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RF design and layout [questions/feedback]
aaronhance:
By using a 500Ohm resistor in place of the 1.5kOhm resistor on the datasheet I've managed to make it unconditionally stable between 10Mhz and 6Ghz. Do I need to worry about stability above this? I also have S21 15dB and s11 -13dB @ 137Mhz, I assume this is OK? If I were to increase the value of the resistor to get better performance it would be conditionally stable, how would I calculate if it would be stable, if it's simply a matter of mismatching the impedance at the conditionally stable frequencies, how much mismatch would be needed?
Nitrousoxide:
--- Quote from: aaronhance on June 28, 2019, 02:23:58 pm ---1. Do I need to worry about stability above this?
2. I also have S21 15dB and s11 -13dB @ 137Mhz, I assume this is OK?
3. If I were to increase the value of the resistor to get better performance it would be conditionally stable, how would I calculate if it would be stable, if it's simply a matter of mismatching the impedance at the conditionally stable frequencies, how much mismatch would be needed?
--- End quote ---
1: Yes. Unconditional stability over a certain range is only guaranteed... in that range. You never know that the amplifier may be unstable at frequencies that are significantly larger than 6Ghz. Now is it likely that it is? Probably not. However, if you find the amplifier outputting a power that is significantly lower than what you'd expect per bias current, then there is some oscillation occurring.
Obviously, the change (and inclusion, i don't see it in the original layout?) of the feedback network will affect the noise figure as the input reflection coefficient is now not solely dependant upon the device, but also the output matching network.
2: S21 (forward gain) looks like about 17.5dB. Considering you've decreased the attenuation (of the FB network) or increased the negative feedback the gain has dropped. Which is to be expected. S11 of -13 looks about right from the datasheet. Again, look at what the datasheet says because it will most likely give a good indication of performance.
3: Apologies as I'm slightly confused by your wording. Are you asking "How do I calculate under what conditions I get a conditionally stable system?" and "What are the tools I can use to do this?". If so:
Conditional stability occurs when the transistor does not behave unilaterally, which is the case with most real transistors. Depending upon how unilateral they are can dictate their stability performance.
There are a few steps for calculating stability, in the order they generally are:
1. Is the transistor unilateral, meaning that S12 = 0? If yes, then it is unconditionally stable. If not, then move on to see if we can ASSUME that the device is unilateral.
2. Calculate the unilateral figure of merit from the S parameters. It is acceptable to assume the device is unilateral if this figure of merit is less than 1dB (some people say 0.5, some say 1.5). :-//
3. If the previous two steps fail, then the device is conditionally stable. It is then important to calculate Rollet's stability criterion for multiple frequencies. If K > 1 and abs(delta) < 1 then the device is stable at that frequency. If either of these conditions are violated. You have found where the device is unstable.
4. Use the formulae to plot the stability circles. Now, if |S11| < 1 and |S22| < 1 then the area defined by the circle boundaries that include the origin of the Smith chart is stable. If |S11| > 1 and |S22| > 1, the area bound by circles NOT including the origin is stable.
Now, how can we vary all this parametrically? This is where mathematics computing software saves the day. You will have to mathematically represent the circuit as such that you can parametrically vary the feedback resistance, thus varying the S parameters and in turn affecting the stability criterion.
Note: There are other stability measures, such as the mu-factor that give slightly more detailed information as to where the instability is occurring but are slightly harder to compute.
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