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Simulators >> Circuit Simulators >> Large Signal Frequency Response
https://designers-guide.org/forum/YaBB.pl?num=1250543357

Message started by kevinvinv on Aug 17^th, 2009, 2:09pm

Title: Large Signal Frequency Response
Post by kevinvinv on Aug 17^th, 2009, 2:09pm

I have an amplifier I'd like to do a LARGE signal frequency sweep on... to ideally get the gain and phase response. Anyone have a suggestion on how best to do this?

Thanks.

Title: Re: Large Signal Frequency Response
Post by buddypoor on Aug 18^th, 2009, 12:12am

Your goal is not quite clear to me.
Both, gain and phase are small signal parameters, because it makes no sense to define a gain as a ratio of two voltages which have not the same shape.
Remember: For large signal excitation the output waveform is different from the input.
More than that, do you ask for a simulation or measurement procedure ?

Title: Re: Large Signal Frequency Response
Post by kevinvinv on Aug 18^th, 2009, 3:16am

Need a way to do it in simulation. I dont understand your points. The signal shape will be the same... say a 5Vpp sinewave... just need to get gain and phase vs frequency... like a network analyzer would do in th lab.

Title: Re: Large Signal Frequency Response
Post by thechopper on Aug 18^th, 2009, 4:38am

Hi Kevinvinv,

What buddypoor suggested is correct: you cannot define a gain as a ratio of two signals which do not have the same shape. In other words, gain is only a valid parameter for linear systems.
When considering LARGE signal frequency response your amplifier will most probably introducing distortion to both amplitude and phase of the output signal (thus a non-linear behavior).

You could, for example, see what the amplitude ratio and phase shift are under LARGE signal conditions for each input amplitude, but cannot define strictly a gain, since such ratio and shift might not be the same if you just change the input signal amplitude.

As for ways of simulating LARGE signal response, I do not know anything but transient simulations under different input signal conditions...

Hope this helps
Tosei

Title: Re: Large Signal Frequency Response
Post by kevinvinv on Aug 18^th, 2009, 5:32am

?? Hmmm.... ok. well- how about we ASSUME that my circuit doesnt distort the large signal and that the output sine wave is still a perfect sinewave? Then any ideas? Are you telling me that people who design audio or other power amps only do smalll signal bandwidth analysis? I find that hard to believe. thoughs?

Title: Re: Large Signal Frequency Response
Post by Frank Wiedmann on Aug 18^th, 2009, 5:42am

Maybe this is what you are looking for: http://www.designers-guide.org/Forum/YaBB.pl?num=1031864062

Title: Re: Large Signal Frequency Response
Post by buddypoor on Aug 18^th, 2009, 7:26am

kevinvinv wrote on Aug 18^th, 2009, 5:32am:

I suppose, the problem in fact is not a problem.
Kevinvinv, if you assume or expect that the output is a perfect sinewave, you are speaking of a "small signal" analysis.
What means "small" ? 1 volt ? Or 5 volts ?
There is no voltage limit ! "Small" means only, that the small signal analysis is applicable because distortions are not yet relevant.
And this can be very different for different circuits. A transistor without negative feedback allows perhaps only 10mV without remarkable distortions - and an opamp with feedback perhaps 5 volts.
Problem solved ?

Title: Re: Large Signal Frequency Response
Post by thechopper on Aug 18^th, 2009, 6:20pm

In addition to what buddypoor described...

If you assume NO distortion for a small or a large signal, that means the transfer function of your amplifier is perfectly linear over the whole signal range.
Thus, the slope of such transfer function is constant over such signal range.....thus the small signal analysis applies for the whole signal range.
Small signal = slope of the transfer function around a certain operating point.

Hope this further clarifies the point.
Tosei

Title: Re: Large Signal Frequency Response
Post by sos on Aug 21^st, 2009, 6:54am

kevinvinv wrote on Aug 17^th, 2009, 2:09pm:

I have an amplifier I'd like to do a LARGE signal frequency sweep on... to ideally get the gain and phase response. Anyone have a suggestion on how best to do this?

Thanks.

Hi Kevin,
There isn't a good way to do this that I'm aware of. The only thing I know to to is put in a large sin wave, measure output gain and phase, repeat for the next frequency. You probably want to automate this... There is a swept sin source, but how fast do you sweep it before you aren't measuring what you really want to? For distortion measurements, I never measure before the fourth cycle of the sin wave. You could get by with a quicker measurement here, but I do not recommend the first cycle.

This is especially a problem with circuits that use active biasing (bias depends on signal level). Try running a sin wave near the full power bandwidth through the circuit and simulate for a dozen cycles. I've often had circuits that decrease in amplitude over the first few cycles, sometimes over the first 6-10 cycles. If you just simulate one or two cycles, you can miss this effect (you are in effect simulating a burst, not a 'continuous' signal). Guess what you measure on the bench when your silicon comes back. Some biasing schemes can sustain for a brief period but after several cycles, errors build and reduce the slew rate.

You've gotten some theoretical responses so far; mine is a practical one, just like you'd measure it on the bench. Even though you are measuring in the large signal region, your output will be pretty much a sin wave. Bandwidth will decrease as you approach slew limit. When the waveform is really distorted, you're not measuring anything useful, but that's past where you're interested in. Remember, when the circuit hits slew limit, the AC response drops like a rock, not like a one pole circuit.

Steve Smith
formerly of Comlinear Corporation

Title: Re: Large Signal Frequency Response
Post by raja.cedt on Aug 24^th, 2009, 8:52pm

hi kevinvinv,
some times people consider large signal gain is ratio of Fourier amplitude of output signal/input signal (i read this in Delta sigma modulator book in the context of MSA)..but even though your circuit is working pretty well from output wave form point of view i think Ac analysis wont give perfect result due to variation of poles with input amplitude

Thanks,
Rajasekhar.