[TheMadMilkman]

I'm trying to understand Negative Feedback as it applies to amplifier design. I know that most amps use some amount of negative feedback, and that one of the advantages is that it lowers the output impedance of the amplifier. What I don't entirely understand, though, is exactly what negative feedback is. I'd also like to know how one goes about designing an amplifier that uses negative feedback...


Any takers?

 

[Az Barber]

Okay, I'll step in it.


First off, I'm not going to try to explain negative feedback. It's very complex and hard to understand.


But I will share what I know about how it applies to audio amplifiers in real world terms.


Most tube amps use very little negative feedback. Single ended triode amps use none. Solid state amps use quite a bit, but it's impossible to know how much because no one will publish specs.


Negative feedback will lower output impedance, which is pretty much the same thing as damping. It also can lead to harmonic distortion, especially when levels go up, as in loud.


This is about as complicated as I can get with this. As difficult as it is to understand, it's even more difficult to compare, because amplifier manufacturers won't release exactly how much negative feedback thier amps use. So there's really no way to compare.


Looking back over this, it's pretty useless. I hope it helps.

 

[TheMadMilkman]

Yeah, I can only think of two companies that I've seen the negative feedback numbers for... BAT, because they use none and like to tell people, and ARC, which uses tubes.


Feel free to throw relatively technical definitions at me. It may take a while, but I'm usually pretty good at fighting my way through them.

 

[Mr.Poindexter]

Negative feedback is when you buy an amp, use it and return it, telling them: "This thing suck like a hoover deluxe!"


Lots of negative feedback impedes the output of the amps, as they do not sell well.


Designers then use that negative feedback in their designs to come up with a better amp.


The common logic among mass produced amp manufacturers is that enough negative feedback used in the design stage will turn your Sony, Yamaha or Pioneer amp into a Classe or a Krell.


While not too technical and mostly in jest, there is a lot of truth to the above...

 

[Az Barber]

Connecting the output of an op amp to the inverted input of that same op amp, is negative feedback. This is also a shunt.


The benefits of negative feedback are primarily stabilization, damping, increasing bandwidth, increasing input impedance, and decreasing output impedance.


The only real disadvantage is a loss of sensitivity.


Sounds great, right.


The problem is how much.


10db of feedback is low enough so that no distortion is heard, but the amp is stabilized.


The problem is that amplifier manufacturers are relying too heavily on negative feedback to make thier amps perform better. 40db is not unusual, and there are many amps with much higher amounts than that, but no one talks about this, so there's no way of knowing which amp does what.


But it's a cheap way out. Sure, a little negative feedback is a good thing. The problem is when it gets out of control as a cost cutting method of making the amp perform better with little regard to sound quality.

 

[TheMadMilkman]

Okay. I'm starting to make some sense out of this... I'm sure I'll have more questions, but those will have to wait while I think for a while.

 

[knutinh]

Think of it as compensating for errors. If you are holding a tray of coffe and orange juice over your spouse in the bed in the morning, you would apply negative feedback to the tray angle....


If one corner dips down, your vison sense this, and you applie the opposite force using your hand to lift it up.


If, however, your bed is vibrating fast (like those motel beds), the tray may move so rapidly that you cant compensate fast enough, you might even make things worse trying to keep the tray level...



best regards

K

 

[jamin]

Local feedback and global feeback:


The individual stages of the amplifier have local feedback to linearize and stabilize their response characteristics.


The amplifier as a whole has feedback to stabilize and linearize it's response.


A little of each is OK, while a lot of either can be bad.


The can be bad part is a function of a pile of stuff, mostly having to do with how well, and in what specific way, the individual stages are designed.


Most of the good and bad with respect to the high and low feedback and it's effects were uncovered in the early 70's and published by M. Otala.


Others followed with refinement and debate. One I remember in particluar was Jung.

http://home.online.no/~tsandstr/OtalaStory.htm


If I can find a decent link on feedback theory I will post it here for you.


EDIT: more background (sorta) http://users.ece.gatech.edu/~mleach/lowtim/bckgrnd.html
http://users.ece.gatech.edu/~mleach/...5/tutorial.pdf

 

[Ethan Winer]

Milkman,


> What I don't entirely understand, though, is exactly what negative feedback is.

 

[lwang]

It is used to reduce harmonic distortion. The output of a amplifier by default has lots of distortion, so that output is inverted and fed back to the input at a reduced level, so the inverted distortion will cancel out the distortion that is yet to be generated by the signal of a fraction of a second later. If it does not completely cancel out, that signal is reduced and inverted again and fed back to the input again. This is a continuous loop, each time at a lower and lower level.


A regular operational amplifier design has a freq response that is shaped like a molehill. To get a flat freq response, you would have to feed back, at a reduced level, the inverted signal of its output. That means at the peaky region, alot of its inverted signal is fed back to the input, creating a flat response. So the next time you see a 10hz-500mhz design, it could be as simple as cranking up the feedback of the amplification stage.


Main issue is Transient IM distortion, which is distortion in the initial increase in signal in which negative feedback has not had a chance to act upon it, all properties of a non-feedback designed amp becomes apparent. This is more prominent in designs that has global feedback that loops around multiple amplification stages.

 

[hometheaterguy]

Negative Feedback is what you give companies, or individual's that do you wrong on eBay. The only problem is if you give negative feedback, you usually get a negative.

 

[Bigus]

Hmm... there really isn't a simple way to explain this, but I will take my best shot.


Control systems of all types use negative feedback. This ranges from a thermostat to an amplifier. Here's the basic idea:


- The base control system is designed to generate a particular output function given a particular input function. The relationship between the two is called the control system's transfer function. The transfer function can be very complicated, or very simple, ranging from non-linear non-continuous transfer functions to simple linear relationships.


- Power amplifiers are designed to have a linear transfer function. That is, the output is designed to always be proportional to the input in a linear fashion.


- All control systems have some error, caused by part tolerance or design, that causes the output function and input functions to be related in a way different from transfer function to some degree. In other words, the effective transfer function in practice isn't identical to the theoretical transfer function.


- It is a often a simple task to measure the actual output, calculate the intended output, and derive an "error signal" that is the difference of the two. This error signal is the basis for feedback in amplifiers. "Negative" feedback refers to the typical use of error signals being subtracted from the input function to correct it (I believe... been a while since I had a control systems course...).


- In control system theory, an error signal added to the input function can theoretically correct the errors in the effective transfer function, such that the output and input functions are then related by the theoretical transfer function as is intended.


- The above works quite well for steady state systems. When the system becomes transient, bad things can happen. Just why is difficult to explain, but the true answer comes from the solution of the control system differential equation. What is found is that the magnitude of negative feedback and rate of change of the input function are inversely related such that they both cannot increase without limit, less the sytem become unstable.


- In layman's term, it is possible for an error signal to "overcorrect" the input signal. This happens when the rate of change in the input signal is to rapid for the error signal to track. The result is oscillation in the feedback system.



So, the greater the magnitude of the feedback signal, the closer the output can track the theoretical function (lower distortion). However, during transients the input may change rapidly enough to cause brief periods of an inability to properly correct the transfer function. The result is transient distortion. Were the rate of input change to continue at that level, the system would become unstable.


Since music is transient in nature, high levels of feedback can lead to higher levels of distortion than were present in the original signal, but only during brief transients. Sine wave measurements will not pick up this distortion. However, since humans also have a limited frequency range of hearing, it is possible to design proper signal filtering and feedback levels such that at all times the distortion levels remain either (1) below the level present in the original signal or (2) well beyond the human hearing range and taken care of by proper filtering.



At least, that's my understanding of it. I'm not an electrical engineer by training, but the differential equations describing fluid, mechanical, and electrical systems are essentially the same. All control systems exhibit this same basic behavior.

 

[TheMadMilkman]

Thanks Bigus. I was waiting for you to show up and pipe in. Your explanations tend to help me a lot. And in this case, I think that I've finally got a pretty good understanding of what's going on.


Thanks to everyone else, too!

 

[Az Barber]

Quote:

Originally posted by TheMadMilkman Thanks Bigus. I was waiting for you to show up and pipe in. Your explanations tend to help me a lot. And in this case, I think that I've finally got a pretty good understanding of what's going on. Thanks to everyone else, too!

He did explain it rather well, didn't he?


Thanks Bigus!