Design of analog predistortion power amplifier for W-CDMA

With the development of wireless communication technology, various linearization technologies for RF power amplifiers have been further studied and applied. In particular, the development of technologies such as narrowband CDMA and third-generation mobile communications has put forward higher requirements for the linearity of power amplifiers. In wireless communication systems such as W-CDMA, if a general high-power amplifier is used, the spectrum regeneration effect will occur due to the intermodulation distortion of the power amplifier, thereby interfering with adjacent channels and even generating bit errors. Therefore, the linearization technology of power amplifiers has received more and more attention. The three commonly used technologies are: feedforward technology, feedback technology, and pre-distortion technology.

Among these linearization technologies, the feedforward method can obtain very high linearity, but the structure is complex and expensive. The feedback method has its fatal defects, such as instability and limited bandwidth. In the pre-distortion technology, the pre-distortion system in the baseband signal needs to correctly compare the source signal and the feedback signal, has high requirements for loop delay compensation, and the system structure is relatively complex; while the analog pre-distortion technology has the advantages of simple structure, low cost, and good linearity, so it has become an ideal technology for linearization of small and medium power amplifiers.

Aiming at the linearity requirement of the downlink of W-CDMA repeater, a power amplifier with an average transmission power of 41 dBm is designed by using predistortion technology. This design uses a pair of anti-parallel diodes to generate nonlinear distortion components, and uses this nonlinear component to compensate for the nonlinear distortion of the power amplifier.

1 Nonlinearity of amplifier

As shown in Figure 1, the input and output relationship of an ideal linear amplifier can be expressed as V _out =K ₁ V _in , where K1 is the gain. However, as a semiconductor device, when the input signal of the power amplifier is large, the tube saturates, resulting in compression of the output signal, generation of high-order harmonics, and distortion. Therefore, the output should include high-order components such as square law terms and cubic terms, and its output can be expressed by power series formula (1):



It can be seen from formula (2) that due to the nonlinear characteristics of the system, in addition to the frequency of the input signal, there will also be new DC components and harmonic components such as 2ω _i , 3ω _i , etc. in the output signal. If the input signal is a dual-frequency signal Vin ₌ A1 _cos ( _ω1t ) + A2 _cos ( _ω2t ), the same analysis shows that the final output component is composed of tributary components, fundamental waves ω1 _and ω2 _, second and third harmonics 2ω1, 2ω2 and 3ω1, 3ω2, second-order intermodulation components ω1 ±ω2, third-order intermodulation components 2ω1 ±ω2 and ω1 ±2ω2. In general, only 2ω1-ω2 and 2ω2-ω1 fall within the passband. Dual _- tone _third - _order intermodulation _is generated _by the _cubic term _in nonlinearity _. Since _it falls _within the _band , _the nonlinear _product is _mainly considered. These nonlinear products will interfere with the carrier signal, causing nonlinear distortion such as intermodulation distortion and harmonic distortion. When multiple carriers are input, the third-order intermodulation distortion has a more serious impact.

2 Principle of analog predistortion

In principle, predistortion linearization technology is the simplest technology to improve linear characteristics. Its principle is shown in Figure 2, that is, a predistorter is added in front of the RF amplifier. The characteristics of the predistorter are precisely matched with those of the RF amplifier. When the signal passes through the cascade system composed of the predistorter and the RF amplifier. Due to the mutual compensation effect between the characteristics of the predistorter and the RF amplifier, the output signal is a completely distortion-free signal, thereby achieving the purpose of linearization. This compensation principle is shown in Figure 3. The curves of Figure 3 (a) and Figure 3 (b) compensate each other, and the output signal of Figure 3 (c) is completely distortion-free.



3 Implementation of analog predistortion

The downlink of the W-CDMA repeater requires a power output of 41 dBm, and both IM3 and IM5 are less than -50 dB. Therefore, the following circuit is designed to meet the above requirements. The entire design is shown in Figure 4.


As can be seen from Figure 4, the scheme adopts a dual-loop structure. The first loop uses a power divider to divide the input signal into two paths, one of which is delayed by a delayer; the other is distorted, attenuated, and phase-shifted by a predistortion generator, and then coupled with the delayed signal to form a loop of the predistortion signal generation part. The second loop attenuates the signal generated by the predistortion loop, and after phase shifting, couples it with the delayed signal input by the power amplifier to form the entire power amplifier part. This structure effectively reduces the influence of the required main signal on the loop, and the distortion part can be fully compensated. At the same time, the circuit is relatively easy to adjust in actual debugging, and is a circuit structure with good practicality.

Since the diode is a nonlinear semiconductor device, for an input cosine signal, the output of the diode is a cosine signal containing nonlinear distortion components. Therefore, the design of the nonlinear generator part is realized by anti-parallel diodes. The implementation circuit is shown in Figure 5.



In Figure 5, 1 and 4 are the input and output ends of the signal respectively; 2 and 3 are the coupling end and direct output end of the signal respectively. Two diodes D1 and D2 connected in anti-phase parallel are used to generate odd harmonic components, which are output through the isolation terminal 4 of the 90° orthogonal bridge as the pre-distortion signal. The ideal 3 dB orthogonal bridge coupling terminal 2 is connected to resistor R to eliminate the residual linear component in the diode relative reflection output signal, while capacitor C is used to reduce the carrier signal so that it is not too high compared with the pre-distortion signal. The orthogonal bridge divides

the input signal equally and transmits it to the coupling terminal and the direct output terminal in orthogonal manner. There is a 90° phase shift between the coupling terminal and the direct output terminal, and there is no output at the isolation terminal. Assume that the input of the bridge input terminal 1 is _Vin (t), then the relationship between the output of the direct output terminal 3 and the input of the two reverse parallel diodes is:



In formula (4): _IS is the reverse saturation current, which is mainly affected by temperature and can be regarded as a constant value in the circuit; VA _is the bias voltage applied to the diode, where VA ₌ V _{di (t). The additional term IR S} in formula (4) can be ignored under small bias current . The input current i(t) is:



Using the Talor series expansion formula (5), we can get:



Because the third-order component and the fifth-order component are the factors that have the greatest impact on the nonlinearity of the amplifier, for the convenience of analysis, only the fifth power term of the expansion is taken. At this time, the diode pair can be regarded as a single-port network, and its admittance is:



4 Experimental results

In order to verify the feasibility of the circuit, the entire circuit is tested by adding a signal. The nonlinear generator part uses Schottky diode HSMS2802 to generate a pre-distortion signal. The Schottky diode has good nonlinearity and can be used for mixing and power detection. When testing the third-order and fifth-order intermodulation components, the input frequency is a two-tone signal of 2.139 5 GHz and 2.140 5 GHz, with a spacing of 1 MHz. When testing ACPR, a W-CDMA downlink signal with a frequency of 2.14 GHz is used, and the results are shown in Figures 6 to 8.




From the output waveform of the spectrum analyzer, it can be seen that before adding the pre-distortion system, the third-order intermodulation component and the fifth-order intermodulation component of the driver stage output 41 dBm have indicators of -31 dB and -40 dB respectively. Correspondingly, when the pre-distortion generator is connected to the system, it can be seen that the output fifth-order intermodulation component and the fifth-order intermodulation component have been significantly improved, respectively, by 14 dB and 11 dB. At this time, the output ACPR of the output stage is less than -52 dB. The experimental results show that the pre-distortion system has greatly improved the nonlinear distortion of the power amplifier and well optimized its linearity.

5 Conclusion

The linearization technology of the power amplifier was studied by using the reverse parallel diode predistorter. The nonlinear characteristics of the diode were used to cancel the odd-order components of the amplifier distortion, and the IM3 and IM5 components of the power amplifier were improved by 13 dB and 10 dB respectively. The ACPR of the system reached below -52 dB after adding the predistorter. The experimental results verified the feasibility of the design and provided a practical basis for finding a better linearization technology. How to better improve the predistortion technology will be a topic for further research.