Integrator in DC servo

Ipanema · 17th May 2005, 04:19 AM

Hi,

Someone pls explain to me

1) why we need an integrator in DC servo instead of just an error correction amplifier?

2) How to implement DC servo/offset compensation in a fully balance system?

An example would be much appreciated.

Thanks.

PRR · 17th May 2005, 04:43 AM

> why we need an integrator in DC servo instead of just an error correction amplifier?

As far as the "error correction" is concerned, our audio is an "error". It would try to reduce this error to zero. No audio output.

What we really want is: DC is an error, audio is not an error. The simplest way to define what is DC and what is audio is with a low-pass filter. And the most extreme low-pass is the integrator, because it tends to infinitely high gain (error reduction) at infinitely low frequency (DC).

> How to implement DC servo/offset compensation in a fully balance system?

You have errors in two terms, you need two servos. There may be some very clever way to share parts between two servos, but it still has at least two inputs, two outputs, and two gain functions. If you are not going into mass production, two or even three servos is the most direct and maybe-successful way to approach it.

peranders · 17th May 2005, 05:16 AM

I agree with Mr. R but one way to avoid those kind of errors is to use precision parts. Do you have any particular mind?

amplifierguru · 17th May 2005, 05:29 AM

You don't need servos at all if you take care of offsets/tracking in the design. The best servo is no servo.

Ipanema · 17th May 2005, 06:57 AM

Hi PRR,

Thanks for your reply. Do you have any examples which have offset cancellation in full balance amplifier? So far I have only seen single ended version.

My concern for offset cancellation in full balance design is to cancel any offset cause by device mismatch in differential amplifier (more severe if high gain), where the usual common-mode feedback (CMFB) is unable to correct. Hope you can understand my desciption.

Thanks.

jcx · 17th May 2005, 03:34 PM

I show a bias scheme for a current mirrored complemetary dual diff pair input amp in:

optoisolator VAS bias for Comp Diff?

current mirror loading assures Ic balance to within a couple of Ib, easily better than 1% Ic match, this should give sub mV input refered offset V - with well matched input Qs

In DIY you have the luxury of discounting labor in favor of craftmanship so matching input transistors should be expected

Manual trimming which is anathema to the production engineer is another DIY option; small emitter degeneration R can be trimmed for 0 offset V somewhat independant of Ic balance (which can/should be tirmmed depending on topology)

17th May 2005, 04:19 AM	#1
Ipanema diyAudio Member Join Date: Jun 2004 Location: KL	Integrator in DC servo Hi, Someone pls explain to me 1) why we need an integrator in DC servo instead of just an error correction amplifier? 2) How to implement DC servo/offset compensation in a fully balance system? An example would be much appreciated. Thanks.

17th May 2005, 05:16 AM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	I agree with Mr. R but one way to avoid those kind of errors is to use precision parts. Do you have any particular mind? __________________ /Per-Anders (my first name) or P-A as my friends call me

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17th May 2005, 04:43 AM	#2
PRR diyAudio Member Join Date: Jun 2003 Location: USA	> why we need an integrator in DC servo instead of just an error correction amplifier? As far as the "error correction" is concerned, our audio is an "error". It would try to reduce this error to zero. No audio output. What we really want is: DC is an error, audio is not an error. The simplest way to define what is DC and what is audio is with a low-pass filter. And the most extreme low-pass is the integrator, because it tends to infinitely high gain (error reduction) at infinitely low frequency (DC). > How to implement DC servo/offset compensation in a fully balance system? You have errors in two terms, you need two servos. There may be some very clever way to share parts between two servos, but it still has at least two inputs, two outputs, and two gain functions. If you are not going into mass production, two or even three servos is the most direct and maybe-successful way to approach it.

17th May 2005, 05:29 AM	#4
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	You don't need servos at all if you take care of offsets/tracking in the design. The best servo is no servo.

17th May 2005, 06:57 AM	#5
Ipanema diyAudio Member Join Date: Jun 2004 Location: KL	Hi PRR, Thanks for your reply. Do you have any examples which have offset cancellation in full balance amplifier? So far I have only seen single ended version. My concern for offset cancellation in full balance design is to cancel any offset cause by device mismatch in differential amplifier (more severe if high gain), where the usual common-mode feedback (CMFB) is unable to correct. Hope you can understand my desciption. Thanks.

17th May 2005, 03:34 PM	#6
jcx diyAudio Member Join Date: Feb 2003 Location: ..	I show a bias scheme for a current mirrored complemetary dual diff pair input amp in: optoisolator VAS bias for Comp Diff? current mirror loading assures Ic balance to within a couple of Ib, easily better than 1% Ic match, this should give sub mV input refered offset V - with well matched input Qs In DIY you have the luxury of discounting labor in favor of craftmanship so matching input transistors should be expected Manual trimming which is anathema to the production engineer is another DIY option; small emitter degeneration R can be trimmed for 0 offset V somewhat independant of Ic balance (which can/should be tirmmed depending on topology)

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