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Old 21st September 2011, 04:02 PM   #251
EUVL is offline EUVL  Europe
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Yes, AD1862 would be great.


Patrick
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Old 21st September 2011, 04:11 PM   #252
EUVL is offline EUVL  Europe
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Joachim,

The ES9018 has a low output impedance of about 800R.
So let's go as far as we can and aim to do Zin < 1R.
Since it is CEN, the PNP & NPN are in parallel for the signal.
That is why I chose E153 as current source (15mA bias), so that Zin is about 0R8.
This has the same function as your BF245 with trimmer.
E153 are widely used in Japan for powering LEDs.
I am sure Davide would be able to get them if necessary.

I was hoping to get zero DC offset by perfect matching.
Anyhow, the schematics demonstrates how it can fulfill all the functional requirements.



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Old 21st September 2011, 04:26 PM   #253
bcarso is offline bcarso  United States
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Joachim, I think you mean 2SC1815 for the NPN devices?
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Old 21st September 2011, 04:34 PM   #254
EUVL is offline EUVL  Europe
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well spotted.

Incidentally the reason why I chose BC550C / 560C is not only they are easy to source, but they have a much higher hfe than the Toshiba's.
That is also the same reason why I did not choose matched BJTs like MAT12 or BCM846.


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Old 21st September 2011, 05:03 PM   #255
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Yes, true. It is a 2SC1815. This is Holgers drawing and i did not double check it. Thanks.
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Old 21st September 2011, 05:14 PM   #256
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EUVL, the is always a small difference in Ube in PNP and NPN bipolars that give an offset in a parallel symmetric BJT transimpedance input. Unfortunately Ube is a week function of idle current so you can not trim it out by asymmetric bias. You could of cause select a pair of transistors that have the same Ube at the required idle current. Good luck. Actually i do not think that this ( small ) DC offset is a problem but on my thread the consensus was that even a small DC offset is not allowed, that in the context of amplifying an MC phono cartridge. I solved that problem with the local loop but curiously the same people that bugged me did not react. No response, nada. That happens on the web quite often. When the readers spot a "problem" it is beaten to death and when the solution comes than it is "obvious". That does not frustrate me in any way, quite the contrary. Without pushing me i get lazy. I will do some calculations how much input impedance you get with how much idle but your estimate sounds about right to me. As i said, low input impedance is the forte of a BJT common base stage with high idle.
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Old 21st September 2011, 05:15 PM   #257
bcarso is offline bcarso  United States
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Were they not getting scarce as proverbial hen's teeth, I'd recommend the Toshiba low rbb' parts, the 2SA1316 and 2SC3329. The blue grades claim 350 - 700 beta for both parts, which if one is lucky enough to get toward the high end of the range is pretty decent. My friend Ben G. bought bags of them some years ago and I don't believe ever used more than a few.

He did report being disappointed that they still had appreciable current noise, but digging a bit deeper it seemed to be well in line with expectations based on the operating point and the typical behavior of bipolar base current. They are not JFETs!

This reminds me of an anecdote, but I've muddied this thread enough already
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Old 21st September 2011, 05:20 PM   #258
EUVL is offline EUVL  Europe
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I also do not think the small offset is an issue.

An easy way to solve it is by trimming the resistor network that provides the Vref offset at the base of the input biasing pair.
ES9018 needs an offset of +1.65V (or 1/2 of +Vs analog).
So one can take the voltage off the regulator, divide by 2 with a potential divider, and trim one of them till the input voltage is zero at ideal.

I know it is better to solve the problem where is arises.
But solving the problem without adding 5 resistors is also quite attractive.

Don't you think ?


Patrick
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Old 21st September 2011, 05:21 PM   #259
EUVL is offline EUVL  Europe
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Don't you guys like the good old BC550 / 560 ?


Patrick
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Old 21st September 2011, 05:25 PM   #260
bcarso is offline bcarso  United States
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Quote:
Originally Posted by Joachim Gerhard View Post
EUVL, the is always a small difference in Ube in PNP and NPN bipolars that give an offset in a parallel symmetric BJT transimpedance input. Unfortunately Ube is a week function of idle current so you can not trim it out by asymmetric bias. You could of cause select a pair of transistors that have the same Ube at the required idle current. Good luck. ...
I was re-reading one of Linsley-Hood's books the other day, and he makes the controversial statement that P and N JFETs are much better complements than PNPs and NPNs. Well, I guess it all depends on what parameters one is concerned about. The mismatch in Vbe is certainly a nuisance for bipolars in many configurations. But the drastic difference in mobility of holes and electrons in majority-carrier devices leads to huge differences in the capacitances associated with comparable-transconductance JFETs. I'm still very glad we have both polarities available, but they are not very complementary.
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