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The nonlinear distortion of the RF power amplifier will cause it to generate new frequency components, such as the second harmonic and the two-tone beat frequency for the second-order distortion, and the third harmonic and the multi-tone beat frequency for the third-order distortion. If these new frequency components fall within the passband, they will cause direct interference to the transmitted signal, and if they fall outside the passband, they will interfere with the signals of other channels. To this end, the RF power amplifier should be linearized, which can better solve the problem of signal spectrum regeneration.
There are five basic methods to improve the linearity of RF power amplifiers: power backoff, predistortion, negative feedback, feedforward and envelope cancellation and recovery (EER).
Fig 1. Higher the Power of the RF Input Signal
1 Power Backoff
Since many nonlinear actions occur in the nonlinear region of the power amplifier, the power back-off technology that allows the RF power amplifier to work far from the nonlinear region is the most direct method in practical applications, that is, using a tube with a large output power capacity to output low power. Generally speaking, when the input power of the power amplifier is reduced by 1dB, the cross-modulation coefficient CM is improved by 2dB. This method needs to make the power amplifier work in a state far from the saturation point, so as to consume more DC power to improve the linearity of the power amplifier.
The power back-off method is simple in design and easy to implement, as long as the static DC operating point and the matching circuit of the power amplifier are well designed, it is an effective method to improve the linearity of the amplifier. However, in one hand, since the operating point is far from the saturation point, the efficiency is relatively low; on the other hand, when the output power falls back to a certain extent, such as when the third-order intermodulation value is less than -45dBc, it is difficult to further improve the linearity of the amplifier by continuing to fall back. For wideband signals, the backoff technique has limited effectiveness due to memory effects. In general, it is suitable for occasions where the linearity requirement is not high and the output power is small.
Predistortion is the preprocessing of the signal with the inverse amplitude and phase of the amplifier before it enters the amplifier. The implementation method is to add a digital or analog nonlinear circuit as a predistorter before the power amplifier to compensate the nonlinear distortion of the RF power amplifier.
The advantage of predistortion is that it does not require power back-off, and the amplifier can work in the vicinity of the ldB compression point or even in the saturation region, so higher efficiency can be obtained. Predistortion is the fastest and currently mainstream linearization method found today. However, the distortion caused by some factors in the predistortion loop cannot play a compensation role, such as the nonlinear fluctuation of the device caused by the change of temperature, DC voltage and the aging of the device.
3 Negative Feedback
Negative feedback is that the nonlinear distortion signal output by the amplifier is reversed and fed back to the input terminal through the feedback network. It is synthesized with the signal at the input terminal as the input signal of the power amplifier, thereby reducing the nonlinearity of the amplifier. The negative feedback signal and the input signal of the amplifier itself jointly control the input of the amplifier, thereby increasing the stability, gain flatness and linearity of the output signal of the amplifier.
The main implementation methods are polarization ring and Cartesian ring. The linearization function of the polarization loop is completed by its modulation and demodulation circuit through phase comparison and peak detection. Another way, the Cartesian loop demodulates the output signal and compares it with the input signal to generate a predistorted signal that is remodulated and amplified by a saturated amplifier. But they are only suitable for narrowband, and because it is a closed-loop, there is instability.
Feedforward is the fastest growing and most advanced method in power amplifier linearization technology. It originated from feedback, the difference is that the output signal is processed and then coupled forward to the output of the power amplifier. After the RF signal enters the feedforward network, the nonlinear distortion entering the power amplifier part will output the required main signal and the third-order intermodulation interference signal, which is extracted with the opposite phase that output to the loop of the main amplifier. Where non-linear components on the main amplifier branch can be cancelled, improving the linearity of the power amplifier.
Feedforward technology has the advantages of higher calibration accuracy and stable signal. However, the cancellation requirements of the feedforward power amplifier are very high, and it requires complete matching of amplitude, phase and delay. Therefore, factors such as operating temperature changes, power changes, and device ageing will cause cancellation failures.
5 Envelope Elimination and Restoration (EER)
In EER technology, the amplitude and phase of the RF input signal are separated, and the phase signal passes through a nonlinear power amplifier. This type of amplifier works in the switching state, so theoretically it will have 100% efficiency. Likewise, the amplitude signal can be separated from the RF input signal before being amplified. In the process of signal amplification, the envelope signal can be restored to the carrier signal. The amplitude signal and the phase signal should be as consistent as possible in terms of time requirements. Therefore, a delay line is added to the phase signal branch to meet the above requirements according to the length of the control line. Of course, EER technology also has shortcomings. When the envelope is restored to the carrier signal, it is achieved by adjusting the bias voltage of the RF power amplifier. In fact, when the drain voltage is adjusted to correct the amplitude of the output signal of the amplifier, the phase itself is also changing. This will extend the spectrum of the useful signal, thereby weakening the linearity of the RF power amplifier. In addition, the dynamic range of the envelope recovery feedback loop is relatively small.
Fig 2. Classic RF Power Amplifier Load Curve
Among the various linearization techniques discussed above, the feedforward linearization technique has wider bandwidth and better linearity improvement, but is not very efficient. Negative feedback technology has an ideal suppression effect on distortion, and can also control the input and output impedance of the power amplifier and reduce the influence of noise, reducing the sensitivity of the power amplifier components to temperature, but the negative feedback technology is required to improve the linearity of the system. The gain of the power amplifier is sacrificed to achieve the distortion of the compressed signal. Negative feedback technology has very limited bandwidth and is not suitable for broadband amplifier circuits. If the phase control is not good, it is easy to generate positive feedback and cause system instability.
The predistortion has the advantages of unconditional stability, low cost and moderate bandwidth. The analog predistortion circuit not only has a simple system structure and low cost, but also has better linearity improvement and moderate bandwidth. But the working bandwidth of analog predistortion is limited by the phase flatness, gain of the analog predistorter and the power amplifier itself.
Through the summary of RF power amplifier linearization technology, we can roughly see some future trends. RF power amplifiers will develop in the direction of low power consumption and high linearity. With the continuous adoption of various linearization modulation techniques, the nonlinearity of RF power amplifiers will become more and more prominent.