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Simulation and Design of Feedforward

Print View , by: iSee ,Total views: 12 ,Word Count: 1050 ,Date: Sat, 27 Jun 2009 Time: 5:23 PM

0 Introduction

With modern communication technology, rapid development, efficient spectrum modulation (QPSK or QAM) need to also enlarge the process to maintain a high linearity. However, almost all the amplifiers have a non-linear characteristics, so, what technology to eliminate such non-linear output signal of the adverse effects as a linear amplifier design a problem that needs to be examined. Currently, linear power amplifier of the main power back-off, negative feedback, predistortion, feedforward, active bias and non-linear amplifier linear components (LINC) technology. Among them, the power back-off technology can effectively improve the linearity of narrow-band signals, but its efficiency is not satisfactory; active bias LINC technology although technology and efficient, but its part of the device or the higher requirements, or the need for complex phase modulation signal and the two match well balanced amplification chain, have led to its use in the design of the need to invest more energy. The pre-distortion and feed-forward technology, because of its high accuracy and stability, taking into account a certain degree of efficiency, to improve the broadband signal has therefore become the main technology linearity. In particular, feed-forward technology, the premise that good design is often able to provide better circuit performance.

1 the basic principle and simulation

Feedforward technology from Bell Labs of HsBlack suggested that early because of their own characteristics, such as: open-loop characteristics of the circuit over time lead to transform the device can not be compensated; band in the whole circuit demanding the transfer of properties; feedforward realization of the need for auxiliary amplifier, designed to improve the cost and complexity of the issues and not be neglected. With the linear-depth technical studies, the inherent advantage of the many potential is gradually being recognized by everyone. Feed-forward the basic principles of technical diagram as shown in Figure 1, since the former feed-forward amplifier design, appropriate choice of coupling factor and the auxiliary amplifier drift characteristics of their own, such as the offset of the error signal has a great impact, because

Here, before the actual design should be simulated circuit design, and in accordance with simulation results to determine whether adjustments need to increase the adaptive circuit.

Simulation and Design of Feedforward

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Simulation of the design of a power output of about 80 W of LDMOS power transistor device as the basis of simulation, simulation focused on a certain power backoff based on the error amplifier gain drift of the entire impact of feed-forward circuit. Simulation results based on the expected settlement in inhibiting the target distortion, the error amplifier gain can be changed to bear. To simplify the simulation process, are neglected in the simulation of power devices, such as the issue of the insertion phase. Figure 2, Figure 3 as the basic circuit diagram of the simulation and the simulation results can be seen from the results in the appropriate coupling factor and gain, the feed-forward can be used to improve the results of cross-transfer inhibition.

Simulation and Design of Feedforward

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Simulation and Design of Feedforward

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Figure 4 thick solid line in the blue part to increase the spectrum after the auxiliary amplifier, the red thread as not part of the auxiliary amplifier output spectrum. As can be seen from the simulation results, the former fed about 50 dB improvement in cross-transfer, but in actual use, the transfer to pay such a huge improvement in the basic non-achievement of suppression, mainly due to its own error amplifier gain fluctuations, and the actual design the use of delay-line amplifier can not be completely equivalent insertion phase.

In the error amplifier circuit to add an adjustable attenuator to simulation error amplifier gain fluctuations, from Figure 4 the simulation results can be that the error amplifier in the volatility of ± 0.2 dB gain, the results of cross-transfer inhibition nearly variation 26 dB. Similarly, the main amplifier gain fluctuations, can also lead to error amplifier input signal to the consumer is not satisfactory, or even failure, thus affecting the improvement of cross-tune the results of inhibition. Therefore, in order to reduce the main amplifier and error amplifier design requirements, and to some extent to meet the current multi-carrier system design requirements, the circuit can be adjusted to increase adaptive circuit, the diagram can be constructed as shown in Figure 5 circuit.

Diagram in Figure 5 an increase of rf pre-distortion devices, can help to improve the loop performance, increase the efficiency of the entire loop. Further method is: in Figure 5, based on the input signal and output signal envelope coupling comparison.

Simulation and Design of Feedforward

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Envelope at the same level of cases, mainly as a result of nonlinear distortion in the amplitude envelope of the compression characteristics of the two envelope by controlling the difference between the smallest signal to adjust the error amplifier gain and phase characteristics.

2 Conclusion

Feedforward amplification of a computer simulation, in the simplified model, the simulation to determine the appropriate coupling factor and the error amplifier gain and downs of the request. For the main, auxiliary gain amplifier that may exist, such as phase shift caused by failure of the error signal offset and so on, given the actual design can be used for feed-forward block diagram of the multi-carrier design circumstances in actual practice, with some help.


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