# Opamp resistor magnitude

Started by April 28, 2014
```I have a question about the magnitude of the two feedback resistors on
opamps.  I am familiar with how to determine the ratio of the
resistors to get the desired gain, whether inverting or not.  What I'm
trying to figure out is what determines the magnitude of the resistors
involved?  I've seen different circuits with resistors of, say
thousands, tens or hundered of thousands and even one that used
Megohm-range resistors.
Why the variation in magnitude of the resistors, assuming a fixed
ratio?  Does it have anything to do with the overall impedance of the
opamp circuit block?
I thank you for whatever illumination that can be provided.

GP
```
```On 2014-04-28, Gone Postal <gone_postal@it.doesn't.exist> wrote:
> I have a question about the magnitude of the two feedback resistors on
> opamps.  I am familiar with how to determine the ratio of the
> resistors to get the desired gain, whether inverting or not.  What I'm
> trying to figure out is what determines the magnitude of the resistors
> involved?

mainly the impedance of the input.
the imput will have high resistance but not infinite.

--
umop apisdn

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
```
```"Gone Postal" <
>
>I have a question about the magnitude of the two feedback resistors on
> opamps.  I am familiar with how to determine the ratio of the
> resistors to get the desired gain, whether inverting or not.  What I'm
> trying to figure out is what determines the magnitude of the resistors
> involved?  I've seen different circuits with resistors of, say
> thousands, tens or hundered of thousands and even one that used
> Megohm-range resistors.
> Why the variation in magnitude of the resistors, assuming a fixed
> ratio?  Does it have anything to do with the overall impedance of the
> opamp circuit block?
> I thank you for whatever illumination that can be provided.
>

** Choosing a particular resistor value for a given spot in a circuit is a
major part of electronics design -  so your Q is a mile wide and goes way
beyond just op-amps.

Try narrowing it down a bit.

....    Phil

```
```On Mon, 28 Apr 2014 16:29:05 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

>
>"Gone Postal" <
>>
>>I have a question about the magnitude of the two feedback resistors on
>> opamps.  I am familiar with how to determine the ratio of the
>> resistors to get the desired gain, whether inverting or not.  What I'm
>> trying to figure out is what determines the magnitude of the resistors
>> involved?  I've seen different circuits with resistors of, say
>> thousands, tens or hundered of thousands and even one that used
>> Megohm-range resistors.
>> Why the variation in magnitude of the resistors, assuming a fixed
>> ratio?  Does it have anything to do with the overall impedance of the
>> opamp circuit block?
>> I thank you for whatever illumination that can be provided.
>>
>
>** Choosing a particular resistor value for a given spot in a circuit is a
>major part of electronics design -  so your Q is a mile wide and goes way
>beyond just op-amps.
>
>Try narrowing it down a bit.
>
>
>
>....    Phil
>
>
>
Thank you Phil.
Apparently I understand less than I thought (a common occurrence), I
had been under the impression my question was fairly narrow, but I can
(sort of) see how other circuit considerations would affect the choice
of specific resistors.  I will spend some more time with LT Spice and
see what I come up with.

GP
```
```On 28 Apr 2014 06:13:27 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

>On 2014-04-28, Gone Postal <gone_postal@it.doesn't.exist> wrote:
>> I have a question about the magnitude of the two feedback resistors on
>> opamps.  I am familiar with how to determine the ratio of the
>> resistors to get the desired gain, whether inverting or not.  What I'm
>> trying to figure out is what determines the magnitude of the resistors
>> involved?
>
>mainly the impedance of the input.
>the imput will have high resistance but not infinite.

Thanks for the answer, that's what I was generally getting at.

GP
```
```"Gone Postal"
"Phil Allison"
>>
>>"Gone Postal"
>>>
>>>I have a question about the magnitude of the two feedback resistors on
>>> opamps.  I am familiar with how to determine the ratio of the
>>> resistors to get the desired gain, whether inverting or not.  What I'm
>>> trying to figure out is what determines the magnitude of the resistors
>>> involved?  I've seen different circuits with resistors of, say
>>> thousands, tens or hundered of thousands and even one that used
>>> Megohm-range resistors.
>>> Why the variation in magnitude of the resistors, assuming a fixed
>>> ratio?  Does it have anything to do with the overall impedance of the
>>> opamp circuit block?
>>> I thank you for whatever illumination that can be provided.
>>>
>>
>>** Choosing a particular resistor value for a given spot in a circuit is a
>>major part of electronics design -  so your Q is a mile wide and goes way
>>beyond just op-amps.
>>
>>Try narrowing it down a bit.
>>
>>

>>
> Thank you Phil.
> Apparently I understand less than I thought (a common occurrence), I
> had been under the impression my question was fairly narrow,

**  Ignorance is bliss.

> but I can
> (sort of) see how other circuit considerations would affect the choice
> of specific resistors.  I will spend some more time with LT Spice and
> see what I come up with.

....  Phil

```
```On Mon, 28 Apr 2014 19:50:26 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

>
>"Gone Postal"
> "Phil Allison"
>>>
>>>"Gone Postal"
>>>>
>>>>I have a question about the magnitude of the two feedback resistors on
>>>> opamps.  I am familiar with how to determine the ratio of the
>>>> resistors to get the desired gain, whether inverting or not.  What I'm
>>>> trying to figure out is what determines the magnitude of the resistors
>>>> involved?  I've seen different circuits with resistors of, say
>>>> thousands, tens or hundered of thousands and even one that used
>>>> Megohm-range resistors.
>>>> Why the variation in magnitude of the resistors, assuming a fixed
>>>> ratio?  Does it have anything to do with the overall impedance of the
>>>> opamp circuit block?
>>>> I thank you for whatever illumination that can be provided.
>>>>
>>>
>>>** Choosing a particular resistor value for a given spot in a circuit is a
>>>major part of electronics design -  so your Q is a mile wide and goes way
>>>beyond just op-amps.
>>>
>>>Try narrowing it down a bit.
>>>
>>>
>
>>>
>> Thank you Phil.
>> Apparently I understand less than I thought (a common occurrence), I
>> had been under the impression my question was fairly narrow,
>
>**  Ignorance is bliss.
>
>
>> but I can
>> (sort of) see how other circuit considerations would affect the choice
>> of specific resistors.  I will spend some more time with LT Spice and
>> see what I come up with.
>
>
>
>....  Phil
>
>
Do you think playing with Spice is the wrong approach?  If so, what
would be a better approach to understanding this?

GP
```
```"Gone Postal  "
"Phil Allison"
>>"Gone Postal"
>> "Phil Allison"
>>>>
>>>>"Gone Postal"
>>>>>
>>>>>I have a question about the magnitude of the two feedback resistors on
>>>>> opamps.  I am familiar with how to determine the ratio of the
>>>>> resistors to get the desired gain, whether inverting or not.  What I'm
>>>>> trying to figure out is what determines the magnitude of the resistors
>>>>> involved?  I've seen different circuits with resistors of, say
>>>>> thousands, tens or hundered of thousands and even one that used
>>>>> Megohm-range resistors.
>>>>> Why the variation in magnitude of the resistors, assuming a fixed
>>>>> ratio?  Does it have anything to do with the overall impedance of the
>>>>> opamp circuit block?
>>>>> I thank you for whatever illumination that can be provided.
>>>>>
>>>>
>>>>** Choosing a particular resistor value for a given spot in a circuit is
>>>>a
>>>>major part of electronics design -  so your Q is a mile wide and goes
>>>>way
>>>>beyond just op-amps.
>>>>
>>>>Try narrowing it down a bit.
>>>>
>>>>
>>
>>>>
>>> Thank you Phil.
>>> Apparently I understand less than I thought (a common occurrence), I
>>> had been under the impression my question was fairly narrow,
>>
>>**  Ignorance is bliss.
>>
>>
>>> but I can
>>> (sort of) see how other circuit considerations would affect the choice
>>> of specific resistors.  I will spend some more time with LT Spice and
>>> see what I come up with.
>>
>>
>>
>>
> Do you think playing with Spice is the wrong approach?

** What was your first clue?

> If so, what
> would be a better approach to understanding this?

opinions.

...  Phil

```
```On Monday, April 28, 2014 12:44:23 AM UTC-4, Gone Postal wrote:
> I have a question about the magnitude of the two feedback resistors on
> opamps.  I am familiar with how to determine the ratio of the
> resistors to get the desired gain, whether inverting or not.  What I'm
> trying to figure out is what determines the magnitude of the resistors
> involved?  I've seen different circuits with resistors of, say
> thousands, tens or hundered of thousands and even one that used
> Megohm-range resistors.
> Why the variation in magnitude of the resistors, assuming a fixed
> ratio?  Does it have anything to do with the overall impedance of the
> opamp circuit block?
> I thank you for whatever illumination that can be provided.
>
I think 10k ohm resistors are my favorite. (then 1 k ohm)
In general I'd like to choose R's that are smaller.
First this tends to reduce the effects of stray capacitance.
(Some RC corner that move to higher frequency with smaller R)
And then if you care, smaller R has less noise.. though most of the time that doesn't matter much.
Now of course you can't make 'em too small.
First the opamp mya not have enough poop to drive it.
and second if the R's get very small (a few ohms) you then
start to worry about the resistance of the traces and contacts.

George H.

>
> GP

```
```On Mon, 28 Apr 2014 06:04:10 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Monday, April 28, 2014 12:44:23 AM UTC-4, Gone Postal wrote:
>> I have a question about the magnitude of the two feedback resistors on
>> opamps.  I am familiar with how to determine the ratio of the
>> resistors to get the desired gain, whether inverting or not.  What I'm
>> trying to figure out is what determines the magnitude of the resistors
>> involved?  I've seen different circuits with resistors of, say
>> thousands, tens or hundered of thousands and even one that used
>> Megohm-range resistors.
>> Why the variation in magnitude of the resistors, assuming a fixed
>> ratio?  Does it have anything to do with the overall impedance of the
>> opamp circuit block?
>> I thank you for whatever illumination that can be provided.
>>
>I think 10k ohm resistors are my favorite. (then 1 k ohm)
>In general I'd like to choose R's that are smaller.
>First this tends to reduce the effects of stray capacitance.
>(Some RC corner that move to higher frequency with smaller R)
>And then if you care, smaller R has less noise.. though most of the time that doesn't matter much.
>Now of course you can't make 'em too small.
>First the opamp mya not have enough poop to drive it.
>and second if the R's get very small (a few ohms) you then
>start to worry about the resistance of the traces and contacts.
>
>George H.
>
>>
>> GP
Thank you George.  I've been following that as a general rule of thumb
(staying in the huuped singled K and the general 10's of Kohm range),
but I was seeing circuits that had other orders of magnatude and I was
just starting to wonder why so high, in particular with two of three
circuits I saw that just struck me as a wee bid odd..

GP
```