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Message started by waseda-rfic on Oct 17th, 2009, 7:47pm

Title: how to simulation the large signal stability of power amplifier?
Post by waseda-rfic on Oct 17th, 2009, 7:47pm
in cadence or ads ,there some tools for  stability such as kf ,stable circular . but it is all small signal stability . is there any method to simulation the large signal stability of power amplifier?  and what about judging by the  transient  impulse output waveform with a input of square waveform ? is it accurate?

Title: Re: how to simulation the large signal stability of power amplifier?
Post by Frank Wiedmann on Oct 18th, 2009, 12:51am
A transient simulation of the step response is the most reliable way of determining stability. It should always be done to verify the stability of the circuit, even if the circuit was designed with small-signal analyses. The disadvantage of the transient simulation is that it usually takes much longer than a small-signal analysis and that it is more difficult to extract a number like the phase margin from the results.

Title: Re: how to simulation the large signal stability of power amplifier?
Post by Ken Kundert on Oct 18th, 2009, 4:17pm
Let me second Frank's suggestion to use transient analysis to look for large signal instability. And it is important to recognize that harmonic balance is no substitute in this case. Harmonic balance is a steady-state analysis and so will appear to give reasonable results even if the circuit is unstable (conceptually shooting methods also suffers from this problem, but you are much less likely to miss instabilities using shooting methods because they are built on transient analysis). Thus, if you design your large signal circuit using harmonic balance, you should perform at least some simulations with transient analysis to look for stability issues.

-Ken

Title: Re: how to simulation the large signal stability of power amplifier?
Post by waseda-rfic on Oct 18th, 2009, 7:03pm

Frank Wiedmann wrote on Oct 18th, 2009, 12:51am:
A transient simulation of the step response is the most reliable way of determining stability. It should always be done to verify the stability of the circuit, even if the circuit was designed with small-signal analyses. The disadvantage of the transient simulation is that it usually takes much longer than a small-signal analysis and that it is more difficult to extract a number like the phase margin from the results.

    Thanks for your suggestion. I still have some question about the stability judging by the transient simulation. For opamp simulation, input is a square wave using vpulse source in spectreRF, and if no ring in the rise time at the output. It is ok! For RF circuit, usually, there is an input match network including capacity for match and dc separation in amplifier and with an output match network after it. In this case, should I still use vpulse source in series with a 50 resistance（vpulse source has no output resistance）, and add the vpulse in the input match and see the wave after the output match? If there is no ring, it is ok?
     Another question is about the stability at the time of power on, should I simulation it to confirm the stability? If I should simulation the state of power on, what input signals source should I choose?

Title: Re: how to simulation the large signal stability of power amplifier?
Post by waseda-rfic on Oct 18th, 2009, 7:21pm

Ken Kundert wrote on Oct 18th, 2009, 4:17pm:
Let me second Frank's suggestion to use transient analysis to look for large signal instability. And it is important to recognize that harmonic balance is no substitute in this case. Harmonic balance is a steady-state analysis and so will appear to give reasonable results even if the circuit is unstable (conceptually shooting methods also suffers from this problem, but you are much less likely to miss instabilities using shooting methods because they are built on transient analysis). Thus, if you design your large signal circuit using harmonic balance, you should perform at least some simulations with transient analysis to look for stability issues.

-Ken

Thanks for your reply! But for shooting methods, it is the response of a fixed sine input.So how to test for a wideband input? and in steady-state analysis, can we see the wave at the initial time like the trans simulation.
By the way, should I consider the state at the power on? If I should simulation the state of power on, what input signals source should I choose?

Title: Re: how to simulation the large signal stability of power amplifier?
Post by Frank Wiedmann on Oct 18th, 2009, 7:34pm
You should always simulate the circuit configuration(s) that you will have in your application, including matching networks and parasitic components from interconnections. This applies for all types of simulations, not only stability simulations.

The same is true for simulations of power-up. Find out the output signal of the power source (and the power network, inlcuding parasitics) at power-up (rise time, possibly overshoot or ringing) and use it in your simulation. If you are designing a catalog part, use the worst-case values (shortest rise time etc.) from the datasheet. Also don't forget to check that you don't exceed the maximum rated values of your components during power-up (and power-down).

Title: Re: how to simulation the large signal stability of power amplifier?
Post by waseda-rfic on Oct 18th, 2009, 8:15pm

Frank Wiedmann wrote on Oct 18th, 2009, 7:34pm:
You should always simulate the circuit configuration(s) that you will have in your application, including matching networks and parasitic components from interconnections. This applies for all types of simulations, not only stability simulations.

The same is true for simulations of power-up. Find out the output signal of the power source (and the power network, inlcuding parasitics) at power-up (rise time, possibly overshoot or ringing) and use it in your simulation. If you are designing a catalog part, use the worst-case values (shortest rise time etc.) from the datasheet. Also don't forget to check that you don't exceed the maximum rated values of your components during power-up (and power-down).

thanks! my input also use square wave,and if no rings, it is ok?

Title: Re: how to simulation the large signal stability of power amplifier?
Post by Frank Wiedmann on Oct 19th, 2009, 12:26am
I would guess so. If you are concerned about stability at certain points inside your circuit, you might also want to probe the corresponding internal nodes and check for ringing there. Depending on the structure of the circuit, not all internal ringing may be visible at the output.

Title: Re: how to simulation the large signal stability of power amplifier?
Post by waseda-rfic on Oct 19th, 2009, 6:19am
Dear Frank , I get it. thank you very much!

Title: Re: how to simulation the large signal stability of power amplifier?
Post by Ken Kundert on Oct 19th, 2009, 12:39pm
Your original question was about large-signal stability. Just applying a square wave and looking for ringing is insufficient. In doing so you are likely just replicating a small-signal stability test. If a circuit is unstable in a small-signal sense, then it generally becomes very obvious in transient, because the circuit breaks into oscillation. However, a circuit that is stable in a small-signal sense and unstable under large signals will not just break into oscillation in transient. You need to provide a signal that will get it into is unstable region. For example, a cohort designed a track and hold that had a feedback loop that contained two op amps. Under small signal conditions, it worked fine. The second op amp was configured as a unity gain buffer that had a bandwidth well beyond the unity gain frequency of the loop. But once that opamp entered slew-rate limiting, its effective bandwidth dropped by several orders of magnitude making the loop unstable. The only way he could see this in simulation was by providing the right large stimulus that put that opamp into slew limiting. So in general, there is no one particular test you can run that if your circuit passes you can say with certainty that your circuit is large-signal stable. Instead, you have to know your circuit and know what states it can get into and test each of them.

-Ken

Title: Re: how to simulation the large signal stability of power amplifier?
Post by Frank Wiedmann on Oct 20th, 2009, 1:38am
When I was talking about the step response, I was thinking of a worst-case step at the input (maximum amplitude, maximum slew rate, both directions). I did not state this explicitly, however. I agree that a small step will just replicate a small-signal analysis.

Title: Re: how to simulation the large signal stability of power amplifier?
Post by waseda-rfic on Oct 22nd, 2009, 11:52pm

Ken Kundert wrote on Oct 19th, 2009, 12:39pm:
Your original question was about large-signal stability. Just applying a square wave and looking for ringing is insufficient. In doing so you are likely just replicating a small-signal stability test. If a circuit is unstable in a small-signal sense, then it generally becomes very obvious in transient, because the circuit breaks into oscillation. However, a circuit that is stable in a small-signal sense and unstable under large signals will not just break into oscillation in transient. You need to provide a signal that will get it into is unstable region. For example, a cohort designed a track and hold that had a feedback loop that contained two op amps. Under small signal conditions, it worked fine. The second op amp was configured as a unity gain buffer that had a bandwidth well beyond the unity gain frequency of the loop. But once that opamp entered slew-rate limiting, its effective bandwidth dropped by several orders of magnitude making the loop unstable. The only way he could see this in simulation was by providing the right large stimulus that put that opamp into slew limiting. So in general, there is no one particular test you can run that if your circuit passes you can say with certainty that your circuit is large-signal stable. Instead, you have to know your circuit and know what states it can get into and test each of them.

-Ken

Dear  Ken ,thank you for your advise! thank you for you example for my understanding of large signal unstable!

Title: Re: how to simulation the large signal stability of power amplifier?
Post by waseda-rfic on Oct 22nd, 2009, 11:55pm

Frank Wiedmann wrote on Oct 20th, 2009, 1:38am:
When I was talking about the step response, I was thinking of a worst-case step at the input (maximum amplitude, maximum slew rate, both directions). I did not state this explicitly, however. I agree that a small step will just replicate a small-signal analysis.

Dear Frank Wiedmann ,thank you again for your additional advice!

Title: Re: how to simulation the large signal stability of power amplifier?
Post by RFrequency on Dec 30th, 2009, 9:56am
What you need to do is just to simulate the large signal [s] parameter (LSSP), then from the large signal [s], you can easily calculate stability factor and so on.

Title: Re: how to simulation the large signal stability of power amplifier?
Post by pancho_hideboo on Dec 30th, 2009, 10:20am

RFrequency wrote on Dec 30th, 2009, 9:56am:
What you need to do is just to simulate the large signal [s] parameter (LSSP), then from the large signal [s], you can easily calculate stability factor and so on.
Generally we can't judge stability from results of LSSP.

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