dac I/V convertion with very low distortion

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[...]Can you post it here ?[...]
OK here it is...balanced, so one leg can be eliminated if one wants to.


[...]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.
Thank you, now I have to think about this, later...
 

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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.
Don´t care about the thread jackers, that happens all the time when you enter an open forum.


thanks Joachim

Fell free to make the changes you want.;)
 
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?

Yes, Is a of folded cascode.

the folded cascode have already low distortion, the key for a even lower distortion is to use very high impedance ccs (a resistor is a bad choice) , and to take care of the distortion caused by the base currents.

As Joachim said, q7 and q8 compensate for the distortion of the base currentes , by going to get current to the opposite side.
hope you understand, Its very difficult for me to explain better, as english is not my natural language (i have some help for google traducter :eek:).
 
with a op amp in the input to lower the input impedance.
I don't mind use op amp , as long as the load impedance is very high ,as in this case.

PS: is no need for the base resistors r18 and r27 and is better to put a 1 ohm resistor between input node and the source, for preventing that capacitors in the source degrade the phase margin of the op amp.
 

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I like that version even better. Now you return the base currents of the input stage too and it is very low noise. Noise may come in from the CCSs. the reason is that R3 and R4 are rather low. In my experience they should be at least 100 Ohm. That will provide that the noise of the CCS transistor is not amplified. Output impedance of the CCS will also be higher in proportion of that resistor. One way is to insert a green Led in the emitters of Q11 and Q12. That will make the voltage over R3, R4 aound 2.7V, the optimum value after Tietze-Schenk. For 50mA we end up with a bit over 50 Ohm for R3, R4. Not 100 Ohm but an improvement.
 
I like that version even better. Now you return the base currents of the input stage too and it is very low noise. Noise may come in from the CCSs. the reason is that R3 and R4 are rather low. In my experience they should be at least 100 Ohm. That will provide that the noise of the CCS transistor is not amplified. Output impedance of the CCS will also be higher in proportion of that resistor. One way is to insert a green Led in the emitters of Q11 and Q12. That will make the voltage over R3, R4 aound 2.7V, the optimum value after Tietze-Schenk. For 50mA we end up with a bit over 50 Ohm for R3, R4. Not 100 Ohm but an improvement.

i agree with you, a low noise voltage reference can also be used instead of the led (lower serie resistance) .

the input offset trimming will be made in the base of q2, q5.
 
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The DACs that you like have DC current on their output pins which will upset the balance of your circuit. The PCM1794a for example puts out -6.2mA of DC current. Something has to be done about that as well.

do you know why?
Yesterday i was think about this, and i have a theory . They do this for biasing the ne5534 in class a, in this way they decrease the thd of the circuit, so changig this amp op with another type can increase distortion.

what you guys think?
 
do you know why?
Yesterday i was think about this, and i have a theory . They do this for biasing the ne5534 in class a, in this way they decrease the thd of the circuit, so changig this amp op with another type can increase distortion.

what you guys think?

No, I don't have a clue why they do that, and I doubt it is for the reason you suggest. All I know is that it will seriously upset this kind of circuit, since I have sim'd it myself.
 
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