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PostPosted: Fri Jul 20, 2007 6:46 am 
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Much more enlightening, these ones… R1/R2=27R.
I will come back later to the CM5 and maybe a few others mirrors configurations.
I focus on low level higher order harmonics

CM1
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CM2
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PostPosted: Fri Jul 20, 2007 6:48 am 
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CM3
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CM4
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 Post subject: As promised...
PostPosted: Fri Jul 20, 2007 5:57 pm 
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... my plots.

All the circuits are the symmetrical ones, small BC transistors, biased at some 3-4mA, +/- 19V supply. A +/-2mA dumped into input, 1k2 to develop the voltage at the output. The diamond structure is chosen for driving capability of its "emitter" input. (These circuits are actually I/V converters so they have to deal with real world DAC output impedances.)

First, the simple mirror as shown previously, 1k both resistors (I actually already posted this to my site), less than that makes the current through the output transistor too small and less than 250R with 3mA makes it really not to work, so:

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Performance (THD 0.012%):

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Practically the same circuit but with four transistors Wilson (“CM3” here) and no emitter degeneration. As opposed to the simple circuit, all the Wilsons here were biased by current sources and not by resistors. The last graph here actually shows a bit better result than that from above, however results I've been getting were not that consistent (I mean between the samples measured, not from time to time), regardless of all the effort put in the parts matching. (Later I moved to Toshiba transistors which proved not only to sound but also to measure better than BC, at least in circuits like this.) Please ignore artifacts at 9kHz, 11kHz and about 20kHz in these graphs, they are not related to this circuit.

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Four transistors Wilson, 100R resistors.

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Four transistors Wilson, 300R resistors.

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PostPosted: Fri Jul 20, 2007 6:59 pm 
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That's a nice simple circuit, Pedja. Has anyone ever tried the OTA section from the OPA660? It's pretty much the same thing, but with the benefit of matched transistors interleaved on one die. It's also really, really fast, which could mean it was optimized for speed, not linearity. Anyway, would make for an interesting comparison.

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PostPosted: Sat Jul 21, 2007 1:18 pm 
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Thanks, Jim. I’ve never tried OPA660 because at the moment I finally intended to do that I realized that it was just going out of production. As I got a bit tired of working with obsolete parts, I’ve given up. (Speaking about this, MAX435 and MAX436, which are the similar parts, are now also obsolete.) I’m however these days fiddling with OPA861 which is unbuffered successor of OPA660.

As for the IC transistor matching and linearity, AD844, which is also similar (diode connected first transistors instead of the real diamond structure), also varies a bit from sample to sample, its current mirrors can show from 0.3% to 0.7% THD under the same conditions as described above. Stereophile measurements of Ayre CX-7 point out the same issue. Anyhow, taking into account internal bias of AD844 (6.5mA for all the arrays), I am convinced that this part can easily do much better, only if someone at AD would like to make possible external biasing. I am not sure they think in this direction though. Other than that, from conversation with Maxim, I indeed realized that linearity wasn’t any priority in case of their 435/436 and for the audio they simply recommend use of feedback.


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PostPosted: Sat Jul 21, 2007 6:50 pm 
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Pedja wrote:
As for the IC transistor matching and linearity, AD844, which is also similar (diode connected first transistors instead of the real diamond structure), also varies a bit from sample to sample, its current mirrors can show from 0.3% to 0.7% THD under the same conditions as described above.

Sorry, it had to read "from 0.03% to 0.07% THD".


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