TABLE OF CONTENTS From the stability chart shown in Figure 2-26, we observed that the circuit would be stable only for those source impedances that were represented by the "stable portion of the Smith chart." In particular, observing the real axis, it is clear that a minimum resistance of approximately 90 ? needs to be added to the source for the circuit to be stable. Do that and look at the result. Figure 2-30 shows the result of adding 91 ? in series with the inductance and source voltage for the circuit example of Figure 2-22. Note how the stability circle now encompasses the center of the chart and that s 22 < 1. This means that the nonshaded area of the chart is now the stable region. This includes (barely) the entire real axis. This circuit is right on the verge of oscillating. The SPICE result for this was shown in Figure 2-20 as a continuous oscillation that does not grow or decline in amplitude. Increasing the resistance to 100 ? will cause the oscillation to damp out with time. Decreasing the resistance would cause the oscillations to grow in value until limited by nonlinearity in the devices. This circuit is still not useful because of its marginal stability. One would have to increase R b considerably in order to guarantee stability and even then there would be a lot of ring and overshoot or the bandwidth would be so poor as...
