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## Potentially unstable device

In a 50 system, the characteristics of a microwave transistor measured at 2.0 GHz is shown as the following.

Design an amplifier with an operating power gain of 8 dB.

Solution

This device is potentially unstable.

The input stability circle on plane has

The corresponding constant operating power gain has

We put both the constant operating power gain and input stability circles as shown in Figure 23. Clearly, the chosen should stay away from the input stability circle and must be on the constant operating power circle. Let's say, we choose . Then we can find the input reflection coefficient

If we use conjugate matching at the input,

Figure 23: The constant gain and input stability circles drawn on plane

We need to examine this to see whether it is in the stable region by means of output stability circle in Figure 24. The output stability circles has

Figure 24: The output stability circle drawn on plane

From Figure 24, is in the stable region. So we can accept and as the design value. Otherwise, we need to choose another value of until the corresponding is in the stable region. The above procedures are used for designing for a specified power gain. If the objective is to design for low noise figure, we should start with in the beginning. Use the output stability circle to check whether the is in the stable region. If is in the stable region, choose , otherwise choose a in the stable region with lowest noise figure with the aid of constant noise figure circle. For a given , find the corresponding conjugate matched load reflection coefficient . Check its stability with the input stability circle. Repeat the process until both and are in the stable regions.

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