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Simulators >> Circuit Simulators >> What is "No DC convergence" in MMIC design
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Message started by santosh31013 on Nov 17^th, 2015, 11:30am

Title: What is "No DC convergence" in MMIC design
Post by santosh31013 on Nov 17^th, 2015, 11:30am

Hi,

I working on GaN based MMIC Power Amplifier design. I could easily design basic power amplifier, But while increasing stages to get high power output (by using combiners and dividers), i am facing convergence problem. Sometimes i am getting error like "no DC Convergence" in command window just after starting simulation.
what is DC convergence problem in power amplifier design? How can i correct it?

Thank you in advance...

Title: Re: What is "No DC convergence" in MMIC design
Post by loose-electron on Nov 18^th, 2015, 6:12pm

the DC operating point is not explicitly defined for all nodes in the circuit (generally it all needs a resistive path to ground), or you have a feedback loop that is not stable. If you got a schematic we can give you better answers

Title: Re: What is "No DC convergence" in MMIC design
Post by Geoffrey_Coram on Feb 18^th, 2016, 7:27am

I would actually suspect the GaN model. If the circuit converges for low power, then it isn't a problem with resistive paths to ground.

What model are you using for the GaN device? Have you looked at the I-V characteristics at the high power levels you're interested in? I know some GaN and GaAs models are table-based or empirical.

Title: Re: What is "No DC convergence" in MMIC design
Post by santosh31013 on Feb 19^th, 2016, 9:37am

I am using GaN pHEMT model.

"No DC convergence":
I am using Agilent ADS tool for my design.
When i was simulating my circuit, sometimes it's showing error message like "NO DC CONVERGENCE" and sometimes it's simulating without errors for the same circuit. so, i have decided that it's not problem with the circuit(topology) and it might be with the tool(selection of simulators, modes). i have tried s many options in that tool. finally I could solve it by changing convergence mode in DC-simulation set up.

As I am trainee and recently entered into this area, i couldn't analyze how it was get solved by changing convergence mode, But temporarily i got solution for this problem.

thank you for your response.

Title: Re: What is "No DC convergence" in MMIC design
Post by santosh31013 on Feb 19^th, 2016, 10:09am

And

Sir, you have asked like " Have you looked at the I-V characteristics at the high power levels you're interested in?".

What is the significance of that? and how to check those?
Generally, i am plotting I-V characteristics for HEMT and by using FET_curve_Tracer option in ADS tool and selecting bias point for PA.
Is this same what you were talking about??

Till now i haven't related I-V plots to output power. just i have used to select operating point.

Thank you

Title: Re: What is "No DC convergence" in MMIC design
Post by Geoffrey_Coram on Feb 22^nd, 2016, 7:03am

Sometimes empirical or table-based models are only valid in small regions, and if your circuit sometimes goes outside the valid regions, the model is not valid, and can in fact give absurd values. If you look at the I-V (and C-V) plots, you can sometimes see that the curves are not well-behaved outside the expected range.