![]() The results after optimization of the lengths of lines in the input and output matching networks are shown in the Genesys screen shown above. The remaining microstrip models comprise the matching networks which were optimized for 10dB of gain and best flatness. The microstrip tee and transmission line models are added to account for the physical structure which is necessary to add the resistors to the amplifier. These capacitors and resistors are evident in the schematic shown above. These will also be a part of the bias scheme. To further enhance stability, resistors to RF ground are added at the input and output. Therefore, we will use a series capacitor at the input and output with the smallest value which does not disturb the desired amplifier. In this case, since the circles above represent the lowest frequency and since the top half of the Smith chart is inductive, stability is enhanced by insuring that the device is capacitively terminated at low frequencies. Similar conditions should be satisfied at the output. To insure stability, the impedance presented to the device at its input terminal should avoid the shaded region of the input plane stability circles. Click the Zoom Maximize button to see the full range of data. Each circle locus is specified via a marker, which selects which frequency is of interest. The shaded regions of the Smith chart represent regions of instability. Shown above are the input and output plane stability circles for an HP/Avantek AT41586 bipolar transistor biased at 8 volts and 25 mA. Stability should be examined over as broad a frequency range as possible, and not just over the range desired for the amplifier. The first step is to examine the stability characteristics of the selected active device before adding additional circuitry. What is it about MSG that makes it the final word on stable gain when, I'll say, the 23 or even the 26dB gain circle seems safely out of the output stability circle's unstable region and seems like there would still be some safe terminating impedances, although not many, available.This example illustrates stability circles and designing an amplifier for stability. ![]() Apparently, gain seems to be infinite with respect to a potentially unstable transistor. Only when I jumped to 50 and 100dB did the gain circles just align with the output stability circle. The 29dB gain circle is dangerously close to but still outside the output stability circle's unstable region. For the heck of it, I plotted gain circles all the way up to the ludicrous values of 50 and 100dB to see what would happen. Notice that some of the 22.1dB constant-gain circle is located pretty far away and outside of the output stability circle's unstable region. I have the output stability circle plotted and the unstable region marked as UR on the Smith chart. The on-line article also said that "one should never try to tease more gain from the transistor than the MSG". For my transistor, the MSG in absolute gain 162.5 or 22.1dB as shown on the attached Smith chart. MSG is simply the ratio of the magnitude of S21 over the magnitude of S12. I recently read on-line that for a potentially unstable transistor the maximum gain that can achieved is the MSG or maximum stable gain. ![]() I was plotting constant-gain circles on the Smith chart the other day for a potentially unstable RF transistor.
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