefore, in the design of RF power amplifier and must be linearized so that the output signal to obtain better linearity. Commonly used linearization techniques include: power back to back, predistortion, feedforward, etc., of which the power regression technique can effectively improve the linearity of narrowband signals, while the pre-distortion technology and feedforward technology, in particular feed-forward technology, because of its higher accuracy, high stability and bandwidth constraints from the advantages of a broadband signal to improve the linearity of the main technologies used in the time. This paper outlines the common Feedforward techniques, and then to improve on this basis, add the adaptive algorithm, and through the signal envelope detection to extract the signal band to adjust, so as to achieve improved linearity of output signal purposes.

2. The basic principles of feedforward

The most basic principle of feed-forward amplifier shown in Figure 1. He is represented by two loop components: loop 1 by a power divider, the main amplifier, coupler 1 and attenuator 1, a phase shifter, delay line 1, the composition of synthesizer 1. RF input signal, that is, two pure carrier signal, by the power splitter into two signals after: the branch of the main power amplifier slip road, pure RF carrier signal generated through the amplification of the slip after the carrier after signal and intermodulation distortion signal; under the branch road for the attached slip, pure RF carrier signal through the delay after the slip, slip out the main power amplifier nonlinear distortion signal by attenuator and phase shifter 1 1 After the slip and attached the output of the signal in the synthesizer 1 synthesis, adjusting attenuator and phase shifter 1 1 so that two signals were of equal amplitude, 180 'phase difference and equal delay. At this time, we can effectively offset the main branch of the RF carrier signal power amplifier, and extract the non-linear amplification as the main amplifier intermodulation distortion generated by the signal. Therefore, the First Ring Road, also known as the RF carrier signal to eliminate loop.

Circle 2, also known as the signal to eliminate distortion loop, the delay line 2, auxiliary amplifier, attenuator 2, phase shifter 2, coupler 2 composition. Similarly, there are two branches of slip: the branch road to the main amplifier output of the nonlinear distortion signal delay given away after the coupler 2; under the distributor road will loop an extracted intermodulation distortion signal amplification, attenuation, phase given away after shift coupler 2, adjust attenuator and phase shifter 2, 2, 2 until the coupler output signal minimum intermodulation distortion signal, IMD is the smallest, then the signal is then amplified output of RF signal.

3. Adaptive feedforward RF power amplifier

3.1 The principle of adaptive feedforward circuit and algorithm

As the feedforward system requirements on the carrier signal offset the high changes in the external environment, such as: the input signal power, DC bias voltage and temperature changes, are likely to cause failure of the carrier signal offset. Therefore, the introduction of adaptive technology to enable timely access to the carrier signal in amplitude, phase and delay the match, it becomes very necessary. Adaptive feedforward system structure shown in Figure 2.

RF power amplifier design for adaptive feedforward

He posed by the three loop: Loop 1 is mainly used for extraction of intermodulation distortion signal, loop 2 is mainly used to eliminate distortion of the signal, while the loop 3 is mainly used for detecting intermodulation distortion signal power.

An input signal for the υin (t), by the main amplifier output signal after the υρα (t), will υρα (t) coupled to the vector modulator part 1, with complex coefficients α 1, on behalf of the modulation vector modulator coefficients, the same time, the main amplifier is reduced to a nonlinear model without memory, its AM / AM and AM / PM transfer function can simply use the complex voltage gain G (χ) to express, on behalf of which χ instantaneous power, then synthesized from the vector 1 output signal υα (t) can be expressed as:

In the specific implementation structure, in the back of another synthesizer adds a power splitter 2, the purpose is to signal υd (t, g, ψ) for power detection, it is clear that if the adjustment makes the synthesizer α 1 2 input signal magnitude, phase and delay matching are achieved, then there will be only the power to detect intermodulation distortion signal υe (t) the average power corpse + and he is very small, in other words, if the detected output of power splitter 2 power sufficiently small, then the regulation of α at this time reached the optimum, the RF carrier signal has been the greatest eliminated, and retained only intermodulation distortion signal υe (t).

Into the loop 2 of the intermodulation distortion signal through the auxiliary amplifier, modulator 2 (the modulation factor for the complex coefficient β), after adjustment, and through the delay line 2 of the main amplifier output signal in the synthesizer 2 synthesis. The Circle of the intermodulation distortion signal amplitude and phase adjustment also use adaptive technology, mathematical principles as mentioned above, but in the realization of the structure, but with a different loop, loop 1 is detected by direct synthesis 1 RF output signal to determine whether the carrier signal is offset to the minimum, while loop 2 in the determination of intermodulation distortion signal was offset to a minimum, it is necessary to introduce a third loop.

We know that the same power output signal, linear envelope signal is greater than non-linear signal envelope, while the difference between the two envelope signal is intermodulation distortion signal, reducing the maximum difference between the envelope value of the signal, we can maximize the output signal to improve linearity, thus reducing the IMD. Circle 3, the working principle is this. He dealt with the way the two signals is linear signal, through the delay lines 3 and 4 of the RF splitter carrier signal, the other way is non-linear signal that the system by the feed-forward loop a and loop 2 after the synthesis 2 output signal. First, the loop on the synthesis of the two signals, signal power detection, and adjust the vector modulator 3, until the detected power of the minimum, then, can be considered linear and nonlinear signal the carrier signal with the same output power . Then, again on the two signals separately envelope detection, extraction of the difference signal envelope, this envelope difference signal acting on the vector modulator 2, which is constantly adjusting to small end-of-band intermodulation distortion signals to minimize Then it will be highly linear output signal.

3.2 Computer Simulation

Apply a peak power of 180W of LDMOS field effect transistor designed on a computer simulation system, a 25 W power amplifier, and two of its input frequency interval of 1MHz carrier signal, to generate third-order and fifth-order intermodulation distortion signal. Figure 3 is not used in the adaptive feedforward technology situation when the output signal. -55dBc IMD3 at this time can only reach around, IMD5 only achieve-56dBc, while the Figure 4 is the use of the output signal of the techniques the situation. Up to -72 dBc IMD3 at this time around, IMD5 up to-76dBc, and as its improvement is obvious.

RF power amplifier design for adaptive feedforward

4. End

In this paper, adaptive feedforward technology and to co-envelope detection technology to design RF power amplifiers. As the technology may take into account the practical problems, thus to simplify complex issues, not only in theory but from practice, confirmed his easy to realize.

Computer simulation results show that: This is indeed adaptive feedforward technique can effectively improve the power amplifier nonlinear distortion. Of course, the technology has yet to be further strengthened.

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