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Potentially unstable device

In a 50 text_wrap_inline2807 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.


This device is potentially unstable.


The input stability circle on text_wrap_inline2885 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 text_wrap_inline2885 should stay away from the input stability circle and must be on the constant operating power circle. Let's say, we choose text_wrap_inline3565. 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 text_wrap_inline2885 plane

We need to examine this text_wrap_inline3569 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 text_wrap_inline2883 plane

From Figure 24, text_wrap_inline3569 is in the stable region. So we can accept text_wrap_inline3569 and text_wrap_inline3565 as the design value. Otherwise, we need to choose another value of text_wrap_inline2885 until the corresponding text_wrap_inline2883 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 text_wrap_inline2883 in the beginning. Use the output stability circle to check whether the text_wrap_inline3319 is in the stable region. If text_wrap_inline3319 is in the stable region, choose text_wrap_inline3405, otherwise choose a text_wrap_inline2883 in the stable region with lowest noise figure with the aid of constant noise figure circle. For a given text_wrap_inline2883, find the corresponding conjugate matched load reflection coefficient text_wrap_inline2885. Check its stability with the input stability circle. Repeat the process until both text_wrap_inline2883 and text_wrap_inline2885 are in the stable regions.

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