Simple enough question. Why not use a 741 op-amp in a target circuit or anyone's target circuit? What are the reasons not to use it? What might be the reasons to still choose this part?
There are many good reasons not to use the 1968-vintage LM741: -
- Minimum recommended power supply rails are +/- 10 volts
- Modern op-amps have power supplies that can be as low as 1.8 volts.
- Input voltage range is typically from -Vs + 2 volt to +Vs - 2 volt
- Modern op-amps can be chosen that are rail-to-rail
- Input offset voltage is typically 1 mV (5 mV maximum)
- Modern op-amps can easily be as low as a few micro volts and have low drift.
- Input offset current is typically 20 nA (200 nA maximum)
- Modern op-amps are commonly available that are less than 100 pA
- Input bias current is typically 80 nA (500 nA maximum)
- Modern op-amps are commonly less than 1 nA
- Input resistance is typically 2 MΩ (300 kΩ minimum)
- Modern input resistance starts at hundreds of MΩ
- Typical output voltage swing is -Vs + 1 volt to +Vs - 1 volt
- Many cheap rail-to-rail op-amps get to their supplies within a few mV
- Guaranteed output voltage swing is -Vs + 3 volt to +Vs - 3 volt
- Supply current is typically 1.7 mA (2.8 mA maximum)
- Modern op-amps with this current consumption are ten times faster and better in many other ways too.
- Noise is 60 nV/sqrt(Hz) for LM348 (quad version of 741)
- GBWP is 1 MHz with a slew rate of 0.5 V/us
The LM741A is slightly better but still a dinosaur in most areas.
Things of importance that the 741 data sheet does not appear to list (and that may depend on the age and manufacturer): -
- Input offset voltage drift versus temperature
- Input offset current drift versus temperature
- Common mode rejection ratio versus frequency
- Output resistance (closed or open loop)
- Phase margin
- likeliness of latchup (and gain reversal)
I can't think of any valid reasons to use the 741 other than "that's all I will ever have or own". Common reasons why they are still used in actual devices appear to be: -
- Someone had a design that they didn't want to change from the 70s
- Someone had millions of them lying around and wanted to put them to use
- Someone actually determined that all the parameters are fine for their design, and at that moment the 741 was the cheapest to acquire and in millions of units it saved a few thousand dollars in total.
I've been an electronics designer since 1980 and I have never used or specified a 741 in any design I've been associated with. Maybe I'm missing out on something?
If there weren't loads of textbooks dating back decades that used the 741 as an example, I'd be surprised if many people knew of it now. It'd have passed into history, like the OC71 which was the BC108 of the mid/late 60s.
One of the reasons I think it persisted for so long beyond the mid-80s, when it was well superseded, was because of its many bad characteristics. It's a good example for teaching students the characteristics to be concerned about, those weaknesses that highlight the inner workings of the op-amp. It has the significantly-large input offset voltages and input currents, needs the offset null, shows the bandwidth/gain product so vividly. Few/no op-amps of 20 years past have brandished so many of these so prominently. In modern devices, many of these characteristics are negligible or practically non-existent.
I've been designing for 30 years, starting with mid-volume equipment in the mid-80s. In my experience, the 741 wasn't anyone's choice then or since - there were always much better and cheaper parts to use. I imagine maybe I've worked on a circuit with one on in that time but I genuinely can't recall it.
Another reason not to use a 741 is that under certain conditions it can go into a latch-up state, where the output saturates and sticks to one of the supply rails until it is powered down. I can't find a reference specifically about the 741, but this page describes something similar: https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/op-amp-practical-considerations/ (search for the word "latch-up" on the page).
Just out of school, I built a thermostat for our fish-tank, using a 741 with a small amount of positive feedback to act as a comparator to switch the tank heater with a relay. Twice the 741 latched up and killed all our fish.
No one has mentioned the social signalling aspect in that a circuit using a 741 will probably work with most any opamp, at least for a relaxed definition of "work". So if you need higher freq or different voltage limitations given that the original design was very vanilla, substituting a somewhat different amp will almost certainly work. This can be extremely educational, if you're trying to build an ultra low noise low pass input then build a boring low pass input with a 741 to wrap your brain around the "low pass" part of the project, then figure out the modifications needed (if any...) to substitute in a low noise amp to divide and conqueror a two part project like that. Lets say you have a very special ultra high input impedance circuit, that is unfortunately static sensitive, you could debug at least portions of the design with a mostly indestructible and very cheap 741, then swap in the ultra high impedance (probably expensive?) op amp at the very end. One reason to still use 741s is noobs and experts think its very predictable. Folks with experience in the middle tend not to like the 741 because they've probably latched a few up or found a way for a project to fail due to lack of rail to rail output. But they're mostly trustworthy and better understood than the rest of the project, most of the time.
Be wary of substitution if the design spec'd some obscure ultra high freq part or an ultra high output current or ultra low noise or chopper stabilized. If you slap in a 741 or possibly any other amp than the specified one, the design is almost certain not going to work.
The safest way to professionally figure out the above paragraphs with an actual design is to figure out the Venn diagram of the two components specs, then figure out how those specs relates to your individual project, then decide if the performance difference is irrelevant or a bit deal. This works as an engineering strategy for almost any component substitution problem you'll ever run into.
What are the reasons not to use it?
High supply voltage requirement, high bias current, not rail-to-rail operation, limited current drive, slow, etc.
What might be the reasons to still choose this part?
For many applications, it gets the job done, respectable DC performance, fast enough, wide availability, cheap, limited current drive, slow, etc.
High performance isn't always needed, and sometimes low performance has its advantages.
Before we harangue too fiercely newbies who specify a 741 opamp in their questions, it's become apparent from reading many of the 'homework' questions that it's the opamp used in their courses, whether highschool or graduate level.
I can only imagine that much of this teaching material was updated when opamps arrived on the scene, and has not been updated since.
Use the 741? It depends. If the 741 meets your requirements, and it's your cheapest/most available multi-sourced solution, by all means use it. A legacy design that uses it can keep using it if it didn't have any identified problems in the field.
New designs? In this year 2022, the 741's well-known limitations will disqualify it from most applications: newer systems tend to be lower voltage and lower power, making the 741 is a poor choice. To meet these needs as well as others there are excellent, easy-to-buy choices that do things the 741 could never dream of. Neat tricks like 'over the top' high-side current sensing, working at 3.3V with a rail-to-rail output swing, having uW standby power, or having nearly-zero input bias current.
In fact it will often be the case that the advantages a modern op-amp brings to your design can reduce your overall system cost, even if the op-amp itself is more expensive than the 741.
The downside? Multiple sourcing: choosing a specific op-amp requires some research and qualification work. That said, 741s from different vendors can also vary in characteristics, so you'd still need to qualify each vendors' part for your Approved Vendor List (AVL). So really, you're not saving any legwork by choosing a 'multiple source' 741 vs. choosing several sources of modern parts with similar characteristics.
Existing designs? Designs that show the 741 as the op-amp may be the designer's way of saying "hey, my design is so tolerant that it can use this old POS op-amp". (Is that really true? Hard to know.) On the other hand, if the designer is counting on a specific characteristic of the 741 (either intentionally or accidentally), beware. Changing the op-amp could cause unexpected behavior.
Either way, when upgrading the 741 op-amp to a more modern type, carefully analyze the circuit.
- Check with the designer if possible to gauge (1) their intent in choosing the 741 and (2) if they knew what they were doing.
- Simulate it using a good sim like LTSpice, comparing the 741 and its proposed upgrade/replacement.
- Make a prototype with the upgraded part and test the snot out of it to fully qualify the new device.
This is standard sustaining-engineering stuff, really.