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Discrete opamp offset trim
Discrete opamp offset trim
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Old 13th February 2020, 10:37 AM   #21
RCruz is offline RCruz  Switzerland
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Discrete opamp offset trim
Hi stein
I chose jfet ips because i need very high input impedance and need to be free to choose feedback circuit independent of input circuit without issues regarding output offset..... and i do prefer jfet sound
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Old 13th February 2020, 12:26 PM   #22
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Discrete opamp offset trim
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Originally Posted by stinius View Post
Hi Ricardo
Thank you, but to see it I have to install LTSpice and that is not going to happen.
I have looked on some of your previous posts and can see your "design".
I think you should try a 10k pot. with the wiper connected at the positive rail and the element between R1 and R2 see the drawing, that should null the DC offset at the output.
I am trying to avoid any imbalance in the currents of the mirror as I found these are determinant for low THD
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(This i my opinion: I would not null the offset like Damian is suggesting and in case you intend to use a DC servo I would not connect it to the negative input.)
Why would you not use the idea provided by Damian ?
Where would you connect the servo injecting current ?
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Originally Posted by stinius View Post

One question why do you select this design and what is the reason you have chosed to use JFET ips instead of BJT?

In my opinion, if you for some reasons prefere JFET ips instead of BJT there are much better ways to design a circuit with lower noise and THD that will fit into a 990 package.

Stein
Would you please suggest a better way to design this DOPA ?

I built it this way because I am familiar with the LIN topology and know how to reduce THD in a blameless type design...
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Old 13th February 2020, 03:12 PM   #23
RCruz is offline RCruz  Switzerland
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Discrete opamp offset trim
I simmed your connection and it works without affecting THD.... Thank you so much

I see that circuit is provided in the OPA627... I added the 2k resistors to avoid accidental large offset when setting the trimmer to one of it's ends.... what would be the best resistor combination to ease the nulling offset trim ?
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Last edited by RCruz; 13th February 2020 at 03:14 PM.
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Old 13th February 2020, 03:19 PM   #24
RCruz is offline RCruz  Switzerland
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Discrete opamp offset trim
Increasing the pot value just makes trimming easier
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Old 14th February 2020, 09:27 PM   #25
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[QUOTE=RCruz;6082849]I simmed your connection and it works without affecting THD.... Thank you so much

I'm glad I could help you.

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Increasing the pot value just makes trimming easier
Yes that is right.

Stein
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Old 14th February 2020, 11:15 PM   #26
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Forgot to mention that it will have close to zero impact on the noise.
Stein
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Old 15th February 2020, 09:18 AM   #27
RCruz is offline RCruz  Switzerland
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Discrete opamp offset trim
I am now populating another board with the modified trimmer arrangement and without servo... hope it works as expected.
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Old 15th February 2020, 03:51 PM   #28
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Quote:
Originally Posted by RCruz View Post
I am now populating another board with the modified trimmer arrangement and without servo... hope it works as expected.
I see no reason why it shouldn't work.
It's a good idea to use a multi-turn pot and trim the DC when the transistors have reached the working temterature.

Stein
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Old 16th February 2020, 10:28 AM   #29
RCruz is offline RCruz  Switzerland
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Discrete opamp offset trim
I do not have enough KSC1845 for the CCS.... can I use 2SC2240 instead ?
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Old 16th February 2020, 05:23 PM   #30
RCruz is offline RCruz  Switzerland
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Discrete opamp offset trim
It works with the 2sc2240 but it becomes prone to oscillations @ 8.5MHz.... cured with an increased input resistor.

I can now null ops offset ... stable @ 0.1mV
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Last edited by RCruz; 16th February 2020 at 05:26 PM.
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