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Old 7th August 2012, 08:36 PM   #31
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Quote:
Originally Posted by smms73 View Post
this topology is not like Dr.Leach. I dont see how to use the current mirror
Are you sure that you understand how it works?
If you can't figure out how to use a current mirror in this circuit after reading the Leach article, then I wonder how much you really understand the circuit you made.
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Old 7th August 2012, 09:26 PM   #32
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smms, that cap would short the bases of the input stage, so there is a low impedance path that lowers the noise. Only the emitter resistors and the base-emitters of the input transistors whould then be in the noise chain. Say we drive the input transistors on 5.5mA each and use BC327-BC337 we end up with 10 Ohm / 6 plus 30 Ohm / 6 = 6.666 Ohm.
That would be a very low noise input.
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Old 7th August 2012, 09:26 PM   #33
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I can try if that works in praxis.
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Old 7th August 2012, 09:39 PM   #34
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by dirkwright View Post
If you can't figure out how to use a current mirror in this circuit after reading the Leach article, then I wonder how much you really understand the circuit you made.
Sorry, i did not pay too much attention to the article. i see what you mean, i will try that, thanks.
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Old 7th August 2012, 10:36 PM   #35
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by Joachim Gerhard View Post
I can try if that works in praxis.
praxis is practice in german ?
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Old 7th August 2012, 10:46 PM   #36
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That was a typo sorry. I use a measurement program called "Praxis" so that intruded into my mind.
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Old 7th August 2012, 10:51 PM   #37
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By the way i find your circuit fascinating, i have not seen it exactly that way and i have seen many. I find it innovative whatever others say. I am just working to make it low noise into low impedances.
Donīt care about the thread jackers, that happens all the time when you enter an open forum.
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Old 8th August 2012, 06:11 AM   #38
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by smms73 View Post
Hi Zinsula,
i was told that the output impedance of the pcm1794 is 2M with 22pf, try to find in internet for the real value, but no luck .
[...]how do you come to 1k , i think is very low for a current source, if someone knows the real value please post.
Oh, I didn't realise, you are designing with a D/S DAC in mind.
I live in the stone age of R2R DAC's (PCM1704 / AD1862 et al). Their output impedance are in the range of 1 to 2kOhms. There is data posted at another forum.

Delta Sigma DAC like PCM1794 work differently and might have quite higher output impedance, but I have no idea how high it is. 2M seems a lot though.
Anyway, they seem to be less picky regarding the impedance they see at the output.
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Old 8th August 2012, 08:36 AM   #39
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by smms73 View Post
i'm going to try to explain how this topology works.
Its very simple, look at this image, we have a current source I2 , this current is divided by the input and output, if the input current increase 1ma the output must decrease 1ma, its similar to a differencial stage, there is no current gain, so this topology is more like a current mirror than a amplifier.
It's a folded cascode stage. Some even replace the ccs (I2 in this simplified schematic) with a resistor, as it forms a ccs anyway.

See here what I had in mind some years ago...you may also see how you can bias the input transistors.

It mirrors the input current to the high impedance node (collector).
I still do not really get what the transistors Q7 and 8 are doing special other than providing a low impedance biasing for the folded cascode transistors Q3 + 6 (kinda darlington folded cascode).
Can you enlighten me?
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Old 8th August 2012, 12:24 PM   #40
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Zinsula, i can not see your circuit because i am not a member of DIY.org
Can you post it here ?
The way i see the smms73 circuit is how he deals with the base currents. Q7 and Q8 pick up the base currents on the emitters and feed a portion in on the other side ( polarity ).
This is a kind of base current feedback.
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