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Old 27th June 2012, 11:54 AM   #541
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Dave Zan View Post
Any one still remember Edmond's front end
Well I still need an OPS for it and I have a question.
How close to minimum phase is a BJT near Ft?

In the LTSpice help they call TF the "transit time"
If it is a real time delay then behaviour near Ft will depart from minimum phase.
If TF is just the time constant of the frequency roll-off then it will be close to minimum phase.
So a second question would be -
How close is SPICE to real transistor behaviour in this area?

This puts an upper limit on loop stability so it's fairly important for optimized compensation.

Best wishes
David
To say it short , mosfets are better in this respect.....
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Old 29th June 2012, 08:08 AM   #542
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Originally Posted by Edmond Stuart View Post
Hi David,
Did you read these comments , where I stated that a clear distinction should be made between time delay and phase lag? Or is it just a coincidence?
It occurs to me that the time delay in the various parts of the amp is a useful way to think about the optimization problem. The actual time delay sets an easily calculated limit to the maximum possible feedback. Time delays are simply additive so the maths should be easy. I expect someone has formulated the multi-loop feedback problem in this way, anybody know a reference?

Best wishes
David.
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Old 29th June 2012, 09:46 AM   #543
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Default different beasts

Hi David,

The reason I emphasized on the difference between propagation delay and phase lag is because some people (NFB haters, for example) erroneously think that in an amplifier the FB signal should always come 'too late', i.e. time delayed and therefore it should be bad.
As also explained by Jan , this isn't the case. For this reason I prefer (and recommend) to think in terms of phase lag or phase delay. Also in mathematical sense phase lag (e.g. (1-exp(-sT))/s) and time delay (exp(-sT)) are two different beasts.

Cheers,
E.
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Old 29th June 2012, 11:06 AM   #544
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Quote:
Originally Posted by Edmond Stuart View Post
Hi David,

The reason I emphasized on the difference between propagation delay and phase lag is because some people (NFB haters, for example) erroneously think that in an amplifier the FB signal should always come 'too late', i.e. time delayed and therefore it should be bad.
As also explained by Jan , this isn't the case. For this reason I prefer (and recommend) to think in terms of phase lag or phase delay. Also in mathematical sense phase lag (e.g. (1-exp(-sT))/s) and time delay (exp(-sT)) are two different beasts.

Cheers,
E.
Yes I understand this, I ended up in software but I studied mathematical physics*
That's why I asked about how close a transistor is to minimum phase. I really do mean the actual delay - of perhaps 5 or 10 nanoseconds for the OPS. This ultimately limits the amount of feedback because it creates excess phase that can not be phase compensated.
The anti-NFB people have an idea that contain some truth - that time delay matters. They misapply the idea because they don't understand the difference between time delay and phase shift. In the attempt not to confuse them a useful concept is avoided. Perhaps if we use it correctly it will actually over-ride the incorrect use and help.
But that's not my reason. Time delay is a conservative quantity so I think it will make it easier to optimize my circuit.

Best wishes
David

*And a lot of maths and stuff, thanks to Australia's absurdly cheap universities at the time. Wasted my chance to do more EE.

Last edited by Dave Zan; 29th June 2012 at 11:12 AM.
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Old 29th June 2012, 11:26 AM   #545
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by Dave Zan View Post
That's why I asked about how close a transistor is to minimum phase. I really do mean the actual delay - of perhaps 5 or 10 nanoseconds for the OPS. This ultimately limits the amount of feedback because it creates excess phase that can not be phase compensated.
The anti-NFB people have an idea that contain some truth - that time delay matters. They misapply the idea because they don't understand the difference between time delay and phase shift. In the attempt not to confuse them a useful concept is avoided. Perhaps if we use it correctly it will actually over-ride the incorrect use and help.
Add to that that is impossible to measure this delay (is it? or just nobody cares?.

I also wonder, since people can hear jitter of few dozens picoseconds, these few nanoseconds of delay in the OPS should be recognizable as well. Maybe less since we are dealing with higher level signals, but still...

And, if we just exclude the OPS from the feedback loop, the NFB of the previous stages (assuming is multistage) of which order of magnitude is?

just my 2c from a non EE
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Old 29th June 2012, 11:42 AM   #546
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Hi David,

As for the actual delay in MOSFETs or BJTs, I really don't know how much it will exactly be. I guess it depends on the mobility of the electrons (or holes) and the distance they have to travel from source to drain, respectively from emitter to collector.
Ovidiu (syn08) should be able to answer this question, as he is an expert in the field of modeling semi-conductors.

Cheers,
E.
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Old 29th June 2012, 12:29 PM   #547
wahab is offline wahab  Algeria
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It takes a few pico seconds for a device to witness a change at its output
related to a change of its input potential.
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Old 29th June 2012, 01:44 PM   #548
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Quote:
Originally Posted by Edmond Stuart View Post
The reason I emphasized on the difference between propagation delay and phase lag is because some people (NFB haters, for example) erroneously think that in an amplifier the FB signal should always come 'too late', i.e. time delayed and therefore it should be bad.
As also explained by Jan , this isn't the case.
It occurred to me that a way to explain it would be to show a phase shifted output and then equalize the phase shift out of it - to show that it is completely restored and there is no delay. Since we don't have a minus delay time machine in the equalizer then there never was a delay in the output.
As to my hope that
Perhaps if we use it correctly it will actually over-ride the incorrect use and help.
It didn't work in time
And thanks for the reference to Ovidiu

Best wishes
David

Last edited by Dave Zan; 29th June 2012 at 01:46 PM.
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Old 29th June 2012, 10:50 PM   #549
PHEONIX is offline PHEONIX  Australia
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Quote:
Originally Posted by Telstar View Post
Add to that that is impossible to measure this delay (is it? or just nobody cares?.

I also wonder, since people can hear jitter of few dozens picoseconds, these few nanoseconds of delay in the OPS should be recognizable as well. Maybe less since we are dealing with higher level signals, but still...

And, if we just exclude the OPS from the feedback loop, the NFB of the previous stages (assuming is multistage) of which order of magnitude is?

just my 2c from a non EE
Hello Telstar

Jitter is not delay, its caused by the noise of the of the oscillator (clock) at close in offsets.

Regards
Arthur
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Old 30th June 2012, 02:26 AM   #550
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Originally Posted by Edmond Stuart View Post
As also explained by Jan , this isn't the case.
Seems that you and Jan have not been able to make them understand over in that thread!
While there I saw the thumbnail of your current SuperTIS and I noticed there is not a TPC load on the super-pair, as recommended in Fig. 6 on your website. Any particular reason or you just decided to keep the circuit "simple"

Best wishes
David
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