Help with DC Servo for Current Feedback Opamp - Page 2

klewis · 26th June 2009, 12:24 AM

Since the non-invert resistor has a much smaller value on a CF amp vs VF amp, I thought I might reduce the resistor values in the servo by an order of magnitude in the inverting servo. I've made a run with this configuration and it seems to correct the offset. I will try to build this configuration tonight and see what happens.

klewis · 26th June 2009, 12:34 AM

here is the plot for the prior schematic

klewis · 26th June 2009, 03:42 AM

Well, that didn't work> I got +4.3v DC offset when i fired it up. Tried several resistor values in place of r22, though r20 was set at 1k.

klewis · 28th June 2009, 08:37 PM

any other thoughts? help...

pacificblue · 28th June 2009, 10:48 PM

The more I think about it the more I come to the conclusion that a DC servo won't work with a current feedback amp. The DC servo gives a voltage feedback, not a current feedback. You will have to inject a current into the non-inverting input to achieve an offset compensation.

I. e. connect a potentiometer (trimmer) between + and -. To the wiper connect a small capacitor to ground and a resistor to the non-inverting input. With the potentiometer you set the voltage to which the capacitor will be charged. The capacitor discharges through the resistor and amplifier's input stage to ground, which leads to the compensation current. Adjust with the potentiometer.

analog_sa · 28th June 2009, 11:55 PM

Quote:

Originally posted by pacificblue
The more I think about it the more I come to the conclusion that a DC servo won't work with a current feedback amp.

Seems to work plenty well with AD815 judging by the number of servoed designs...

klewis · 29th June 2009, 12:30 AM

Quote:

Originally posted by analog_sa

Seems to work plenty well with AD815 judging by the number of servoed designs...

Analog,

can you point me to a one?

thanks,

Ken

klewis · 29th June 2009, 12:36 AM

Quote:

Originally posted by pacificblue
The more I think about it the more I come to the conclusion that a DC servo won't work with a current feedback amp. The DC servo gives a voltage feedback, not a current feedback. You will have to inject a current into the non-inverting input to achieve an offset compensation.

I. e. connect a potentiometer (trimmer) between + and -. To the wiper connect a small capacitor to ground and a resistor to the non-inverting input. With the potentiometer you set the voltage to which the capacitor will be charged. The capacitor discharges through the resistor and amplifier's input stage to ground, which leads to the compensation current. Adjust with the potentiometer.

Pacificblue,

Thanks for thinking about this. I got a servo to work, the values for the resistors are quite different from servos for vf amps. But the resistor value changes move the Fc way up to around 1Khz. Which then boost the lower frequencies - not good. So, I'm currently simulating a third stage in the servo to bring the voltages back up... it's sort of working. I'll build one next weekend.

Ken

sonidos · 21st May 2010, 08:46 PM

klewis,

I started playing with CFB op amps, too. Any luck on your servo after a year? I contacted Nat Semi and they sent me right back over here. ha!

jcx · 21st May 2010, 09:43 PM

by wrapping the CFA in the fedback loop of a good "audio" op amp you can get your "DC Servo" action for free and gain other advantages from the multiloop composite amplifier

compensation is a little more challenging if you want to use more of the CFA gain in the loop but doing so "improves" the performance of the input op amp as well

Walt Jung describes several "multiloop" composit amplifier circuits in his "Op Amp Applications" book
http://www.analog.com/static/importe...0Ch6_final.pdf

28th June 2009, 10:48 PM	#15
pacificblue diyAudio Member Join Date: May 2008	The more I think about it the more I come to the conclusion that a DC servo won't work with a current feedback amp. The DC servo gives a voltage feedback, not a current feedback. You will have to inject a current into the non-inverting input to achieve an offset compensation. I. e. connect a potentiometer (trimmer) between + and -. To the wiper connect a small capacitor to ground and a resistor to the non-inverting input. With the potentiometer you set the voltage to which the capacitor will be charged. The capacitor discharges through the resistor and amplifier's input stage to ground, which leads to the compensation current. Adjust with the potentiometer. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

21st May 2010, 08:46 PM	#19
sonidos diyAudio Member Join Date: Jul 2007 Location: Round Rock, TX	any luck? klewis, I started playing with CFB op amps, too. Any luck on your servo after a year? I contacted Nat Semi and they sent me right back over here. ha!

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26th June 2009, 03:42 AM	#13
klewis diyAudio Member Join Date: Sep 2007 Blog Entries: 36	Well, that didn't work> I got +4.3v DC offset when i fired it up. Tried several resistor values in place of r22, though r20 was set at 1k.

28th June 2009, 08:37 PM	#14
klewis diyAudio Member Join Date: Sep 2007 Blog Entries: 36	any other thoughts? help...

21st May 2010, 09:43 PM	#20
jcx diyAudio Member Join Date: Feb 2003 Location: ..	by wrapping the CFA in the fedback loop of a good "audio" op amp you can get your "DC Servo" action for free and gain other advantages from the multiloop composite amplifier compensation is a little more challenging if you want to use more of the CFA gain in the loop but doing so "improves" the performance of the input op amp as well Walt Jung describes several "multiloop" composit amplifier circuits in his "Op Amp Applications" book http://www.analog.com/static/importe...0Ch6_final.pdf

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