Hi dirkwright,
I don't think they are similar.
If i remove R1 and R2 , it will be difficult to control the current variation due to temperature, doubling up the input BJT's is a valid choice, and i think will lower the noise.
Its a very good solution , have you already construct that? don't you have problems with stability (low phase margin)?
OK. I think that using the current mirror version will regulate the current, won't it? Also, the matching of the transistors plays a major role in this circuit.
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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.
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.
Attachments
I read it in a paper by Analog Devices. They may have been referring to an older DAC architecture, I don't know.
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?
Mouser sells some dual nxp transistores that have 98% matching hfe and vbe for under 0.2 euros
the input is a common base amplifier, and of course that there is something mysterious
... the way that q7 and q8 are connected, without them is impossible to achieve such low distortion.Hi Joachim
Unfortunately, i don't know anything about MC phono stage, but if you tell me what you need, i will be happy to help.
You can shunt r9, but i really don't see why.
the input is a common base amplifier, and of course that there is something mysterious
Q7 and Q8 provide bias to the bases of Q6 and Q3, and also appear to receive a feedback signal on their collectors. There's nothing magical about it, though I may not completely understand it.
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.
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.
That would be a very low noise input.
Sorry, i did not pay too much attention to the article. i see what you mean, i will try that, thanks.
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.
Don´t care about the thread jackers, that happens all the time when you enter an open forum.
Don´t care about the thread jackers, that happens all the time when you enter an open forum.
Oh, I didn't realise, you are designing with a D/S DAC in mind.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 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.
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?
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.
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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