having a hard time finding....

unclejed613 · 7th August 2007, 11:41 PM

an externally compensated FET input op amp. i have an amp design i've been modelling, and most of the time it's unstable, and compensating the VAS has no effect. i finally figured out that it's most likely that the op amps i'm using in the model are internally compensated, so compensating "after the fact" won't have an effect.
i've done searches on ti.com, linear.com, and national.com with no results.

gootee · 7th August 2007, 11:50 PM

You could also look at analog.com; one of the best.

Maybe I don't understand what you mean when you say that compensating an internally-compensated opamp won't have an effect. They're usually just internally compensated to be stable with a gain as low as 1. But you can still also compensate them externally.

A few details of your circuit might help to expose some other problem, in case there is one.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

djk · 8th August 2007, 03:30 AM

Use a slower opamp.

When using an opamp for the front end of a power amplifier, a slower opamp will give better stability when you close the loop around the power stage.

(can't remember if that came from Linear or Analog app notes).

7th August 2007, 11:41 PM	#1
unclejed613 diyAudio Member Join Date: Dec 2006	having a hard time finding.... an externally compensated FET input op amp. i have an amp design i've been modelling, and most of the time it's unstable, and compensating the VAS has no effect. i finally figured out that it's most likely that the op amps i'm using in the model are internally compensated, so compensating "after the fact" won't have an effect. i've done searches on ti.com, linear.com, and national.com with no results. __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: http://www1284177414881.v-dc.net/

8th August 2007, 03:30 AM	#3
djk diyAudio Member Join Date: Feb 2001 Location: USA	Use a slower opamp. When using an opamp for the front end of a power amplifier, a slower opamp will give better stability when you close the loop around the power stage. (can't remember if that came from Linear or Analog app notes). __________________ Candidates for the Darwin Award should not read this author.

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7th August 2007, 11:50 PM	#2
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	You could also look at analog.com; one of the best. Maybe I don't understand what you mean when you say that compensating an internally-compensated opamp won't have an effect. They're usually just internally compensated to be stable with a gain as low as 1. But you can still also compensate them externally. A few details of your circuit might help to expose some other problem, in case there is one. - Tom Gootee http://www.fullnet.com/~tomg/index.html

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