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4th June 2019, 06:48 #1
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Stability of a circuit and its role in design steps
Hi,
This may be a very basic question but I always had my own views on it. When should one check for stability of circuit in the design process, is it before matching the circuit or after?. Usually I check the kfactor before input and output matching, and then make the circuit stable (say by resistive source degenration). After that I perform matching. However, after matching the circuit even if I remove the degenrative resistor, the circuit is still stable. So I ould like to know what is the ideal way of doing it. Thanks

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4th June 2019, 07:46 #2
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Re: Stability of a circuit and its role in design steps
Overall Stability depends also on "Viewed Impedance" by active device.in fact there are 2 stability metrics..
Intrinsic Stability of the active device that is variable by Presented Impedance to the active device ( hint : Stability Circles )
Overall Stability of the circuit..( Intrinsic Stability+Extrinsic affects ) .This is more valid for single stage circuits.For more stages , NDF technique can be applied.
So, the design should following steps for stability..
*Assure the Intrinsic Stability as much as possible ( if the stability is not maintained, second step can still also be applied )
*Design Matching Network by aid of first step
*Check overall stability
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22nd June 2019, 16:25 #3
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Re: Stability of a circuit and its role in design steps
Yeah the viewed impedance has to be greater than 0.
So it doesn't matter whether the circuit is stable or not, we can still design matching network. I have two questions here
1. You said "Assure the Intrinsic Stability as much as possible" How do you do that ? increasing transistor width?
2.What if the circuit is not stable at the end? So far I used degenerative inductor only to stabilize the circuit. Which else techniques do you

23rd June 2019, 13:45 #4
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Re: Stability of a circuit and its role in design steps
You have to learn followings.
 Uncoditionally Stable
 Conditionally Stable
See https://designersguide.org/forum/Ya...1218635777/0#5
Conditionally Stable.
Large width results in more unstable, since large feedback capacitor and large phase delay.
It can not be.
Frequency dependent dumping.
Neutrization.
Search my very old posts, more than 10 years ago.Last edited by pancho_hideboo; 23rd June 2019 at 13:55.

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22nd July 2019, 06:07 #5
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Re: Stability of a circuit and its role in design steps
I see it as well. The losses in the matching network cause the amplifier to be stable. This is what I did for single ended amplifiers:
1. Design the amplifier with a series gate resistance/source degeneration such that the amplifier is stable. I try to keep Kf close to 1 and not overdo it
2. Design the output and input matching networks, and make sure to include losses in them.
3. Check the stability. You will now see that the amplifier has a much larger Kf and the gain is reduced.
4. I remove/reduce the degeneration resistance and then tune the parameters of the input matching network so that s11 reduces.
5. I disconnect the output matching network, rerun the loadpull and then design the output matching network.
And then I keep iterating between steps 4 and 5.
But for mmwave, the differential topology is better as I can use capacitive neutralization.
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7th August 2019, 08:18 #6
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Re: Stability of a circuit and its role in design steps
Actually I realized that to do Harmonic balance simulation the circuit should be stable but for this I need to extra component to make it stable. But how to do I find the exact Output power of a gain stage if I connect the extra components .
   Updated   
I also kept series gate resistance to make it stable but how is going to impact the output power I get from the gain stage. Because I am only interested in the power offered by the gain stage without the resistor

7th August 2019, 11:35 #7
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Re: Stability of a circuit and its role in design steps
You only place the gate resistor to make it stable (Kf=1) which means your MAG and MSG are equal. You cannot get a gain more than MSG anyway. This is just to aid in the first iteration of doing a loadpull. Later on when you start introducing losses in your matching network, you can get rid of your series gate resistance. That gain will be typically lesser than the gain you get when you use the series gate resistance to get a Kf=1.
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7th August 2019, 13:55 #8
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Re: Stability of a circuit and its role in design steps
No.
Do pure frequency domain HB Analysis without transient assist, it can converge even if your circuit is unstable.
Probably you can not understand anything at all.
Learn and understand HB Analysis surely.
https://www.edaboard.com/showthread.php?385490#4
Even if HB Analysis can converge, your circuit gives oscillation in real world.
So your circuit is not useful.Last edited by pancho_hideboo; 7th August 2019 at 14:17.
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7th August 2019, 20:26 #9
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Re: Stability of a circuit and its role in design steps
I know its not relevant to this thread particularly, but it is particularly relevant to HB not converging for conditionally stable circuits.
I was once designing a PA using bipolar devices at mmwave frequencies. When I used the Gummel Poon model (which does not capture a lot of high current effects which are very relevant to the operation of a PA) using pure frequency domain HB analysis, even if the circuit was not unconditionally stable, the simulation converged. But when I switched to HiCUM model (which is a very detailed model that includes a lot of high current effects), if it was not unconditionally stable, it would fail and mostly for high power points. So I don't think its just pure stability. The model files used also have a role to play.
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