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Old 5th April 2012, 06:45 AM   #22041
PMA is offline PMA  Europe
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Some simulation results.
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Old 5th April 2012, 07:58 AM   #22042
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PMA,

What your simulator shows when you add emitter degeneration resistors to the current mirrors? I am very curious...

Best,
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Old 5th April 2012, 08:00 AM   #22043
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Quote:
Originally Posted by john curl View Post
I might clarify what I mean by 'noise gain' or whatever it is called formally.
The first estimated example of this, I learned from a classroom handout of the UA741 op amp that had been analyzed by the best SPICE emulation at the time. It was shown that the bipolar active loads actually ADDED noise to the op amp, because they were not degenerated enough to kill their noise gain. This was a revelation to me, in 1971. I would have never thought it would be important, until it was discussed in class.
You see, we normally think of the input device(s) as the primary noise generator, because we normally use a resistive load. However, when we use an active load, then the load itself can amplify its own self noise, and sometimes overwhelm the input noise. The easiest solution is to add emitter resistors in order to reduce the intrinsic gain of the active load. However, too much emitter resistance will cause voltage drop problems that could effect the common mode range, or the maximum output swing of the circuit. This is where 'finesse' is required, to keep the potential problems almost negligible.
Interesting. Similar results were found by Ovidiu Popa in his Linear Audio Vol 1 article. He looked at the noise tradeoff in a low noise input stage with a CCS load with the usual FET + source resistor. He found that with a CCS you got (of course) much higher stage gain but also much higher stage noise because the CCS had a 'noise gain' of itself. So the S/N didn't change whether you used the CCS or - get this - a resistor of the value used to set the CCS current. Another nice example of noise gain indeed.

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Old 5th April 2012, 08:56 AM   #22044
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Quote:
Originally Posted by mullardel34 View Post
Also, since it's your design, have you ever assigned a name to the circuit (as in a Curl "curl")? I've read that Grey Rollins asked you for a circuit name several years ago.
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Old 5th April 2012, 09:05 AM   #22045
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That active loads amplify their own noise has been discussed in
the Gray&Maier text book on analog chip design some 30 or 40
years ago, at length.

(Meier sp??)

And noise gain is a well-defined terminus technicus in feedback theory,
it should not be abused here.

regards, Gerhard
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Old 5th April 2012, 09:17 AM   #22046
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Quote:
Originally Posted by elektroj View Post
PMA,

What your simulator shows when you add emitter degeneration resistors to the current mirrors? I am very curious...

Best,
As low value as possible in the output stage emitters. In simulation, the standing current is stable with temperature. It would need to build a real sample.

Noise simulation says 2.4nV/rtHz ref input.
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Last edited by PMA; 5th April 2012 at 09:20 AM.
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Old 5th April 2012, 11:38 AM   #22047
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Quote:
Originally Posted by gerhard View Post
That active loads amplify their own noise has been discussed in the Gray&Maier text book on analog chip design some 30 or 40
years ago, at length. (Meier sp??)
[snip]
Yes, it's very hard these days to find something that is not really 50+ years old, including live audio engineers ;-)

Quote:
Originally Posted by gerhard View Post
And noise gain is a well-defined terminus technicus in feedback theory, it should not be abused here.

regards, Gerhard
Agree again. I should have put it between quotes to avoid sloppy use. My bad (and John's! ).

jan
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Old 5th April 2012, 03:02 PM   #22048
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Thanks Gerhard. In fact, it was Dr. R.G. Meyer's class at UC Berkeley in 1971 that I learned this. I was very impressed at the time. I will look at my copy of G&M for the reference pages. Dr. Meyer, who is slightly younger than me, still taught me a great deal in these classes I attended more than 40 years ago.
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Old 5th April 2012, 03:12 PM   #22049
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I found the pages in Grey and Meyer. They are: 761-768. Of course, I learned in in class before the book was written from the class notes.
If you really want an example of noise problems due to active loading, analyze a UA740 sometime (no that is not a typo). '-)
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Old 5th April 2012, 05:46 PM   #22050
bcarso is offline bcarso  United States
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Groner also has a discussion of current mirror noise in his critical review of one of Self's books.

If one thinks of it qualitatively, a unballasted current mirror is an example of a "translinear" circuit, where the nonlinearity of the diode-connected transistor predistorts the input signal to the other device. A diode-connected transistor is a shunt-feedback amplifier, and has the noise of the transistor, the base-spreading resistance thermal noise and the "half-thermal" of the equivalent emitter resistance. These r.m.s.-add to determine the voltage at the collector-base connection.

Now this is the input to the output device, which converts it to a current at the collector in accordance with its transconductance. And this device also has its own noise. Overall the two contributions are about equal.

The effect of such a current mirror on the overall differential stage noise is to raise the net noise current at the output of the stage by 3dB, for an unballasted differential pair. If, otoh, the pair has emitter degeneration, the degradation is correspondingly higher. And Samuel points out that sometimes Self errs a bit on too-low ballasting in his current mirrors for overall "blameless" noise performance.

Now, as JC has mentioned, one does not simply make the mirror resistors indefinitely large as this can cut into the available voltage swing. As well, resistors slow the mirror down, so there is also a tradeoff with overall bandwidth. In the typical current-feedback amplifiers the schematics at least usually show no ballasting (they also show the use of Wilson 3-device mirrors, which have their own tradeoffs).


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