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Old 16th August 2012, 10:31 AM   #201
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by Joachim Gerhard View Post
Something practical. LowDist I/V MC nearly ready.
Hope it don´t end up to be a oscillator, you have to slow it down, do not forget.
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Old 16th August 2012, 11:54 AM   #202
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Yes, the circuit i build makes some trouble. I have one or two ideas to try though.
I will publish what i have at the moment. Maybe somebody has an idea how it can be made to work.
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Old 17th August 2012, 12:05 PM   #203
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I have decided to follow Calvin suggestion of using a current source to cancel the dac offset current.
The current offset and dc out is now control by a R29 (that will be a 1k resistor in serie with 1k potentiometer ).

I have added C8/R25 C10/R26 C4/R24 for stability.
And now every thing seems to be very stable, the most sensible nodes are the nodes named UP and DOWN.
If you wish, the input zobel can be omited, but i keep them, because i feel it add stability.

The main current sources ware also changed (v5 and V4 will be two green led in serie).
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Old 17th August 2012, 12:10 PM   #204
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Quote:
Originally Posted by Joachim Gerhard View Post
Yes, the circuit i build makes some trouble. I have one or two ideas to try though.
I will publish what i have at the moment. Maybe somebody has an idea how it can be made to work.
What type of troubles ?
I will be glad to help if i can.

I start already to make the pcb for the circuit above.
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Old 17th August 2012, 12:16 PM   #205
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I bassically build your input stage but with a floating single supply like the Zen,Cen.
Ether the circuit does not work this way and needs a plus-minus supply or i have problems with stability. The scope shows some wild waveforms.
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Old 17th August 2012, 12:31 PM   #206
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Below are the frequency response of the circuit before the modifications and after.
As you see the old version start to misbehave above 20Mhz With two enormous peaks at 33Mhz and 88Mhz .

The new version has a much stable frequency response

Note that this frequency response are measure without the output capacitor C1. The last figure is the response with the output capacitor.
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Old 17th August 2012, 12:34 PM   #207
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Quote:
Originally Posted by Joachim Gerhard View Post
I bassically build your input stage but with a floating single supply like the Zen,Cen.
Ether the circuit does not work this way and needs a plus-minus supply or i have problems with stability. The scope shows some wild waveforms.
if you post a schema maybe i can help.

Edit: I have to take a better look to the sen/cen circuit as it seems are a lot of people interested.
If i understand you . you change the jfets by the bjt´s.

Last edited by smms73; 17th August 2012 at 12:47 PM.
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Old 17th August 2012, 01:07 PM   #208
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Is this what you intend to do ?
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Old 17th August 2012, 01:50 PM   #209
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Yes, that is the circuit. I reduced R13, R14 to 2kOhm and put a ballast resistor after the electrolytics of 220 Ohm. The electrolytics are 470uF Nichicon bipolar.
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Old 17th August 2012, 01:51 PM   #210
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I see you have also a ballast resistor R6.
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