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Old 12th September 2012, 09:18 PM   #2451
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Looking at BJT logarithmic amplifiers might provide some ays to stabilize bias ithout orrying about drift. For instance, look at figure 21 here:

http://sbisc.sharif.edu/~comcir/read...er-gilbert.pdf

As BJTs are logarithmic for several octaves I think they may be relatively accommodating, if e think like mathematicians. After all, the repeatable logarithmic behavior is the only reason e use current mirrors.

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Old 12th September 2012, 09:38 PM   #2452
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Nice paper less than 20 years old. But I have to admit some of todays technologies reinvent the wheel
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Old 12th September 2012, 11:47 PM   #2453
Struth is offline Struth  Canada
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Hi Guys

Doug, wouldn't having asymmetric loading on the comp-diffs cause asymmetric slew rates?

Have fun
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Old 13th September 2012, 07:06 PM   #2454
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Quote:
Originally Posted by Struth View Post
Hi Guys

Doug, wouldn't having asymmetric loading on the comp-diffs cause asymmetric slew rates?

Have fun
Kevin
Probably, but so long as both directions are fast enough, with a healthy safety margin, who cares?
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Old 13th September 2012, 07:22 PM   #2455
jcx is online now jcx  United States
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you automatically loose a factor of 1/2 of the current from the diff pair with R collector load driving one VAS vs the current mirrored diff pair

and some more from the current shunted by the collector R


I assume anyone who cares to use the complementary diff pair would find this unacceptable - even if not objectively justifiable

Last edited by jcx; 13th September 2012 at 07:29 PM.
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Old 13th September 2012, 09:50 PM   #2456
Struth is offline Struth  Canada
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Hi Guys

All of that is a given, jcx. Slew rate for most circuits I referred to above is often 60V/usec, so obviously anything above or asymmetric is likely of little concern.

It must have been Marshall Leach who did not like asymmetric signal paths because he felt they were harder to stabilise. I thought there might be a pithy comment about that, even though the asymmetry here is not so bad.

To me the comp-diff input + push-pull VAS + comp output falls in line with Otala's recommendations much better than his own circuit. It seems crazy that three gain stages within the loop would become the standard for hundreds of products, despite the difficulties of stabilising such a beast. Although it does reflect latter day tube practices which also employed three stages within the loop. (I don't count followers as gain stages since their gain is unity and nominally nonproblematic)

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Old 13th September 2012, 10:37 PM   #2457
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There is the possibility of combining to different types of transistors for the complementary VAS, say Jfet and BJT, and bias them so that nominally they have equal gain. This ay, due to the asymmetric transfer curves, hen bias creeps up, it also produces offset hich must be corrected in the feedback loop. This correction returns bias to normal. It seems that controlled asymmetry is the key to this idea, and combining devices ith different gm curves is a clever ay of achieving this ithout having a grossly unbalanced complementary VAS. Does anyone understand hat I'm saying?
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Old 14th September 2012, 10:08 AM   #2458
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Old 14th September 2012, 06:17 PM   #2459
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Don't forget the useful and versatile 1-transistor gyrator. Don't do this ith any high-voltage transistors, or transistors ith bad Vcesat behavior (2N5551...). I think 1N4148's generally have greater breakover than a BJT, but anyone building this ould need to make sure so that VAS quiescent isn't too high right after turn-on. Sort of ugly I kno. Remember you sa it here!

The RC can cut into LF OLG if not large enough. I have not examined this circuit for undesirable behavior, ho knos hat ill happen under clipping conditions. R1 may need to be much larger, not sure, but if it is it ill increase VAS bias tempco.
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Old 27th September 2012, 02:02 PM   #2460
Zero D is offline Zero D  United Kingdom
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Hi Bob, please see my PM

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