You think of op amps as amplifiers because, no kidding, it is right in the name. But just like some people say, “you could do that with a 555,” [Doctor Volt] might say, “you can do that with an op amp.” In a recent video, you can see below, he looks at simulations and breadboards for five applications that aren’t traditional amplifiers.
Of course, you can split hairs. A comparator is sort of an amplifier with some very specific parameters, but it isn’t an amplifier in the classic sense.
In addition to comparators, there’s a flip flop, a few oscillators, and a PWM audio over optical transmitter and receiver. If you want to test your understanding of op amps, you can try to analyze the different circuits to see if you can explain how they work.
Op amps are amazing for analog design since you don’t have to build up high-quality amplifier blocks from discrete devices. Even the worst op amp you can buy is probably better than something you have the patience to design in a few minutes with a FET or a bipolar device. Fair to say that we do enjoy these oddball op amp circuits.
A clue to why op amps aren’t simply a traditional amplifier (and never have been) can be found in their name, which is “operational amplifier”.
“A comparator is sort of an amplifier with some very specific parameters”
Yes, and a very standard op-amp circuit. As are flip-flops, Schmitt-Triggers.
Is it good to show this to people and remind them that stuff can be pretty simple? Yes. And people should really look at the circuits and try to understand them. It’s been a while for me as well (2nd year EE, I think).
On paper at arms length and squinting, a comparator might look like an op-amp, but woe betide you if you try to sub one in for the other in a real circuit and expect the desired performance (unless your expectations are low). Saturation recovery is a big difference, among other parameters.
There is nothing “odd” about these circuits. They’re all pretty common opamp applications.
But a word of caution about the schmitt trigger. This circuit can not be understood if you make the common but false assumption that “an opamp tries to make it’s inputs equal to each other”.
A much better way to analyze opamp circuits, is to look at what an opamp actually does. It takes the difference between the two inputs, and amplifies that with a big number. (approx 100.000 is common). Next step is to take another specification of an opamp, and that is the maximum rate at which the output can change, this is called the “slew rate”.
Combine these two, and you get: When the non inverting input is higher then the inverting input, the output goes up at the rate of the slew rate, if the inverting input is higher, then the output goes down with the slew rate.
This analysis always works because it simply is what opamps do. It also helps with analyzing when there is for example a BJT at it’s output, which in some configurations “inverts” how an opamp works. The opamp itself does not know anything about the external circuitry. It just has it’s inputs and output (And it’s power supply, possible compensation nodes and other details that are not significant to understanding the basics of a circuit).
Excellent summary. Short ,detailed and easily understood.
It’s true that it’s false about an op-amp in isolation. It’s misleading, and arguably false, that it’s false about circuit with an op-amp in negative feedback configuration.
Silliest op-amp circuit I’ve done: An oscillator and driver solely to drive a capacitor-isolated voltage doubler, to power a digital panel meter that needed to be isolated. One-chip, $0.40 solution and I had it on hand, saved the $3 +1 day solution to get a ‘real’ one from Digikey. 15 years later, it’s still running.
My favorite “odd Op Amp circuit” Is the simple frequency discriminator.
Don’t try this with a FET input op amp it won’t work. This only works with bipolar transistor front end op amps. Where I use to work we built a transmitter (FM) that used an RCA CA748 op amp as a frequency discriminator. The key to making this work was the parallel tuned circuit across the inverting and non inverting inputs. The circuit worked well into the lower UHF range.
We had a line of transmitters that used this circuit for frequency control in our frequency locked loop transmitters, we also had a linear version that would generate a voltage proportional to the frequency. We used this provide reverse bias for a VCO for an L-band 100 channel FM transmitter.
The transmitters wee modulated by using a varactor diode in that parallel tuned circuit I mentioned above.
This was the preferred method of modulation although we did build transmitters that modulated by other means.
I miss the days when the strange and weird things could be found in microwave circuits; now it’s just nameless, and faceless black silicon dots with stubs and combs and striplines….
Thank you Conic Data Systems for the memories….
Where’s Tom Cat???
Good old Conic.
Scrapped a ton of their stuff into useful parts (lol) when I was growing up in San Diego in the 80s and 90s.
Tons!
There was a lot of good parts to be had.
Back in those days if you worked on the production floor you had access to a lot of free stock parts for some of your own projects…. (nobody abused it)
I left there for more money, and went to work in the two way radio business….
There are times I regretted it….
All it took was the six dollar per pay difference and the regrets vanished.
I did manage to buy one of the transmitters I worked on back in the day.
Found it on Ebay for about 2 percent of it’s original cost.
now I have a working S-band transmitter (2277.0MHZ for $45.00 USD. Makes a nice paperweight. :)
*Operational amplifiers
Operational amplifiers (op amps) are analog circuit blocks that take a differential voltage input and produce a single-ended voltage output
^
Check National Semi AN-32 here. From 1970. http://bitsavers.informatik.uni-stuttgart.de/components/national/_dataBooks/1973_National_Linear_Applications.pdf
heh it’s a couple years ago now that i realized an op amp and a comparator are the same thing
Except when you try to make one act like the other… They really are different things, with performance criteria optimized for the intended use. OK for demonstration and low-speed, low-performance projects, but otherwise poor substitutes for each other.