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In Reply to: Re: I fixed a bias problem in my Mosfet follower (experiment #1 continued) posted by mfc on July 01, 2003 at 18:58:45:
>What effect does feedback have when it isn't
exactly 180 degrees?<If it's 360 degrees it becomes an oscillator. If it's not exactly 180 degrees as most feedback loops get into near the point where the gain gets to one, the difference from 180 degrees is called the phase margin and this results in various amounts of overshoot on a transient response. So any deviation from 180 degrees of feedback is subject to some miscues in transient performance.
With respect to how much steady state distortion reduction you get, I believe you can take the real part as the sole contributor. The amount of imaginary part will have no help nor hindrance in that regard I believe because it does not oppose the signal nor increase the signal - it is "orthogonal" to the signal, which is therefore neutral in steady state conditions. I'm not reading any technical manual, I am just thinking out loud here. Scott will probably give us the exact answer to that if he is so inclined to do so.
Follow Ups:
I think the real part can show a negative feedback or a positive feedback. If for example the feedback is less than +/-90 degrees it can be considered positive feedback (negative resistance in the feedback loop) and if it is more than +/-90 degrees then it is negative feedback (positive resistance). Even with positive feedback it might not oscillate. For oscillation to occur the real feedback times the loop gain must be greater than or equal to one. Again, I don't think the imaginary part plays much of a role in the steady state.
Hi Kurt,I guess I should clarify. I wasn't referring to Nyquist or
Bode diagrams or any of the basics. I was more thinking about
phase modulation of the signal, when the feedback deviates
just slightly from perfect.Mike
When the phase modulates, IMD occurs. The spectrum of phase IMD contains the same frequency components as amplitude IMD, but their phases differ by pi/2. AIMD is usually associated with signal-induced gain variations (e.g., transistor beta nonlinearity); PIMD is usually associated with signal-induced variations in shunt C (e.g., Cc-b). If you want to crunch the numbers see Cherry, "Amplitude and Phase of Intermodulation Distortion," JAES, May 1983.For an interesting discussion of cathode followers vs anode followers driving capacitive loads at high f's, see Keen, "Anode-Follower Derivatives," Wireless Eng, Jan 1953. Hint: one does better on negative cycles, the other on positive cycles.
For cathode followers driving C with large signals see Cocking, "Cathode-Follower Dangers," Wireless World, Mar 1946.
Hi Scott,> The spectrum of phase IMD contains the same frequency components as
> amplitude IMD, but their phases differ by pi/2.Ahh, very interesting. Would the amplitude of PIMD depend on the
amount of phase deviation from 180?PIMD seems like a real interesting challenge to look at. I think
I'll try a few things and then report back. It would certainly be
worth reading those articles, thanks for providing the references.
Exactly along the lines of what I was hoping for. Do you get these
real old articles online, or thru a library?
Thanks,Mike
"Would the amplitude of PIMD depend on the amount of phase deviation from 180?"Mike, the amplitude of PIMD depends on the sensitivity of the amplifier to fractional changes in gain-BW product brought on by signal-induced capacitive variations in devices such as BJT's. You can see how this sort of thing relates to complex frequency and group delay. Think of it as signal-induced *changes* in group delay.
PIMD is seen most commonly in poorly-executed feedback amplifiers but can occur elsewhere. Wide open-loop BW amplifiers with high feedback factors reduce PIMD right along with all the other IMD's (excepting TIMD which has to be dealt with at the feedback summing junction). This is another argument for reducing driver impedance, as jc suggested earlier for another purpose.
"Do you get these real old articles online, or thru a library?"
I get 'em the old-fashioned way, by standing for hours over a hot mimeo-, er, photostat machine at the periodicals archive. A comparatively small number of vintage articles have appeared online. I had to go to the Imperial College basement archive (next to the Science Museum in Kensington) to get some of these British articles of c. 1920. Found some more at the Royal Society, and a few more at the London Patent Office. Mostly I get 'em closer to home: e.g., from SCU, SJSU, Stanford, and UCB.
Hi,> Think of it as signal-induced *changes* in group delay.
OK, I get the picture now. Thanks
It seems like there is a lot of rediscovering of old ideas that
have been forgotten. I guess you can just ignore them altogether and
mix things up and see what happens. I think it is fun to be able to
do both.
"It seems like there is a lot of rediscovering of old ideas that
have been forgotten."Yeh, but it's a helluva lot easier to rediscover an old idea than to discover it in the first place. The thing is, old ideas are "in the air". Even if the full-blooded idea no longer circulates, parts of it still exist to tickle the fancy and spark the "new" discovery.
What bothers me is people who don't look very hard for their precedents.
> I was more thinking about phase modulation of the signal, when the feedback deviates just slightly from perfect. <But I think it's the same situation. Phase modulation is a transient property, and goes away in the steady state condition. The phases will be altered by step conditions and ring and settle back to no phase modulation. Phase modulation is also a noise induced property, but that's another story, as noise introduces unsettled conditions constantly.
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