A New Loop Control Algorithm of Feedforward Power Amplifier Fei You Songbai He 1 Xiaohuan Yan 2 Jingfu Bao 1 School of Electronic Engineering 1 ，School of Communication and Information Engineering 2 University of Electronic Science and Technology of China, Chengdu, China Abstract- This paper mainly proposes an algorithm which can measure the indeterminate phase shift in the measure path and make the final result attain expected accuracy of phase matching. The measure architecture is based on I/Q demodulation, and in the paper, the classical architecture of feedforward amplifier is slightly modified at the error extraction loop and error cancellation loop to get better performance.. Keywords- adaptive algorithm, feedforward power amplifier, phase mismatching, I/Q demodulation I. INTRODUCTION Theoretically speaking, accurately tuned feedforward power amplifier can archive excellent performance on the IM3 (3rd order intermodulation) suppression. But the pre-tuned amplifier’s parameters will vary with the ambient temperature and the aging problems, so it is necessary to adopt adaptive control method. There does exist one important factor, the accuracy of phase matching, which dominates the final performance of the IM3 suppression. 0.5 degree phase mismatching can lead to rapid decline of the performance [1]. This paper will propose a new architecture which can measure the phase deviation at the power combiner in feedforward power amplifier. It can make the vector modulation to attain better performance of IM3 suppression with inaccurate tuned delay line and indeterminate phase delay in the measure path. II. BASIC ARCHITECTURE AND LOOP CONTROL ALGORITHM In order to get the phase and magnitude errors, the classical quadrature demodulation is used to extract theses important parameters. The block diagram of the feedforward with quadrature modulation detect is shown in Figure.1(a). The main idea of the feedforward linearization is to produce another IM3 signal with the same magnitude and opposite phase to the corresponding components of the output of the nonlinear power amplifier. Then, when added together, the output can be linearized. The classical I/Q demodulation method is shown in Figure.1(b), but the architecture which can carry our point is slightly changed compared to the classical architecture, and it is shown in Fig.2. The only difference is that we add a switch in the path of vector modulator. i.e. in the path of the coupled output of main amplifier and the error amplifier’s output. Now firstly construct a simple model to begin our analysis. Two signals to be demodulated are coupled before and after the in-phase combiner. Assume that the two added signal are expressed as follows: direct signal: , tuned signal: *This paper is supported by UESTC-VARA lab. Because the cancellation performance is very sensitive to phase deviation, this factor must be put into our analysis. And all the indeterminate phases are combined into one term, i.e. we assume that only the coupled channel of output of the in phase power combiner is affected by that. (a) (b) Figure 1. (a)The blockdiagram of feedforward with quadrature modulation detect method(b) classical measure architecture The coupled signal of the output of a loop is expressed as follows, ) cos( ) cos( [ 2 1 β θ β α + + + + = wt V wt V V RF (1) Where β represents the indeterminate phase shift of couple path, and the couple factor is, the amplifier’s gain is A. At first, the switch is turned off. Following the principles of I/Q demodulation, we will get the DC components, β α cos 4 2 1 AV V off I = , β α sin 4 2 1 AV V off Q - = (2) And then, the switch is turned on, in the same way, we can get another result: 0-7803-9433-X/05/$20.00 ©2005 IEEE. APMC2005 Proceedings