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a Ϫ3-dBm input level. Figure 3 shows the resultant improvement in the 3 rd -order intermodulation products for the linearized and unlinearized cases.For such a case where the circuit has been optimized for 10 GHz, in addition to the significant improvement in 3 rd -order intermodulation at the optimization frequency, the result also shows improvement over at least the 2-14-GHz range. The complex combination of the amplitude and phase balance in the first and second loops determines this bandwidth.The rapid deterioration of the improvement in 3 rd -order intermodulation above 14 GHz is primarily due to a phase imbalance in the second loop. This imbalance is caused by the phase shift of the artificial transmission line in the auxiliary amplifier, which becomes increasingly nonlinear with frequency as the cutoff frequency of the line is approached. EXPERIMENTAL EVALUATIONFor experimental verification of the technique prior to MMIC implementation, an MIC version of the four section distributed amplifier was implemented using Celeritek CF004-03 pHEMT chip devices mounted in slots inserted between the gate and drain lines on 0.010-inch-thick RT/Duroid 5880 substrate. Although it is a relatively low-power amplifier, it is suitable for demonstration of the concept.The feedforward loop circuitry was implemented as shown in Figure 1 using commercially available connectorized components for the directional couplers, in-line coaxial phase shifters, and an auxiliary amplifier. Delay lines were implemented via length adjustment of coaxial lines and variable attenuators were used to set the signal amplitude where appropriate.Two-tone tests were carried out at 12 GHz with two carrier separations of 1 and 5 MHz tested. The loops were optimized in a manner similar to that in [7].Given a 1-MHz signal separation and input power level at 1-dB gain compression, Figure 4 shows the output spectra for the amplifier without the feedforward linearization and Figure 5 compares the output spectra with linearization. It can be seen that a reduction of 27 dB in the 3 rd -order intermodulation level has been achieved. A similar comparison for 5-MHz separation and an input corresponding to 1-dB gain compression is shown in Figures 6 and 7. Here, 15-db reduction of the 3 rd -order intermodulation level is achieved. The difference between the reduction achieved for the 1-MHz and 5-MHz separations can at least partially be attributed to the gain ripple in the distributed amplifier, which was a result of the nature of its assembly structure. An MMIC is a more suitable method of implementation. CONCLUSIONThe concept of applying feedforward linearization to a distributed amplifier has been investigated. The distributed amplifier can be adapted to form an integral part of the feedforward loop by utilizing the signal on the gate-line output. This novel adaptation simplifies the loop circuitry and lends itself to MMIC implementation. The results of simulations of the feedforward-linearized distributed amplifier have demonstrated the potential of the new...

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