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Choosing an op-amp for your project

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Re: Choosing an op-amp for your project

#11

Post by stephen_g »

Majromax wrote: post #9
peteforde wrote: post #7

I've watched the videos and agree that the OPA2323 does appear to sound amazing by subjective comparison to the TL071. However, it's not enough to say that it costs ~8.7x as much in local tokens because that doesn't reflect how much it costs as a percentage of the other components around it, many of which have a more profound impact on the outcome. This wouldn't stand as a good argument except that in many devices, y…

Is the cost delta really 12×? Looking at Mouser Canada, it looks like the cheapest 071 is the TL071CDR, at $0.138/each (Canadian) in quantities of 5k. The OPA2323IDDFR is $0.49 in the same quantity. > In the end, a 12x cost bump is a lot to swallow for a feature that most consumers are physically incapable of distinguishing between. I think that the performance of an op-amp should very rarely have user-visible effect…

Yes, and if you are actually buying in that kind of quantity you should be able to do even better. TI’s budgetary pricing estimate is US$0.252 per 1K for the OPA2323IDDFR and US$0.067 per 1K for the TL071CDR.

Re: Choosing an op-amp for your project

#12

Post by Archit3ch »

bratwurst3000 wrote: post #6
perriera wrote: post #5

>f you’re designing a new op-amp circuit, here are some decent, all-around alternatives made in the 21st century None of the listed opamps would be an alternative to where an LM324 or TL071 are typically used - low cost, 20V-30V supply, infinite/guaranteed availability. They are 5V only and cost 10x as much.

i build and repair audio electronics and the tl071 is used in abundance in audio circuits and is a bad choice imho. But good opamps for audio are at least a few dollars

If you want the mirror the OpAmp distortion from the original units for authenticity/character, it makes sense to use the TL071, no?

Re: Choosing an op-amp for your project

#13

Post by spamatica »

bratwurst3000 wrote: post #6
perriera wrote: post #5

>f you’re designing a new op-amp circuit, here are some decent, all-around alternatives made in the 21st century None of the listed opamps would be an alternative to where an LM324 or TL071 are typically used - low cost, 20V-30V supply, infinite/guaranteed availability. They are 5V only and cost 10x as much.

i build and repair audio electronics and the tl071 is used in abundance in audio circuits and is a bad choice imho. But good opamps for audio are at least a few dollars

Any suggestion for a (reasonably) good and cheap one for new designs?

Re: Choosing an op-amp for your project

#14

Post by dragontamer »

Vos doesn't matter until it does. I find it surprising to see Vos mentioned as likely unimportant right next to CMRR, as CMRR and Vos are innately related in some circuits.

In particular, any low voltage current sense circuit is going to require very precise Vos. Let's say you have a 0.01 Ohm current-sense resistor on a 5-Amp or so circuit.

Your current-sense is now in the range of +/- 0.05Volts (!!!!). So a Vos of 0.005 would represent a 10% error. Likely too much for most applications.

In effect, CMRR has become hyper-sensitive to Vos in this particular use case, to the point that Vos is suddenly the most important statistic.

Fortunately, there are specially designed low-offset chopper or auto-zeroing OpAmps like MCP6V26 or whatever out there.

MCP6V26 has Vos of 2uV, or in relatable terms... 0.000002 Volts (!!!!). Meaning it is more than sufficient at reliably making this current-sense application. Indeed, you can drop down to 0.0001Ohm resistance and still have high accuracy (and power savings compared to the earlier assumption).

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Alas, nothing is ever free in life. Chopper Amps have noise issues and other designs have very very low bandwidth (which is truly an important statistic for most circuits).

Choosing a chopper amp specifically is making a Vos tradeoff with Bandwidth. So only choose if you know what you are doing (aka, dealing with very low voltages and needing the precise zero).

Re: Choosing an op-amp for your project

#16

Post by dragontamer »

kragen wrote: post #8

I found this post extremely interesting and informative, well above my expectations even given the eminence of its author. I'm not sure that everything in it is covered in The Art of Electronics — it's the kind of stuff the book covers, but I learned things from this post I didn't learn from the book. (But possibly it's information that's in the book that I just failed to absorb the last few times.) I was recently lo…

> Then I decided I'd screwed up my design sketch by requiring one of the opamps to sink significant current very close to the negative rail, which is something even "rail-to-rail" opamps can't do; I was planning to use millivolts from ground to represent measured nanoamps.

Get an OpAmp specifically designed for current sense applications.

OpAmps for current sense applications have high accuracy near 0 Volts and Vos measured in single digit microvolts.

Oh, you WILL lose bandwidth with these designs. So make sure you are allowed to be much much slower.

Re: Choosing an op-amp for your project

#17

Post by Cervisia »

kragen wrote: post #8

I found this post extremely interesting and informative, well above my expectations even given the eminence of its author. I'm not sure that everything in it is covered in The Art of Electronics — it's the kind of stuff the book covers, but I learned things from this post I didn't learn from the book. (But possibly it's information that's in the book that I just failed to absorb the last few times.) I was recently lo…

Nowadays, it is hard to recommend a general-purpose opamp. Just plug the desired parameters into the search function and sort what's left by price.

(Distributors like DigiKey and Mouser have somewhat adequate search functions; I usually have to go to manufacturers' web sites like https://www.ti.com/amplifier-circuit/op-amps/general-purpose... to be able to filter by all important parameters. I'm mentioning TI because they have a large selection and a good search; even when you do not end up selecting on of theirs, you see what is possible.)

___

If you need only a small negative supply and have nothing else, the LM7705 charge pump can generate −0.23 V. (This is designed to fit into the typically allowed 5.5 V range of a nominal 5 V opamp.)

I do not know what a "significant current" is for you, but there are opamps with strong outputs. (When comparing opamps, you usually have to estimate the drive strength from the short-circuit current.)

Re: Choosing an op-amp for your project

#18

Post by nine_k »

dragontamer wrote: post #14

Vos doesn't matter until it does. I find it surprising to see Vos mentioned as likely unimportant right next to CMRR, as CMRR and Vos are innately related in some circuits. In particular, any low voltage current sense circuit is going to require very precise Vos. Let's say you have a 0.01 Ohm current-sense resistor on a 5-Amp or so circuit. Your current-sense is now in the range of +/- 0.05Volts (!!!!). So a Vos of 0…

For current measurement, I'd rather use a Hall sensor. But to measure the signal from that sensor a sensitive opamp is needed, and your comment applies :)

Re: Choosing an op-amp for your project

#19

Post by shermantanktop »

Archit3ch wrote: post #12
bratwurst3000 wrote: post #6

Earlier quoted context omitted.

i build and repair audio electronics and the tl071 is used in abundance in audio circuits and is a bad choice imho. But good opamps for audio are at least a few dollars

If you want the mirror the OpAmp distortion from the original units for authenticity/character, it makes sense to use the TL071, no?

Based on my experience with vintage music electronics, it wouldn’t be good enough to use a component that just sounds bad like a TL071; it needs to be a TL071 so that the marketing copy can claim “vintage-correct parts.”

At that point you are engineering with a completely different set of tradeoffs than would be expected for a modern ground-up design effort.

Re: Choosing an op-amp for your project

#20

Post by rasz »

peteforde wrote: post #10
Majromax wrote: post #9

Earlier quoted context omitted.

Is the cost delta really 12×? Looking at Mouser Canada, it looks like the cheapest 071 is the TL071CDR, at $0.138/each (Canadian) in quantities of 5k. The OPA2323IDDFR is $0.49 in the same quantity. > In the end, a 12x cost bump is a lot to swallow for a feature that most consumers are physically incapable of distinguishing between. I think that the performance of an op-amp should very rarely have user-visible effect…

All good points! And I am no expert. FWIW, here is what I was going off of, price wise: https://www.digikey.ca/en/products/detail/texas-instruments/... https://www.digikey.ca/en/products/detail/texas-instruments/... In other words, the cheapest TL071 variant and the cheapest OPA2323 variant on Digikey Canada, in a quantity of 1 (ie wildly expensive). $0.31 vs $2.70 means that I shouldn't attempt math before coffee; 8…

If you are making a one-off $3 is laughable non issue, you will spend more for a lunch drink. If you are manufacturing something in the thousands the difference goes down to 0.252 vs 0.067 so merely $0.2 BOM bump. Also a no brainer if performance is on the line.
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