Electronics > Beginners
Are amplifier circuits like this still used/built?
bonzer:
Hello everyone! I think this isn't an amplifier that has any class (like class A,B etc), it's just a circuit. To be honest it even might be useless, only for studying purpose, I don't know. But anyway if it wasn't it feels like all what we studied recently about analog at university is kinda obsolete. I'd like to understand what is the trend today? For example is a circuit like what I attached still used and built in any case of amplifier design or they use only mosfets today or there's even some digital implication (like in the case of digital filters).
This is an amplifier with constant gain (25 dB from what I remember) that has a working band of 250Hz-400kHz. What would this circuit look like in the majority of today's cases for the most of purposes?
dzseki:
Every amplifier has a class, here Q1 is the gain element, and is operating in class A.
The trick of this circuit is that it takes a ground referenced input and has two outputs with inverted phase, in other words it has symmetrical output. An engineer today probably would use two single opamps or one differential output opamp for this purpose, but depending on the application the posted circuitt can work just as well.
Berni:
This is a class A design. But its not only a amplifier but also a single ended to differential converter.
What makes you think that BJT transistors are obsolete? They are still used in all sorts of places. Its just that for high power applications they are usually not the best choice.
But yes most analog stuff is now done using Opamps because they are easier to use, cheap and can offer some very impressive performance if you are prepared to pay enough for a fancy kind of opamp. But there are still cases where analog transistor circuitry is the better choice. Perhaps the opamp that can do it costs too much or uses too much power, or maybe there is no opamp out there with the right specs (Like perhaps you need +/- 100V output swing).
Its not useless knowledge. If you work on analog electronics enough it eventually comes in useful for a tricky situation where a chip doesn't do the job. Its just another useful tool in your circuit design toolbox. Tho its only really useful if you understand how these analog transistor circuits actually work rather than copy pasting the design.
bonzer:
thanks for your answers. I didn't know high power op.amps exist and are used in these cases.
Sorry if I go offtopic but talking about op. amps, observing them from the outside what would be a good way to rappresent them as real? A part from input resistance and output resistance like in this image, aren't there other parassite capacitances to be considered? I see the inputs go to internal bjts. It is usually necessary to consider bjt's capacitance in high frequency otherwise it's getting hard to get the desired bandwith or cutoff frequency. I don't know if here it works the same as the transistors are internal so these capacitances could have very low value (I suppose) that could have time constants that activate even further than the open loop gain of the op-amps, so there might be no reason to consider them.
Besides - maybe I said some stupid stuff, if that's the case than sorry for that I'm new into this, I'm not an expert.
dzseki:
I wouldn't call the picture as a "real opamp" it is rather a less ideal opamp :)
Of course more and more complex models are possible to make of a particular opamp, but in most cases (especially when learning the basics) this is absolutely not neccesary.
As for high frequency performance you really don't have to know how the internal parasitics of a particular opamp looks like, because you can't do much about that anyway.
The physical layout of the circuit (as on PCB) is often critical, and the datasheets are often very clear on this. For high bandwidth usualy the problem is not getting up to the frequency but to keep the system stable. The system stability is usualy most affected around the inverting pin of an opamp, so it is a good general idea to avoid stray capacitances as much as possible there.
Navigation
[0] Message Index
[#] Next page
Go to full version