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Old 22nd July 2012, 09:56 PM   #1
kees52 is offline kees52  Netherlands
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Default current servo not voltage.

Hi All

I have a hybrid who use a servo for offset nulling, it works trough the cathode follower input, it do work fine, but.

I did read something about a current servo who has less impact on sound eventually, I did this tryout by using a fet between the current source tube cathode and gate, see schematic, if i do simulate it it works fine, but in the real world not, I did see that I eventually has to change tho opamp inverting input, because on the negative side of the amp there is no inverting or how I do call that in english. ps I did try to invert the opamp input, no result.

maybe some has a idea, voltage servo,s do have impact, and so I want to try what current does.

thanks.
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Last edited by kees52; 22nd July 2012 at 10:22 PM.
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Old 22nd July 2012, 10:18 PM   #2
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Hi Kees,

It looks to me like you either need some kind of level shifter between your op-amp and your PMOS, or you need to replace your PMOS source follower with something that can work with a control voltage low enough for the op-amp.

Your present circuit as an op-amp of which the output can't swing below -16 V. It drives a PMOS source follower whose source must end up a few volts above -120 V. That means its gate has to swing close to -120 V, while it can't go below -16 V.

If you are satisfied with a very limited control range, maybe you can put a resistive voltage divider between the op-amp and the source follower: a big resistor from op-amp to PMOS gate and a much smaller resistor from PMOS gate to -120 V.

Regards,
Marcel
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Old 22nd July 2012, 10:27 PM   #3
kees52 is offline kees52  Netherlands
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Quote:
Originally Posted by MarcelvdG View Post
Hi Kees,

It looks to me like you either need some kind of level shifter between your op-amp and your PMOS, or you need to replace your PMOS source follower with something that can work with a control voltage low enough for the op-amp.

Your present circuit as an op-amp of which the output can't swing below -16 V. It drives a PMOS source follower whose source must end up a few volts above -120 V. That means its gate has to swing close to -120 V, while it can't go below -16 V.

If you are satisfied with a very limited control range, maybe you can put a resistive voltage divider between the op-amp and the source follower: a big resistor from op-amp to PMOS gate and a much smaller resistor from PMOS gate to -120 V.

Regards,
Marcel
Hi Marcel

And if I try a drain follower there? or even a bipolair transistor, a level shifter feeding with the -120 volts can work best?

I don,t now if I am right with the impact, but I think current way of nulling offset is better.

Be the way, do you agree that windows 7 get,s not better after updates, I have crashes the last time, 3 times i did lost a schematic (blue screen) and reset of bios, but maybe you like me not so pc minded.

The schematic in this post is the version I did use for some years now. R31 is not used in present scheme.

thanks.
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Last edited by kees52; 22nd July 2012 at 10:35 PM.
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Old 22nd July 2012, 11:27 PM   #4
kees52 is offline kees52  Netherlands
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The last test, now I go sleep. coming week, I go think further.

i have now use also a bjt and a fet in negative supply.. R7 is grounded.

and now time for dreaming.
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Old 23rd July 2012, 07:25 AM   #5
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some food for thought as they say.
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Last edited by jerluwoo; 23rd July 2012 at 07:35 AM.
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Old 23rd July 2012, 09:39 AM   #6
kees52 is offline kees52  Netherlands
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Originally Posted by jerluwoo View Post
some food for thought as they say.
If I do that, then cathode resistor of lower tube 100 ohm has 47 volts that is much, but I think the simulation is not oke.

use a zener work, but there is 47 volts on cathode for 21 mA current, maybe a voltage driven current source is a good way to go,
maybe a hybrid current source who can be voltage driven.

amp is dc coupled and current need 20 mA for driving mosfets, you have a scheme who has much lower current, but I presume
that it is as a example.

Last edited by kees52; 23rd July 2012 at 09:52 AM.
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Old 23rd July 2012, 12:25 PM   #7
kees52 is offline kees52  Netherlands
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I have test now with a P mosfet, it,s measurement give now 67 uV on schematic.

A hybdride current source will maybe the way to go, it is now a kind of the same
but to simpel, anyway it do work in multisim, but not in real.
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Old 24th July 2012, 03:43 PM   #8
kees52 is offline kees52  Netherlands
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Hi All again.

I have done it this way (see picture) a mosfet to eart and let it snoop some current from the tube ccs cathode, but maybe a danger, if the mosfet fails the resistors will burn, but this afcourse is only if mosfet turn fully on, that is why a resistor of 1 M is from gate to earth, what do you think, it has a offset with it at 8 uV on simulation, but like first in real time what will it do.

thanks
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Old 24th July 2012, 07:09 PM   #9
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Hello Kees,

I see two inversions in your loop (op-amp and PMOS Q5), so doesn't this circuit latch up?

If you replace the integrator with a non-inverting one, then it should work, or at least then I don't see a reason why it shouldn't anymore. You can then protect the op-amp against excessive input voltages by using a resistive voltage divider between the amplifier output and the plus input of the op-amp.

By the way, you could add a resistor in the drain lead of Q5 to limit the current if it should somehow get into the fault condition you described.

Best regards,
Marcel

Last edited by MarcelvdG; 24th July 2012 at 07:12 PM.
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Old 24th July 2012, 07:37 PM   #10
kees52 is offline kees52  Netherlands
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Quote:
Originally Posted by MarcelvdG View Post
Hello Kees,

I see two inversions in your loop (op-amp and PMOS Q5), so doesn't this circuit latch up?

If you replace the integrator with a non-inverting one, then it should work, or at least then I don't see a reason why it shouldn't anymore. You can then protect the op-amp against excessive input voltages by using a resistive voltage divider between the amplifier output and the plus input of the op-amp.

By the way, you could add a resistor in the drain lead of Q5 to limit the current if it should somehow get into the fault condition you described.

Best regards,
Marcel
Hi Marcel

I have only simulate it there it did work but maybe if the offset get positive it don,t.

I have see it, I will make the opamp non inverting it is easy then simulate again, a n fet is not the option.

thanks
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