We all have to start somewhere.
I’ve been thinking about my posts as of late. Moreso, how I can offer something of substance to readers. At least substance that is usable on a daily basis. What do EEs want to read when they get to this site?
I came up with part reviews. I started something similar at my other project, Electricio.us. Spec sheet analysis is a critical skill for any analog engineer. It’s also is time consuming and hard to keep up with. So why not have a few choice parts highlighted on here? I don’t plan on reviewing the newest and hottest parts…but maybe the most useful. So let’s start with the LM741.
A 741? These are parts I joke about, not parts I use on an everyday basis. But like I said, we all have to start somewhere. I’m sure many of us started using this part in electronics labs and at home in hobby projects. It’s the quintessential beginner op amp and it was one of the first ever designed.
LM741 datasheet from National Semiconductor
Pros:
- Cheap — Being made by multiple vendors (necessarily called the LM741 but probably containing the number 741 somewhere in the title) really can help with cost.
- Standard pinout — Sometimes even more valuable than having something cheap in the first place, being able to find a drop-in replacement can be a godsend when in a bind. This has been doubly true with the economic downturn. Sure, the part being out for 42 years doesn’t hurt either. People managed to figure out it’s pretty popular since it first arrived on the scene and duplicated this trailblazing part’s pin assignments.
- Nulling circuit — As we’ll see in the “cons” about this part, the offset voltage is horrible, but the tuneability of the part is a nice feature if you have to use this part. The “offset null” pins give you access to the 1K emitter resistor circuit, basically allowing you to drive the input voltage on one side of the op amp higher by giving a different resistance for the bias current on the input stage. This effectively raises the voltage on one side of the input or the other (inverting or non-inverting). Pretty neat stuff!
- Bandwidth — For an old part, the bandwidth on the 741 is 1.5 MHz. This is set by the internal capacitor, which acts as a pole inside the circuit and limits how fast the circuit can respond. This is extra interesting because the 741 was the first part to ever do this inside of the IC itself; previously you had to set the compensation capacitor externally. At 1.5 MHz, this old geezer of a part can still get up and move like a jackrabbit.
Cons:
- Offset voltage — As you can see above, the nulling circuit is necessary because the offset voltages can get pretty extreme on the inputs of this op amp. “So what?” you say. Well, this causes issues when any kind of precision is required. If you put in a 1 volt signal into an LM741 that is set up in a buffer configuration, the 5 or so mV of offset voltage between the inputs will get passed directly to the output! That’s .5% of your signal right off the bat! And if there is any gain in the circuit (i.e. it being set up in an inverting or non-inverting configuration), then your offset gets multiplied by whichever gain you apply to the circuit! That means for a gain of 10, your 1V input signal now has an output offset of 50 mV! Say goodbye to DC accuracy!
- Offset voltage drift –Even if you decide to “dial in” an offset null resistance on the proper pins, this is only accurate at the temperature you were at while correcting the offset. For every degree change, there is another 15 uV of voltage offset.
- Bias current — We all know that in an ideal model of an op amp, you assume there is no current flowing into the inputs terminals. Well, that’s never really the case, and having any current flowing into the terminals can cause DC offsets once that current flows through resistors (such as in a non-inverting amplifier configuration). The standard bias current spec’d on this part is .1 to 1 uA. That’s nothing to write home about, especially if you care about DC accuracy. The downer on this poor spec is that there isn’t an internal “knob” to help with the bias current; you just have to deal with it and try to design around it.
- PSRR — Though it’s not the worst I’ve ever seen, the PSRR on this part isn’t great either. At a typical value of 77 dB (use 80 for easy math) that means that any noise appearing on the power rails will show up 10^-(80/4) times on the output. So if you see a 1V transient on a 15V supply rail (gah! huge!), then you will have a 10 mV spike on the output. This can cause some serious noise problems down the line and most op amps I’ve been looking at these days have been well north of 100 dB PSRR.
This part is old, no way to overlook that. But it is still relevant part and is the basis for many parts that exist today. That’s pretty impressive some 40 years later. The downsides show the part’s age but are a symptom of the technology and the time this part was developed. The transistors are BJT, compared to the MOSFET based parts of today; many of the specs are guaranteed to be worse on this fact alone (though not the speed nor the power handling ability).
If you have a simple need for an op amp and you know how to properly account for all the shortcomings in this op amp, it’s a fine choice. Like I said above, it’s low cost, easy to use and plentiful in supply. And if history has shown anything, it’s that this part is not going away anytime soon.
So this was my first part review. I think in the future instead of running down the list of a lot of the specs like I did here, I would focus instead on one or two of the spectacular properties of a chip or the extremely underwhelming properties of a chip. Comparing all the middle of the road specs is a waste of everyone’s time. I wanted to make sure I covered a few things here as a baseline for future reviews though. I’d also like to step outside the bread and butter parts of an analog electrical engineer’s part drawer (the op amp) and review other types of components.
If you have any you would like to see reviewed in the future or have any thoughts about this review, be sure to leave a note in the comments.