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Old 20th April 2010, 10:45 AM   #1201
bocka is offline bocka  Germany
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Joachim

I took a closer look to your circuit and I'm not shure if it works correctly. When you inject about 40ma through T6, T3 must source the same amount of current. This current has to be delivered by R1/T7 from the positive power supply. As you are using a current mirror for your jfet the current is limited to the LTP current source from T4/T5. With 180 ohms on the emitter this current source will deliver 3.5ma max. In this way I think T6 will saturate. In addition (when the current for T6 is less than T4) the jfet diff amp is never balanced as the additional current for T6 has always delivered by T7.

Just my 2 ct.

Last edited by bocka; 20th April 2010 at 10:47 AM.
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Old 20th April 2010, 03:26 PM   #1202
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Yes, i think you are right. An output buffer is anavoidable if i like to drive a low impedance RIAA. I will opt for the doube foldet cascode topology you are using and take the current mirrors away.
Thanks for helping me.
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Old 20th April 2010, 04:25 PM   #1203
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Here is my second try. Do you think this will work better ?
I am not sure about the open loop compensation.
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File Type: pdf Discrete Opamp 2.0.TSC - TINA.pdf (61.9 KB, 137 views)
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Old 20th April 2010, 06:24 PM   #1204
bocka is offline bocka  Germany
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Yes, I think this op-amp should work fine. You don't need a lot of open loop compensation. My folded cascodes work fine with as little as 47pF for C3, but without this cap the amp oscillates. You should use a little more current for T3/T10, maybe 5ma in this case (R1/R4 = 220R). In this way both transistors are always class-A biased.

In addition you can drive the LSK389C much harder. When you set the current to 20ma (R1 = 33R) or so and increase the current for T3/T10 to 20 - 30ma (R1/R4 = 39R) I think you can omit the output buffer. Although the JFETs are slightly forward biased under worst condition in this way, I wouldn't expect any problems. Maybe John Curl knows more about this.

When doing so I'd also change T3, T10, T12, T13 and T14 to BD139 / BD140 or similar, as the power dissipation rises to about 0.3 - 0.5w for each transistor. A little bit too much for the BC550 / BC560. This is also the reason for T12. When you omit this transistor T3 dissipates twice the power of T10 so slight imbalances can occur when warmed up.
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Old 20th April 2010, 07:49 PM   #1205
jeepee is offline jeepee  Netherlands
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Default Equivalent input noise current

Hi Bocka
Quote:
Originally Posted by bocka View Post
I'm working on something similar as I/V stage for a DAC.
Did you consider the noise generated by R1, R2, R17 & R18 in your I/V stage?

Since the cascode and folded cascode stage have a current gain of 1x, these resistors effectively appear parallel to the input signal of the amplifier. The combined parallel resistance is only 34 Ohm [1/(2/100+2/220)], resulting in 40nV/sqrtHz at the output of the amplifier (R_IV=1.8K). From my own simulation experiments I found it is beneficial to have huge voltage degeneration for those current sources (>1kOhm), using higher rail voltages (e.g. 15V to 20V).

PS: this is a great thread, a source of inspiration.

Last edited by jeepee; 20th April 2010 at 08:01 PM.
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Old 20th April 2010, 07:51 PM   #1206
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I asume you mean R2 = 33 Ohm and not R1 = 33 Ohm ?
Is there enough voltage gain in the Fet stage left when R1/R4 are as low as 39 Ohm ?
Thanks again for good sugestions. This way the project goes forward very fast.
I am very happy about that. Another Diy fellow is doing a Microcap simulation until tomorrow. This is a great forum with a lot of knowledgeable people here.
Could i also use 2N4401 / 4403 instead of BC550C/BC560C. They are a bit more robust and i find the BD139/140 kind of big in terms of size. Of cause you already recommended similar devices in smaller packages.
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Old 20th April 2010, 07:55 PM   #1207
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Whould it be another option to use more LEDs for higher voltage over R1 / R4 ?
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Old 20th April 2010, 09:21 PM   #1208
bocka is offline bocka  Germany
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Quote:
Originally Posted by Joachim Gerhard View Post
I asume you mean R2 = 33 Ohm and not R1 = 33 Ohm ?
Yes, that's a typo.
Quote:
Originally Posted by Joachim Gerhard View Post
Is there enough voltage gain in the Fet stage left when R1/R4 are as low as 39 Ohm ?
As the jfet drives a common base transistor the input inpedance of this cascode is only 2 ohms or so. But your right, 39 ohms looks very low on a first view. Maybe a current source is better here. I don't know, maybe it's worth trying both variants. More LEDs are worth trying, too.

The 2N4401/4403 looks fine especially when driving currents in the range of 30ma. Seems to be a good choice.

@jeepee
No, I did't make a noise analysis yet, as my posted circuit is currently conceptional only. I did it some days ago and it's not tested. It has some other weakness, too. PSRR is not optimal. But I think this can be solved. But you're right, 40nV/sqrtHz is a little bit high.
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Old 20th April 2010, 09:42 PM   #1209
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Quote:
Originally Posted by bocka View Post
Yes, that's a typo.
As the jfet drives a common base transistor the input inpedance of this cascode is only 2 ohms or so. But your right, 39 ohms looks very low on a first view. Maybe a current source is better here. I don't know, maybe it's worth trying both variants. More LEDs are worth trying, too.

The 2N4401/4403 looks fine especially when driving currents in the range of 30ma. Seems to be a good choice.

@jeepee
No, I did't make a noise analysis yet, as my posted circuit is currently conceptional only. I did it some days ago and it's not tested. It has some other weakness, too. PSRR is not optimal. But I think this can be solved. But you're right, 40nV/sqrtHz is a little bit high.
Thanks again Bocka. I will start to build something and try to find out the best solution.
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Old 21st April 2010, 02:59 PM   #1210
bocka is offline bocka  Germany
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As my circuit is not phono related I've started a new dac i/v thread here. This should it make easier to seperate the discussion.

@Joachim
I think you will also have some noise issues when you lower the values as I've suggested. The more voltage you have across the emitter resistors of the current sources or the current mirror the better. jeepee is right here. In this way I'd choose the LED string.
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