ADS Tutorial Stability and Gain Circles
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The first step in designing the amplifier with the S parameter method is to determine whether the amplifier is unconditionally stable or potentially unstable. This can be easily estimated using the K stability factor and delta. The amplifier will be unconditionally stable if:山楂的药用价值
K > 1 and mag(∆) < 1.
The factors are easily calculated using Measurement Equations in the ADS schematic panel. K is pre-programmed in as stab_fact which can be lected from the S-parameter palette on the left. Delta can be programmed yourlf using a blank MeasEqn. The maximum available gain (MAG) and maximum stable gain (MSG) can also be calculated using the max_gain function.
When swept over a range of frequencies, it can be clearly en where the device will be unconditionally stable (above 1.5 GHz for this example). Note that the ur-defined equation capability of the display panel is ud to also calculate and plot MSG and the intrinsic transducer gai
n (GTi) with both ΓS and ΓL = 0. In the regions where K < 1, the max_gain function plots MSG. When unconditionally stable, it plots MAG.
ADS tip: When you want to plot from your ur-defined equations, you need to lect the equations datat in the plotting panel.
Next, you could check (as the book suggests) to e if the device is unilateral. (this is
rarely the ca). Evaluate the Unilateral Figure of Merit, U, at the design frequency using a Measurement Equation. Let’
告别的意思s choo 500 MHz for our example.
Show the result in a table in the display panel. Unstable
MSG
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Here, we e that at 500 MHz, the device is clearly not unilateral. The unilateral approximation would have an error of over 1 dB.
清朝图片Bilateral design. If the device is unconditionally stable at the design frequency, then the input and output can be conjugately matched as shown in Section 3.6 of Gonzalez. ΓMs and ΓML can be determined uniquely. The input and output VSWR would = 1 in this ca. This would be applicable for this device in the region where K > 1. The conjugate match reflection coefficients can be calculated on ADS using the Smgm1 and Smgm2 measurement equation icons in the S-parameter palette.
However, at 500 MHz, we find that K = 0.75. We must take into account stability as well as gain. It is wi to design the amplifier for less than the MSG to allow margin for stability. In this ca, we need a systematic design method, becau changes in ΓS will affect ΓOUT and changes in ΓL will affect ΓIN.
The operating power gain, G P, provides a graphical design method suitable for bilateral amplifiers. Gain circles can be calculated that show contours of constant operating power gain. G P is uful since it is independent of the source impedance; the gain circle reprents the gain what would be o
btained if a magic genie adjusted ΓS = ΓIN* for each value of ΓL on the circle. Then, G P = G T, ie. the operating power gain equals the transducer gain. Let’s illustrate.
Since we will need to evaluate the load plane for stability, so stability circles should also be calculated. This is done by using the LstbCir function. Source stability circles should also be calculated.
I have found it more convenient to calculate the operating power gain circles on the data display rather than on the schematic. On the data display, you can change the gain values without having to resimulate the amplifier. U the Eqn function to write gain circle equations. The syntax is: gp_circle(S, gain, # points on circle) where S is the S-parameter matrix. In the example below, the gain circles at MSG, and 1 and 2 dB below MSG are plotted. A marker is placed on the –2 dB circle.
ΓL can be read off the display as magnitude = 0.057 with angle = 22 degrees. If the input is conjugately matched (and stable), then the gain should be maxg – 2 = 20.4 dB.
The load stability circle is also shown. The load impedance can be chon away from this circle to maximize stability.
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Next, calculate the input reflection coefficient for your choice of ΓL and make sure it is stable. To do this, plot the source plane stability circle and compare with ΓIN. You can u the marker (m2) to compare values of the source stability circle reflection coefficient with gamma_IN. We can e that this choice will be stable at the design frequency.
Biasing and matching network design . The next step in designing the amplifier will be the implementation of the input and output matching networks such that they provide reflection coefficients ΓS and ΓL as determined above. This can be done using the Smith chart. In general, there may be veral solutions possible using lumped or distributed L networks. In lecting a design, you need to consider how you will bias the amplifier. You have 2 choices: bias with RF chokes and blocking capacitors or bias through the matching network elements.
When possible, biasing through the matching network will often minimize the number of components and may prove to be easier to implement. Consider the load plane.
幼师培训心得简短The marker is on the –2 dB operating power gain circle and reprents our choice of ΓL . Notice that we have many options for ΓL as long as we keep a respectable distance from the load stability circle. As shown above, if we choo a ΓL at the location of m1, we are on the unit constant conductance circle (g=1). Adding a shunt inductance across the 50
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