Any advice to make this Op-Amp temperature controller circuit work?

[DaveE]

BTW, for these low frequency mV level sensors, we would use one of the single supply auto-zero op-amps for the input stage, like the OPA335. Undoubtedly not available for schools, and not necessary for learning in the lab. But in the pro EE world you would normally choose one of the thousands of op-amps that fits the need best. Op-amps like the 741, 324, OP07, etc. really only fit the low cost niche, there are better choices for nearly any spec on the data sheet.

What most analog EEs spend their time doing is reading datasheets, choosing appropriate parts, and designing "on paper", then maybe simulating the tricky bits (or not), and only then building it. Many routine portions of a circuit are never breadboarded or simulated if you are confident in your design work. Still, students do have to learn how to work in the lab. Also, maybe, the cost and pain of doing it wrong. All of us have destroyed more parts than we can count, LOL.

BTW, the days of +/- power supplies for op-amps is mostly past. Extra power supplies cost a lot and usually aren't necessary.

 

[Wrichik Basu]

The best version would be a comparator, like LM311, LM339, etc. with some positive feedback as I sketched before.

I was going through the datasheet of the LM311, but failed to see its advantage over a good op-amp with a transistor. This is the schematic of the LM311:

At the end, it's basically an op-amp with a transistor in a single package. Maybe a goof op-amp, much better than LM741. And maybe will result in less wiring. But other than that, what's the advantage of these over regular op-amp + transistor circuit?

BTW, the days of +/- power supplies for op-amps is mostly past. Extra power supplies cost a lot and usually aren't necessary.

Then how do you power the op-amps? I am almost going to buy an SMPS (Morsun LM60-12A15 with ±15 V, 2A) so that I can use op-amps at home. Even the LM311 requires a -15V supply.

 

[DaveE]

what's the advantage of these over regular op-amp + transistor circuit?

Comparators are basically op-amps without the internal dominant pole frequency compensation. This compensation lowers the gain at high frequencies to make them inherently stable. However, it also slows them down. The LM311 can switch from -15V to +15V in 115nsec (slew rate = 260V/μsec). The OP07 has a slew rate of 0.3V/μsec. So it's much slower, both in switching and the delay to get to the active region of the next stage. This means that the inputs stay in/near the linear (active) range longer for the op-amp with positive feedback. In some designs you can eliminate the external positive feedback because of this, although I would usually include it, especially for slowly changing inputs.

Many comparators also have stronger output stages to drive more current which helps with capacitive loads, although the LM311 isn't a great example of that.

However, with external positive feedback in a slow application like yours, I don't think there's much wrong with using an op-amp. You'll see people do this when they want parts commonality for low cost or size. Like with a dual or quad op-amp IC. If the designer knows the difference and the consequences, I think it can often work well.

You are correct that the extra gain of the transistor makes this less important.

Then how do you power the op-amps?

You choose one that works well with a single power supply. Even the LM324 does, which is ancient. Although there are better, newer versions, like rail-to-rail versions. There are some issues that you need to pay attention to with single supplies and/or low voltage operation though.
https://www.ti.com/lit/ml/sloa030a/sloa030a.pdf?ts=1680287582984

Same for comparators, BTW.

 

[Baluncore]

Comparators are basically op-amps without the internal dominant pole frequency compensation. This compensation lowers the gain at high frequencies to make them inherently stable. However, it also slows them down.

The fundamental difference between a voltage comparator and an op-amp is the design of the input, gain, and output structures.

An op-amp is designed to operate with little input voltage difference. Some early bipolar op-amps would invert their output when the inputs differed by more than a few volts, while the input bias current would then also rise significantly.

Some voltage comparators have an internal 1 mV of hysteresis in the gain stage, that is used to speed up their response. That is never done with op-amps.

The output stage of a voltage comparator is a bipolar digital driver.
The output stage of an op-amp is a linear voltage follower.

Don't get voltage comparators and op-amps confused. Their only similarity is that they can share the same symbol on a circuit diagram.

 

[DaveE]

Some early bipolar op-amps would invert their output when the inputs differed by more than a few volts

Yes, but only if you exceed the common mode input range. It's really the same thing, they just write the spec to cover their...

This reminds me of a run-in I had with the LM6144. They included a great feature to eliminate this "phase reversal" (that's the jargon to search for), to increase slew rate for large input differences, and drive capacitive loads well. That all sounds great in the intro text on the data sheet. What they don't say on the first page is that they do that by diverting current from the input stage which blows up the input current specs. It's buried in the datasheet. I spend about 2 days being confused about why things weren't right in my circuit. That's like 18 hours spent in the lab on one single amp stage, which practicing EEs don't have time for.

It reminds me of the old grad school saying "3 weeks in the lab can save you from spending an hour in the library". Read the datasheet. The whole thing. Even the footnotes (especially the footnotes!). Don't stop when you think you know enough. Some things like this are buried.

I will never use an LM6144 again, mostly because of PTSD. They aren't bad parts if you know what they do, but I rarely would need that.

PS: Anyway... The op-amp specs I think newbies don't really pay enough attention to, and absolutely should, are the input range and output stage drive (I-V) specs. This very much applies to both the single supply issues and the comparator/op-amp differences.