DESIGN OF AN ULTRALINEAR WIDEBAND FEEDFORWARD AMPLIFIER USING EDA TOOLS.
In many digital communication systems, linear amplification is required for linear modulations such as quadrature phase-sift keying (QPSK) and quadrature amplitude modulation (QAM). In these cases, nonlinear amplification yields intermodulation distortion (IMD) products and produces unacceptable levels of spectral regrowth in adjacent channels. The third-order IMD (IMD3) is usually the most significant nonlinear distortion and, as a result, is the most important specification in power amplifiers. Generally, the intermodulation products in base station power amplifiers with multiple carriers must be less than -60 dBc. However, most of the high power amplifiers cannot satisfy such a high linearity requirement without backing off the output power sufficiently or adopting linearization techniques.
Output power back off is a simple and inefficient method to achieve high linearity. For a power amplifier in the class A configuration, if the output power is backed off by 1 dB, the IMD3 products drop by 2 dB. Usually, the IMD3 of a power amplifier (class A) is approximately -20 dBc at the power where the amplifier's gain compresses by 1 dB (P1dB). Therefore, the output power is required to be backed off by at least 20 dB to achieve the -60 dBc IMD3 level. According to this relationship, a 10 W high linearity power amplifier requires a power amplifier of 1000 W P1dB. Moreover, it is known that the bias current of an amplifier in the class A configuration maintains the same value even though the output power changes. On the other hand, class AB power amplifiers have better efficiency and the bias current drops together with the output power back off. However, improvement of the IMD performance of a class AB power amplifier by a power back off method is limited. Usually, the level of the IMD3 products of a class AB power amplifier is -35 dBc when the output power is backed off by 3 to 5 dB and the IMD performance does not increase with further back off of the output power. To improve the efficiency and lower the cost of such linear amplifiers, linearization techniques are usually employed.
Various techniques have been developed to reduce the IMD products in high power amplifiers. Feedforward, feedback and predistortion are three commonly used linearization methods. An adaptive technique is usually employed to improve the linearization performance so as to extend the operating temperature range and widen the frequency band. Class AB operation is usually selected for the final stage of the power amplifier. Recently, high power laterally diffused metal oxide semiconductor (LDMOS) amplifiers with much better IMD performance have attracted much attention.
Feedforward linearization is one of the commonly used approaches and provides significant improvement in the linearization of power amplifiers. A feedforward amplifier achieves its linearity improvement by canceling the intermodulation products produced by the main amplifier. A signal composed of only the distortion produced by the main amplifier is generated by the carrier cancellation loop. The error signal is adjusted to be equal in amplitude but 180[degrees] out of phase with the IMD products of the main amplifier. By adding the error signal to the signal of the main amplifier, IMD cancellation is achieved. The operation of the feedforward amplifier is shown in Figure 1.
A feedforward amplifier consists of two loops. The first loop (Loop I) is a carrier cancellation loop, which is used to cancel the carrier and obtain the IMD products of the main amplifier (denoted as the error signal). The second loop (Loop II) is the IMD cancellation loop, which is used to reduce the output IMD products with the error signal. The amount of correction is limited by the ability of the two loops to match gain and phase between the main signal and error paths. When only a small amount of gain and phase error is achieved, the correction is determined by
[delta]IMD = -10 log / 1 + [10.sup.[delta]G/10] -2
* [10.sup.[delta]G/20] cos([delta][phi])
[delta]IMD = amount of IMD improvement in decibels
[delta]G = amplitude error between the main and error paths
[delta][phi] = phase error between the main and error paths
Typically, the cancellation in each loop is limited to approximately 30 dB if the phase mismatch is 2[degrees] or the amplitude mismatch is 0.25 dB. The feedforward amplifier is quite complex and difficult to realize. First, the gain and phase of each component must be kept flat to extend the cancellation bandwidth. In addition, the linearization components must have near-perfect IMD performance.
It is simple to determine the analytical expression between the IMD improvement and the gain and phase error of the loops in a feedforward amplifier. It is well known that the IMD performance of linearization components is of great importance to the cancellation. However, the analytical expression of such an effect is very difficult to determine because of its complexity and nonlinearity. With microwave EDA tools such as the Hewlett-Packard Advanced Design System, the simulation is quite simple and accurate.
Figure 2 shows the schematic of a 460 MHz feedforward amplifier. The parameters of the components are selected as close as possible to the measured data. Figure 3 shows the simulation results when all of the parameters are fixed to their optimal values. The output spectrum of the feedforward amplifier together with that of the main amplifier and error amplifier is displayed. The carrier suppression of Loop I is greater than 35 dB. The IMD3 performance of the feedforward amplifier is improved by approximately 30 dB. It also can be seen that the feedforward amplifier introduces higher IMD5 products.
The sensitivity of the feedforward amplifier is analyzed by sweeping the component parameters. The IMD3 simulation results vs. the delay in the delay lines in Loops I and II are shown in Figures 4 and 5, respectively. Figures 6 and 7 show the IMD3 sensitivity vs. attenuation in Loops I and II, respectively. The sensitivity to phase shift is shown in Figures 8 and 9, and IMD3 products vs. output third-order intercept point (OIP3) of the auxiliary amplifier in Loop I and the error amplifier in Loop II are shown in Figures 10 and 11, respectively.
It was determined from the simulated results that the performance of the feedforward amplifier is extremely sensitive to the length of the delay lines in both loops. Aside from the phase shifters and the attenuators that are used to adjust the amplitude and phase in cancellation, the IMD performances of the error amplifier in Loop II and the auxiliary amplifier in Loop I are greatly affected. Typically, the higher the OIP3, the higher the obtained IMD3 performance. On the other hand, higher OIP3 requires higher cost. Therefore, a trade-off is required when the error amplifier and auxiliary amplifier are selected.
Figure 12 shows measured results for the 460 MHz feedforward amplifier. The output power of the amplifier is 10 W. The original IMD3 of the main amplifier is approximately -34 dBc and the IMD3 of the linearized amplifier is approximately -66 dBc. Using the feedforward technique, the IMD performance of the power amplifier is improved by more than 30 dB. During the development, the lengths of the two delay lines are carefully adjusted to meet the performance according to the simulated results.
A feedforward amplifier with very high linearization performance for multicarrier applications has been designed using detailed simulation made possible by EDA tools. The simulated results offer important information for the design of the actual high power feedforward amplifiers.
Jianyi Zhou received his ME from Southeast University, People's Republic of China in 1996. He is now an engineer in the State Key Laboratory of Millimeter Waves, Southeast University. Zhou's current research interest focuses on the development of RF systems in wireless communication.
Limin Feng received his degree in electronics from Tsinghua University, People's Republic of China in 1964. He is now a senior engineer in the State Key Laboratory of Millimeter Waves, Southeast University. Feng's current research interests focus on microwave measurement, active and passive microwave circuits, and microwave systems.
Xiaowei Zhu received his ME from Southeast University, People's Republic of China in 1996. He is now an associate professor at Southeast University. Zhu's current research focuses on wireless communication systems.
Wei Hong received his PhD from Southeast University, People's Republic of China in 1988. He is now a professor at Southeast University. Hong's current research focuses on wireless communication systems, microwave systems, antennas, electromagnetic problems and EDA.
(1.) J. S. Kenney and A. Leke, "Design Considerations for Multicarrier CDMA Base Station Power Amplifiers," Microwave Journal, Vol. 42, No. 2, February 1999, pp. 76-86.
(2.) Y. Hau et al., "Sensitivity of Distortion Cancellation in Feedforward Amplifiers to Loops Imbalances," IEEE MTT-S International Microwave Symposium Digest, June 1997, pp. 1695-1698.
(3.) S. Kang and I. Lee, "Analysis and Design of Feedforward Power Amplifiers," IEEE MTT-S International Microwave Symposium Digest, June 1997, pp. 1519-1522.
(4.) A. Katz, "SSPA Linearization," Microwave Journal, Vol. 42, No. 4, April 1999, pp. 22-44.
(5.) D. Wills, "A Control System for a Feedforward Amplifier," Microwave Journal, Vol. 41, No. 4, April 1998, pp. 22-34.
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|Comment:||DESIGN OF AN ULTRALINEAR WIDEBAND FEEDFORWARD AMPLIFIER USING EDA TOOLS.|
|Author:||ZHOU, JIANYI; FENG, LIMIN; ZHU, XIAOWEI; HONG, WEI|
|Date:||Jan 1, 2000|
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