[sdiy] basic inverting op amp question 101

Next Expanse nextexpanse at gmail.com
Mon Dec 30 17:38:45 CET 2019


(On top of what has been said about current v noise and bias current...)

The output stage of op amps is generally class AB, and so greater loading
puts that A part of the section in a smaller part of the signal, and
therefore might cause more distortion. TL07x’s in particular are known to
get much better results for lower output currents. Doug Self published
somewhere (that I can’t find again) that in tests, a 22k feedback gave the
least THD + noise for the TL07x opamps. Unfortunately, (IIRC) he didn’t say
much about the current draw of the following stage, whether this was for
unity gain or something else, etc., so I’m not sure how useful that number
is. At any rate, it’s a decent ballpark figure. This is part of why the
TL07x often comes out worse in tests than people experience it in real
world circuits; most other audio opamps can do at least 10k if not 1K with
no degradation, and I’ve yet to see a comparative test of the TL07x that
gives it optimum loading.

Second thing (and I’m still chewing on this one so corrections are very
much appreciated), distortion aside, there’s actually an optimum feedback
impedance for the lowest noise characteristics for an op-amp, and that’s
equal to the voltage noise divided by the current noise. This doesn’t take
into account the Johnson noise of the resistor itself. Basically, you can
think about this by imagining a constant voltage at the inverting input,
tied via a resistor to another constant voltage. There’s then some current
flowing into the inverting node, which has to be equal to the current
flowing out of it, across the feedback resistor, to the output of the op
amp. That current plus the feedback resistance determines the output
voltage. Now think about voltage noise on the inverting node. Tiny changes
in voltage make tiny changes in current in accord with V/R=I. So the higher
the resistance, the less effect voltage noise has on the output.
Conversely, if we think about current noise, this is going to require
voltage changes on the output in accord with I R = V, so lower resistance
is going to give less effect to the current noise. The optimum is at R =
Vn/In. For ne5532, that’s about 6.8k, and you’ll see this value in a lot of
circuits. However, because this doesn’t take Johnson noise into account,
the value’s going to skew a bit lower, depending on total noise of the
opamp, gain, etc.

The good news is that rarely do you have to worry about any of this all
that much if you aren’t designing a mic preamp. Just don’t use anything too
low. Often people just use 10k for everything that isn’t a TL07x, and 50k
or so for those. Actually, in the synth world, people often just use 100k
for everything and they seem to be happy, but I wouldn’t go quite that far,
personally.

-Evan

On Mon, Dec 30, 2019 at 6:58 AM Spiros Makris <spirosmakris92 at gmail.com>
wrote:

> On top of what Tom has mentioned, the input impedance of the inverting
> stage is depended on one of those two resistors. Sometimes a high input
> impedance is needed and thus you are forced to either use larger resistors
> or more opamps, depending on the case.
> Furthermore, the bias input current of the opamp is converted into a
> voltage by passing through that input resistor- a higher resistor leads to
> a proportionately higher offset. In FET input opamps the bias currents are
> some pA and can usually be ignored, but BJT opamps will be prone to that
> and need some additional care.
>
> On Mon, Dec 30, 2019 at 4:07 PM Tom Wiltshire <tom at electricdruid.net>
> wrote:
>
>> At the simplest level, it’s a trade-off between current and noise. Lower
>> value resistors generate less noise, but use more current (and provide
>> lower input impedance) and higher value resistors do the reverse. So you
>> tend to see “compromise” values around 10K-100K. 1Meg is getting “big” and
>> 1K or less is getting “small”.
>>
>> Years ago I had the same question and since the internet didn’t exist
>> yet, I did some experiments on my breadboard with a 741 and discovered that
>> 1M upwards was noticeably noisier, but that anything between a few K and a
>> few hundred K didn’t really seem to make much odds.
>>
>> Tom
>>
>> ==================
>>        Electric Druid
>> Synth & Stompbox DIY
>> ==================
>>
>> On 30 Dec 2019, at 13:17, bbob <fluxmonk at gmail.com> wrote:
>>
>> (please excuse the noobiness)....
>>
>> in a basic inverting op-amp circuit, where the input and feedback
>> resistors are equal, the gain will be -1... but what are the pros/cons that
>> determine the choice resistor value?  i commonly see/have used 1k, 10k,
>> 47k, 100k in synth circuits, but what design considerations drive those
>> choices of values?   my immediate application is basic output buffers on a
>> LFO, and 1k seems to be working fine, but it got me to thinking (uh oh).
>>
>> thx, b
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