### Author Topic: op amp feedback resistor power rating  (Read 451 times)

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#### electrolust

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##### op amp feedback resistor power rating
« on: July 18, 2017, 04:31:25 am »
For a non-inverting op amp configuration, is it correct that the output current is RL || RF?  Let's say the output goes to the next stage amp, "infinite resistance", so then we can just consider RF by itself.

So say the output is 2.5V DC, and RF is 20k, then the current is V/R = 125µA and power is V*I = 3125µW?

#### hermit

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##### Re: op amp feedback resistor power rating
« Reply #1 on: July 18, 2017, 04:50:39 am »
Without doing the math (you seem to know the formulas).....   Opamps are generally voltage devices with minimal current so power ratings on feedback resistors aren't normally a concern.  If you are calculating a low current this would be normal.

#### Benta

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##### Re: op amp feedback resistor power rating
« Reply #2 on: July 18, 2017, 06:11:21 am »
For non-inverting, the current drawn by the feedback network is what runs through the feedback resistor in series with the ground resistor.
If you redraw the circuit, so the feedback network is a voltage divider from output to ground, it's apparent.

#### LabSpokane

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##### Re: op amp feedback resistor power rating
« Reply #3 on: July 18, 2017, 07:39:24 am »
A drawing will be of  help. I'm assuming from your description that RL is between the output and ground, but hate to guess.

#### mikerj

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##### Re: op amp feedback resistor power rating
« Reply #4 on: July 18, 2017, 07:47:44 am »
For a non-inverting op amp configuration, is it correct that the output current is RL || RF?  Let's say the output goes to the next stage amp, "infinite resistance", so then we can just consider RF by itself.

So say the output is 2.5V DC, and RF is 20k, then the current is V/R = 125µA and power is V*I = 3125µW?

You are a factor of ten out:

P=V^2/R

2.5^2/20000 = 312.5µW

#### Hero999

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##### Re: op amp feedback resistor power rating
« Reply #5 on: July 18, 2017, 07:53:19 am »
Generally there's no point in calculating the power dissipation in op-amp feedback resistors, because the power dissipation is usually far below the maximum rating of even tiny resistors. The only time I'd do it, is if the op-amp had a high voltage buffer, in the loop, although I'd try to keep the resistor values as high as possible, so power dissipation a non-issue.

Smf