An alternative stability factor |K.sub.A~ is presented that provides a definitive indication of stability and permits the design engineer to concentrate stability circle investigations on those frequency ranges where further insight is needed. This process can be tedious, particularly since k tends to predict false alarms, that is, frequencies at which the network is not unconditionally stable, but is in fact stable when operated within the range of expected source and load reflection coefficients. The layout after resolution of the rubber band lines is given below.Stability is an important consideration when designing an amplifier.|1-3~ Both the overall amplifier and each individual stage must be determined to be stable.|4,5~ Unless the traditional stability factor|6~ k in conjunction with|4~ |B.sub.1~ indicates that a network is unconditionally stable, and both a passive source and load are expected, stability circles must be computed.|2~ These circles are then displayed simultaneously with the source and load reflection coefficients. Also notice that the sweep range for the amplifier gain and match is from 2000 to 2800 MHz, but the sweep range for the stability analysis is from 100 to 6000 MHz, the entire range for which S-parameter data was available. Notice that the entire Smith chart region, which represents any possible passive load, is stable for both the input and output. 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. This example illustrates stability circles and designing an amplifier for stability.
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