Schematic for Servo opamp. - Page 2

sajti · 25th July 2003, 08:15 AM

My amplifier is DC coupled too. But the input stage is designed to be useful for DC servo. The first is paralell 22kohms, and then 2k2+68pF low pass filter, than connect the DC servo with 100kohms, directly to the base of the input LTPs.
I have 10kohms+8.2uF RC filter at the DC servo's output. With this values the maximum offset I can compensate:
290mV. It's enough for most application.

About the ringing: The ringing caused by the instability, due the low phase margin. If You push away the two filter's frequency, there will be large phase margin.

Sajti

capslock · 25th July 2003, 08:24 AM

Sonny,

sorry, didn't want to spoil the fun. And I am all in favor of building things from scratch where it makes sense to me. In this case, I didn't see the point, so I asked.

Regards,

Eric

sonnya · 25th July 2003, 08:38 AM

Quote:

Originally posted by sajti
My amplifier is DC coupled too. But the input stage is designed to be useful for DC servo. The first is paralell 22kohms, and then 2k2+68pF low pass filter, than connect the DC servo with 100kohms, directly to the base of the input LTPs.
I have 10kohms+8.2uF RC filter at the DC servo's output. With this values the maximum offset I can compensate:
290mV. It's enough for most application.

About the ringing: The ringing caused by the instability, due the low phase margin. If You push away the two filter's frequency, there will be large phase margin.

Sajti

Thanks for the tip on the input... have some how been blind for this.

The instability start at 180 degress phase shift.. right... how much phasemarging do you prefer? 60degress? ... I can follow you... just have to make some extra calculation...

sonnya · 25th July 2003, 08:51 AM

Quote:

Originally posted by capslock
Sonny,

sorry, didn't want to spoil the fun. And I am all in favor of building things from scratch where it makes sense to me. In this case, I didn't see the point, so I asked.

Regards,

Eric

Just wanted make a simple gain block every one could build, which could work as a servo, but also with very small changes resistor values could do as much faster opamp.

sonnya · 25th July 2003, 09:18 AM

Quote:

Originally posted by sonnya

The instability start at 180 degress phase shift.. right... how much phasemarging do you prefer? 60degress? ... I can follow you... just have to make some extra calculation...

I have just digged in my notes....

i can correct up to 10V offset on the output.

at a phase margin of 90degress i have a gainmargin of 1.3

sajti · 25th July 2003, 09:16 PM

For the phase margin: More is better. If You set the cutoff frequencies same for both filters, You can get 90 degree. It looks enough, but I like more than enough

Sajti

sajti · 25th July 2003, 09:19 PM

"i can correct up to 10V offset on the output."

It's more than enough, looks OK

It would be nice if You can show some schematics

Sajti

sonnya · 26th July 2003, 04:38 PM

it is soon to come....

When i have made a production PCB i will post a schematic...

I will then be able to offer the PCB, without components,with standard and premium grade components. As a kit or as a finished board.

Sonny

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25th July 2003, 08:15 AM	#11
sajti diyAudio Member Join Date: Apr 2003 Location: Vác, Hungary	My amplifier is DC coupled too. But the input stage is designed to be useful for DC servo. The first is paralell 22kohms, and then 2k2+68pF low pass filter, than connect the DC servo with 100kohms, directly to the base of the input LTPs. I have 10kohms+8.2uF RC filter at the DC servo's output. With this values the maximum offset I can compensate: 290mV. It's enough for most application. About the ringing: The ringing caused by the instability, due the low phase margin. If You push away the two filter's frequency, there will be large phase margin. Sajti

25th July 2003, 08:24 AM	#12
capslock diyAudio Member Join Date: Feb 2002 Location: Germany	Sonny, sorry, didn't want to spoil the fun. And I am all in favor of building things from scratch where it makes sense to me. In this case, I didn't see the point, so I asked. Regards, Eric

25th July 2003, 09:16 PM	#16
sajti diyAudio Member Join Date: Apr 2003 Location: Vác, Hungary	For the phase margin: More is better. If You set the cutoff frequencies same for both filters, You can get 90 degree. It looks enough, but I like more than enough Sajti

25th July 2003, 09:19 PM	#17
sajti diyAudio Member Join Date: Apr 2003 Location: Vác, Hungary	"i can correct up to 10V offset on the output." It's more than enough, looks OK It would be nice if You can show some schematics Sajti

26th July 2003, 04:38 PM	#18
sonnya diyAudio Member Join Date: Oct 2001 Location: Denmark	it is soon to come.... When i have made a production PCB i will post a schematic... I will then be able to offer the PCB, without components,with standard and premium grade components. As a kit or as a finished board. Sonny

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