| Electronics > Beginners |
| What's the use of OpAmps? |
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| Darkwing:
Hey mates! Well, I don't get it ... I've now read a lot of times about OpAmps, but I don't see the use of them, when I can do amplifications of all sorts with MOSFETs, BJTs and Darlingtons or even Optocouplers (which have an advantage of their own). What is this mystic operation, that is best to be amplified via an Operational Amplifier? Can someone please point out? Do they have an advantage in audio in terms of noise or the like? Side quest 1: What's this "rail-to-rail" thing? Side quest 2: Can you name your favorite OpAmp? Should there be such a thimg, what could be the best go-to jelly bean I should have in my box? Thanks y'all! :) |
| Dubbie:
Sure you can make a basic amplifier out of a bunch of BJTs, but by the time you have designed it to be as quiet, stable, precise, efficient, small as a 20cent op amp, you would have effectively designed your own inferior op amp anyway. Opamps are for people who have more productive and interesting things to spend their design efforts on. Of course there are applications where a custom amplifier design is necessary and an op amp won’t do, but for everything else there is an opamp. |
| dmills:
Indeed, the selling point of an opamp is that it is (more or less, ok actually less then more) an ideal gain block, within certain limits you can assume open loop gain is large, input impedances are high and bias currents are small (but never zero). This means that rather then winding up with circuits where beta matters I can assume (again within limits) that the closed loop gain is set almost solely by the feedback network, epic maths time saver. Also often my other option is to build an inferior difference amp out of discrete components (that will not match near as well as the ones in the opamp), it will be bigger, probably noisier (unless I am good) and worse behaved, worse, it will take me a day two design it. Regards, Dan. |
| soldar:
The ideal op-amp has infinite gain and infinite input resistance. Then the gain of that stage is controlled by the feedback circuit. You can design an amp of any gain you want just by using a couple resistors for the feedback. Try to do that with a transistor or two. |
| IDEngineer:
Another advantage of opamps over discrete amplifiers is matching of their components. Process variations mean you generally can't guarantee absolute values, but relative values on the same die can be amazingly close without trimming. And for even tighter matching things like laser trimming are a matter of automation. If matching is important for your homebrew discrete amplifier (think: differential input amplifier), you'll be mucking around with matched components or trimpots and the like just to achieve what a single monolithic opamp delivers for remarkably low cost. Monolithic opamps are one of the modern miracles of electronics. The same three terminal device can yield hundreds of different results based solely on the feedback loop you put around it. Think of them as the analog equivalent of the microprocessor... both incredibly flexible building blocks, one in the analog domain, the other in the digital. EDIT: As for "rail to rail", that refers to an opamp whose input and/or output voltage swings more closely approach the supply rails. Early opamps required some margin from the rails, but rail to rail has been available for literally decades and gives you more dynamic range within the supply rails. |
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