Electronics > Beginners

Purpose of this amplifier circuit with op. amplifiers

<< < (5/6) > >>

ogden:

--- Quote from: Zero999 on February 05, 2019, 12:34:57 pm ---The simulation clearly shows the one with the two op-amps in the same loop, #2, will oscillate.

--- End quote ---

To make your blanket statement work, you "conveniently forgot" to include capacitor "C2" of original circuit :)


Zero999:

--- Quote from: ogden on February 05, 2019, 03:45:06 pm ---
--- Quote from: Zero999 on February 05, 2019, 12:34:57 pm ---The simulation clearly shows the one with the two op-amps in the same loop, #2, will oscillate.

--- End quote ---

To make your blanket statement work, you "conveniently forgot" to include capacitor "C2" of original circuit :)




--- End quote ---
Yes, that capacitor was a bodge and isn't needed if the local feedback resistor network is configured correctly. Now the circuit is stable and there's no need to mess around with compensation capacitors.

Yansi:

--- Quote from: nick_d on February 05, 2019, 10:50:47 am ---The link to the 50W/channel composite amp is golden. If I was going to build a class B amp I would definitely build it like this. Actually, it's sort of logical because it makes sense to drive the power stage with an op-amp IC rather than building your own driver stage. The fact that the power stage is designed to work as effectively an op-amp, is just an added bonus that makes this circuit a lot cooler.

--- End quote ---

Not sure which link, but most composite amplifiers suck.  There are more issues brought to the design, rather than solved, when a complex feedback loops with additional gain stages after the opamp.

There is normally no point in building composite audio amps like this. There is nothing that difficult on designing a nice stable discrete amplifier, if enough care is given to it and circuit built wisely.  Opamps does not really make that any easier.

There are numerous fully discrete designs availabel for free on the web, some of them very well executed, across wide variety of topologies (even CFA).

nick_d:

--- Quote ---Not sure which link, but most composite amplifiers suck.  There are more issues brought to the design, rather than solved, when a complex feedback loops with additional gain stages after the opamp.
--- End quote ---
I'm not an expert on this so I don't doubt what you say, however I would be keen to know what these issues are and why. I understand there are things like crossover distortion, offsets, noise and so on. Also, that various kinds of distortion products are easier or harder to get rid of. I also understand that feedback doesn't always remove distortion fully, especially in conditions where the open loop gain is low.

On the other hand, the arguments advanced previously seem convincing: the feedback compensates for wonkiness in the second stage, such that only the first stage has to be a high quality amplifier. The criticism is that it requires a complicated feedback network and will have overall slower response, but this seems surmountable with correct design (and audio doesn't require a super high bandwidth in any case).

cheers, Nick

Zero999:

--- Quote from: nick_d on February 06, 2019, 02:50:59 am ---
--- Quote ---Not sure which link, but most composite amplifiers suck.  There are more issues brought to the design, rather than solved, when a complex feedback loops with additional gain stages after the opamp.
--- End quote ---
I'm not an expert on this so I don't doubt what you say, however I would be keen to know what these issues are and why. I understand there are things like crossover distortion, offsets, noise and so on. Also, that various kinds of distortion products are easier or harder to get rid of. I also understand that feedback doesn't always remove distortion fully, especially in conditions where the open loop gain is low.

On the other hand, the arguments advanced previously seem convincing: the feedback compensates for wonkiness in the second stage, such that only the first stage has to be a high quality amplifier. The criticism is that it requires a complicated feedback network and will have overall slower response, but this seems surmountable with correct design (and audio doesn't require a super high bandwidth in any case).

cheers, Nick

--- End quote ---
Adding multiple op-amps in the same feedback loop complicates frequency compensation, making oscillation more likely.

It's normally better to just use a more expensive op-amp, with wide bandwidth and low distortion.

The only legitimate reason I can think of for having multiple amplifiers in the same feedback loop, is an op-amp, followed by a discrete power stage. Most op-amps are only rated to 40V maximum and the output current is typically limited to under 20mA, so using one to drive a power stage makes perfect sense.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod