Question 12.7: PERFORMANCE AND OPTIMIZATION OF A BALANCED AMPLIFIER Use the...

The amplifier of Example 12.4 was designed for a gain of 11 dB at 4 GHz. As seen from Figure 12.8c, the gain varies by a few dB from 3 to 5 GHz, and the return loss is no better than 5 dB. We can design a quadrature hybrid, according to the discussion in Chapter 7, to have a center frequency of 4 GHz. Then the balanced amplifier configuration of Figure 12.11 can be modeled using a microwave CAD  package, with the results shown in Figure 12.12. Note the dramatic improvement in return loss over the band as compared with the result for the original amplifier in Figure 12.8c. The input matching is best at 4 GHz since this was the design frequency of the coupler; a coupler with better bandwidth will give improved results at the band edges. Also observe that the gain at 4 GHz is still 11 dB, and that it drops by a few dB at the band edges.

Most modern microwave CAD software packages have an optimization feature with which a small set of design variables can be adjusted to optimize a  particular performance variable. In the present example, we will reduce the gain specification to 10 dB, and allow the CAD software to adjust the four transmission line stub and line lengths in the amplifier circuit of Figure 12.8b to give the best fit to this gain over the frequency range 3–5 GHz. Both amplifiers in the balanced circuit remain identical, so we should still see the improved input matching. The results of this optimization are shown in Figure 12.12, where it can be seen that the gain response is much flatter over the operating band. The input match is still very good in the vicinity of the center frequency, with a slightly worse result at the low frequency end. The optimized stub and line lengths for the amplifier matching networks are listed below:

These represent fairly small deviations from the lengths in the original matching networks.