Output resistor in feedbackloop?

Lovan · 13th April 2007, 08:16 AM

I'm making a general headphone-amp using an opamp and stumbled upon the question if the outputresistor should be in the feedbackloop or outside?

/Niclas

AndrewT · 13th April 2007, 09:02 AM

Hi,
are you referring to the resistor that sets the output impedance and also gives a resistive load to improve stability and a bit of short circuit/muting protection?
Then it goes outside the loop.

If the resistor is inside the loop then NFB tries to reduce it's effect to near zero.

Lovan · 13th April 2007, 10:46 AM

Hi Andrew

Exactly my thoughts. Thanks for confirming them!

/Niclas

jcx · 14th April 2007, 05:21 PM

you can have it both ways

resistive feeback around the series R reduces ouput impedance at low frequencies

C feedback from the chip output can dominate the feedback at high frequencies where the series R isolates the op amp output from destabilizing C load

more complicated with typical power chip amps that are not unity gain stable but still possible

13th April 2007, 08:16 AM	#1
Lovan diyAudio Member Join Date: Oct 2003 Location: Sweden	Output resistor in feedbackloop? I'm making a general headphone-amp using an opamp and stumbled upon the question if the outputresistor should be in the feedbackloop or outside? /Niclas

13th April 2007, 09:02 AM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, are you referring to the resistor that sets the output impedance and also gives a resistive load to improve stability and a bit of short circuit/muting protection? Then it goes outside the loop. If the resistor is inside the loop then NFB tries to reduce it's effect to near zero. __________________ regards Andrew T.

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13th April 2007, 10:46 AM	#3
Lovan diyAudio Member Join Date: Oct 2003 Location: Sweden	Hi Andrew Exactly my thoughts. Thanks for confirming them! /Niclas

14th April 2007, 05:21 PM	#4
jcx diyAudio Member Join Date: Feb 2003 Location: ..	you can have it both ways resistive feeback around the series R reduces ouput impedance at low frequencies C feedback from the chip output can dominate the feedback at high frequencies where the series R isolates the op amp output from destabilizing C load more complicated with typical power chip amps that are not unity gain stable but still possible

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