Svart

If CFB designs are better, why use VFB designs?
« on: December 06, 2005, 01:28:35 PM »
So it seems that current feedback designs are better, then why use voltage feedback designs at all?

Discussion please:

 :thumb:
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


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If CFB designs are better, why use VFB designs?
« Reply #1 on: December 06, 2005, 02:15:17 PM »
While CFB is better at providing higher slew rate, more gain and lower distortion using fewer voltage amplifying stages it is not better at every possible parameter that may count to the designer. Generally, with VFB one can achieve better parameters of input noise and output DC offset. If your design criteria requires to have the lowest possible of those two you are better off going with VFB. Also, if you need precise setting of gain like an analog system controller for hospital equipment you want to stay with VFB.
I tend to favor CFB because I really like the HF reproduction by these topologies. DC can be blocked with capacitors and the noise is not so bad in most audio applications.
One more caveat with CFB is that the bandwidth is highly dependent on the feedback network used. High impedance networks can cut back on the bandwidth severely.

Cheers,
Tamas

If CFB designs are better, why use VFB designs?
« Reply #2 on: December 06, 2005, 02:20:48 PM »
I don't know if I would say that one is better than the other, because each has strengths and weaknesses.

CFB is quite fast, and, to my ears, much more desirable as an audio amplifier.  But they aren't stable at unity gains, which means they can't be used as buffers.  They differ from VFB's in the sense that  impedance at the inverting (negative) input sets the bandwidth and therefore the stability of the amplifier, and it should be resistive, not capacitive.  My experience is that CFB's seem to dislike capacitors in the feedback loop.  They hate resistive variations in the feedback loop as well, or rather, resistive variations in the FB loop will vary the bandwidth along with the gain, and from what I've read, small FB R's will cause instability.

Edit--looks like I got beaten to the punch :wink:

Svart

If CFB designs are better, why use VFB designs?
« Reply #3 on: December 06, 2005, 05:31:03 PM »
nice!  keep going guys!
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

BradAvenson

If CFB designs are better, why use VFB designs?
« Reply #4 on: December 06, 2005, 06:21:58 PM »
Quote from: "featherpillow"

 But they aren't stable at unity gains, which means they can't be used as buffers.  They differ from VFB's in the sense that  impedance at the inverting (negative) input sets the bandwidth and therefore the stability of the amplifier, and it should be resistive, not capacitive.  


They can be stable at unity, but you can't short the out to the -in.  you have to put a bandwidthlimiting resistor.  Check out these docs:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1146,D4234
http://www.intersil.com/data/an/an9787.pdf
http://www.intersil.com/data/an/an9663.pdf
http://www.web-ee.com/primers/files/Current_feedback.pdf

PRR

If CFB designs are better, why use VFB designs?
« Reply #5 on: December 07, 2005, 12:13:30 AM »
> current feedback designs are better, then why use voltage feedback designs at all?

So-called "current feedback" just combines the inverting input and the compensation network onto one (often poor-performing) pin.

Often the difference is just a matter of attitude. The standard transformerless XLR mike input is usually looked-at as VFB, but is usually really working CFB over most of the gain range.

It only looks better when compared with 30 years of opamps fixed-compensated for worst-case use because we were too lazy to compensate them ourselves. Dig out the 709 opamp: it will give you a ton of bandwidth and a better inverting input than many "current-feedback" chips. Yes, today's CFB chips will give even more bandwidth, but they are made on much better processes.

> why use voltage feedback designs at all?

Many EQ circuits are perfect with VFB, impossible with CFB. There may be a dual of the VFB topology that will work as CFB; it may lead to unreasonable cap/choke values.

I was reminded today that the VFB (or CFB) op-amp is NOT the ultimate amplifier for all uses. We thought it was heaven back in 1972, but this was folly. Op-amp theory requires that we assume infinite gain: this never happens. It may happen close-enough for many uses, but not really good enough for others. And many times the only reason to dip into op-amp theory is to avoid actually thinking what you are doing. There are many other amplifier building-blocks; unfortunately, dollar op-amp chips drove most of them out of our awareness.

Figure out what you really want to do. See if you can do it in 3 or 4 transistors. A lot of things can be (example BA283; which BTW is "CFB"). Look at a few chips and see if they are better. A 40-transistor chip is not better than a 4-transistor amp if the chip does not do what you want. The chip may still be better for reduced assembly cost, more compact and consistent layout, intrinsic matching.

If you would rather cut disks than fool with design, go for a chip. Don't obsess about VFB CFB or XYZ "features". Take one with good performance, no bad habits, and just do it. Most chips simplify design-time and get you ripping sooner. 99% of your audience won't know the difference between Gold-doped JFETs and a 741.

bcarso

If CFB designs are better, why use VFB designs?
« Reply #6 on: December 07, 2005, 02:53:54 AM »
quote PRR:  "I was reminded today that the VFB (or CFB) op-amp is NOT the ultimate amplifier for all uses. We thought it was heaven back in 1972, but this was folly. Op-amp theory requires that we assume infinite gain: this never happens. It may happen close-enough for many uses, but not really good enough for others. And many times the only reason to dip into op-amp theory is to avoid actually thinking what you are doing. There are many other amplifier building-blocks; unfortunately, dollar op-amp chips drove most of them out of our awareness.

Figure out what you really want to do. See if you can do it in 3 or 4 transistors."

Amen.  Op amps helped many people to believe they were analog engineers, and made well-deserved money for companies like Philbrick and their successors like Analog Devices, Fairchild, National, et al.   The half- or smaller-fractional truths that made the first-order theory so seductive fostered a sense of power and helped a youthful generation pooh-pooh the preceding couple, who had learned to do more with less, especially important when tubes were the only option and still of some value when transistors were expensive.

Unity-gain-stable voltage-feedback op amps are wonderful for quick and dirty jobs, and may well satisfy all of the required performance criteria.  They can always be made to oscillate with sufficiently wacky feedback components, but require significant provocation.

If one has access to the whole circuit including the interstices of the forward gain path, the additional degrees of freedom can be used to advantage.  Since this access is usually only granted nowadays via our beloved discrete designs, that is one reason for our tendency to think of these as superior.  But there are many advantages to monolithic fabrication, especially those of inherent parts matching and thermal tracking from sheer proximity.  So the best of both worlds is a bit of both.

A sad aspect to much of this:  for some really interesting things where one attempts to combine high frequencies with high accuracy at low, you simply can't get the d*mn parts---you have to have the formidable resources to pay for monolithic design and fabrication using the latest and greatest "processes."

Fortunately most of audio design, at least analog audio design, doesn't suffer much from this restriction.


 

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