How to linearize the RF power amplifier

The nonlinear distortion of the RF power amplifier will cause it to generate new frequency components. For example, for the second-order distortion, it will produce the second harmonic and two-tone beat frequency. For the third-order distortion, it will produce the third harmonic and multi-tone beat frequency. 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 other channels' signals. For this reason, it is necessary to linearize the RF power amplifier, which can better solve the problem of signal spectrum regeneration.

The nonlinear distortion of the RF power amplifier will cause it to generate new frequency components. For example, for the second-order distortion, it will produce the second harmonic and two-tone beat frequency. For the third-order distortion, it will produce the third harmonic and multi-tone beat frequency. 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 other channels' signals. For this reason, it is necessary to linearize the RF power amplifier, which can better solve the problem of signal spectrum regeneration. The principle and method of the basic linearization technology of the RF power amplifier is nothing more than taking the amplitude and phase of the input RF signal envelope as a reference, comparing it with the output signal, and then producing appropriate corrections. There are three commonly used techniques to achieve linearization of RF power amplifiers: power back-off, predistortion, and feedforward.

Power fallback

This is the most commonly used method, which is to use a larger power tube as a small power tube, in fact, at the expense of DC power consumption to improve the linearity of the power amplifier.

The power fallback method is to reduce the input power of the power amplifier from the 1dB compression point (the amplifier has a linear dynamic range, within this range, the output power of the amplifier increases linearly with the input power. As the input power continues to increase, the amplifier gradually enters In the saturation region, the power gain begins to decrease. Usually the output power value when the gain drops to 1dB lower than the linear gain is defined as the 1dB compression point of the output power, expressed by P1dB.) Backward 6-10 decibels, working at far At a level less than the 1dB compression point, the power amplifier is moved away from the saturation zone and enters the linear operating region, thereby improving the third-order intermodulation coefficient of the power amplifier. In general, when the fundamental wave power is reduced by 1dB, the third-order intermodulation distortion is improved by 2dB.

The power back-off method is simple and easy to implement, and does not require any additional equipment. It is an effective method to improve the linearity of the amplifier. The disadvantage is that the efficiency is greatly reduced. In addition, when the power falls back to a certain extent, when the third-order intermodulation reaches below -50dBc, continuing to fall back will no longer improve the linearity of the amplifier. Therefore, it is not enough to rely solely on power back-off in applications with high linearity requirements.

Predistortion

Predistortion is to add a nonlinear circuit before the power amplifier to compensate for the nonlinear distortion of the power amplifier.

The advantages of predistortion linearization technology are that there are no stability problems, a wider signal frequency band, and the ability to process signals with multiple carriers. The cost of predistortion technology is low. Several carefully selected components are packaged into a single module and connected between the signal source and the power amplifier to form a predistortion linear power amplifier. The power amplifier in the handheld mobile station has adopted predistortion technology, which reduces the intermodulation products by a few dB with only a few components, but it is a critical few dB.

The predistortion technology is divided into two basic types: RF predistortion and digital baseband predistortion. RF predistortion is generally implemented by analog circuits, which have the advantages of simple circuit structure, low cost, easy high-frequency, broadband applications, etc. The disadvantage is that the spectrum regeneration component is less improved and the high-order spectrum components are more difficult to cancel.

Digital baseband predistortion can be realized by digital circuits due to its low operating frequency, and it has strong adaptability. It can also offset high-order intermodulation distortion by increasing the sampling frequency and increasing the quantization order. It is a promising method . This predistorter is composed of a vector gain adjuster, which controls the amplitude and phase of the input signal according to the content of the look-up table (LUT), and the magnitude of the predistortion is controlled by the input of the look-up table. Once the vector gain regulator is optimized, it will provide a non-linear characteristic opposite to that of a power amplifier. Ideally, the output intermodulation product at this time should be equal to the output amplitude of the dual-tone signal through the power amplifier but opposite in phase, that is, the adaptive adjustment module is to adjust the input of the lookup table to minimize the difference between the input signal and the output signal of the power amplifier. . Note that the envelope of the input signal is also an input of the look-up table, and the feedback path samples the distortion output of the power amplifier, and then sends it to the adaptive adjustment DSP through A/D conversion to update the look-up table.

Feedforward

Feedforward technology originated from "feedback", it should be said that it is not a new technology, as early as the 1920s and 1930s by the American Bell Labs. Except that calibration (feedback) is added to the output, it is completely "feedback" in concept.

The feedforward linear amplifier forms two loops through a coupler, attenuator, synthesizer, delay line, power divider, etc. After the RF signal is input, it is divided into two channels by the power divider. All the way into the main power amplifier, due to its nonlinear distortion, in addition to the main frequency signal that needs to be amplified, there is also third-order intermodulation interference at the output. A part of the signal is coupled from the output of the main power amplifier, and the main carrier frequency signal of the amplifier is cancelled by loop 1, leaving only the inverted third-order intermodulation component. After the third-order intermodulation component is amplified by the auxiliary amplifier, it cancels the intermodulation component generated by the nonlinearity of the main amplifier through loop 2, thereby improving the linearity of the power amplifier.

Feedforward technology not only provides the advantages of higher calibration accuracy, but also does not have the disadvantages of instability and limited bandwidth. Of course, these advantages are exchanged for high cost. Since the output calibration requires a higher power level, the calibration signal needs to be amplified to a higher power level. This requires an additional auxiliary amplifier and requires the auxiliary amplifier itself. The distortion characteristics should be above the index of the feedforward system.

The cancellation requirements of the feedforward power amplifier are very high, and the matching of amplitude, phase and time delay needs to be obtained. If power changes, temperature changes and device aging, etc. occur, the cancellation will disappear. For this reason, consider adaptive cancellation technology in the system, so that the cancellation can keep up with changes in the internal and external environments.