Very simple circuits with minimal components sometimes have no feedback. The simplest circuits are limited to an active component and practically nothing else. Usually they include biasing and coupling components, but not always. Such simple circuits have fixed gain of hfe or mu and input and output impedance equals the impedance of the active device inputs and outputs.

Some biasing components will introduce negative feedback when signal current flows through them causing a voltage drop that biases the active device in a negative direction. This is actually very common. A bypass component has to be added to the circuit to reduce negative feedback. When components are placed around the active device, it chances the circuit's gain, frequency response and input and output impedance. These are usually required and their effects are usually desirable.

Negative feedback can be employed on a single stage, as is the case when an emitter or cathode resistor is used without a bypass capacitor. It can also be employed globally, by running an output signal back to the input in reduced amplitude and opposite phase. This is generally done on more complex amplifiers.

Negative feedback reduces distortion, but it does so at the expense of reduced gain. That increases the number of gain stages required. Positive feedback increases gain, but it does so at the expense of increased noise, distortion and reduced circuit stability. Excessive positive feedback will cause an amplifier to oscillate. But it was a common technique used in early radios to get enough gain; Global positive feedback was used in old radio circuits called "regenerative" receivers. Hetrodyne and Superhetrodyne circuits made them obsolete.

There are pros and cons of each kind of amplifier circuit configuration. I'd like to hear from some of the tube gurus on this one, because I think the topic of feedback is one that's not well understood.

Not in typical RC-coupled circuits. An active plate or collector load or else a coupling transformer or choke is required to get the full device gain as voltage gain.

>Some biasing components will introduce negative feedback when
>signal current flows through them causing a voltage drop that
>biases the active device in a negative direction

Sometimes called degenerative feedback.

>Global positive feedback was used in old radio circuits
>called "regenerative" receivers. Hetrodyne and Superhetrodyne
>circuits made them obsolete.

In most radio receivers, yes.
But simpler TRF and regen circuits are still seen today, esp. in specialized applications.

Feedback is a well-understood mechanism. If used properly, it is an extremely useful tool. But no tool is perfect. (That's what decent engineering textbooks usually say.)

Feedback, not carefully implemented, can cause phase margin errors at the frequency-response extremes. It can cause the amp to be slightly unstable with some load impedances (esp. certain costly monkey-coffin speakers with complex crossovers).

I once saw a Kenwood receiver, brand new out of the box, whose power amp oscillated at ~ 4 MHz--regardless of the load placed on it or the source material.
It made a pretty good shortwave jammer, when hooked to a speaker with a long cable.
They sold tens of thousands of that model line circa 1990.
As a special bonus, each channel oscillated at a slightly different frequency--giving some truly unpleasant IM distortion.
Most customers were perfectly happy with them....they were quite unreliable, though, for an obvious reason.
So one hopes that line, probably designed by a junior engineer at the factory (typical Japanese practice), is in the landfill. But don't think that'll be the last one!

The advantage of tube amplifiers is primarily in two areas:
1) low distortion of many triodes (and even some pentodes) allows the design of low feedback circuits;
2) tubes do not (usually) have slew-rate issues, and they never have voltage variable device capacitances, which semiconductors often have--in abundance, esp. in cheap devices intended for audio use.

And tubes still find a home in audio equipment because:
3) feedback has been (and still is) misused in solid-state amps.

The problem does not lie in the evil of feedback; the problem is inside the heads of arrogant, self-important design engineers who successfully managed to campaign in the 1960s and 70s that high-feedback transistor amps gave perfect sound. They often did not, and even today mid-fi amplifiers and professional sound reinforcement amps often have horrible feedback implementations.

A few names come to mind. Names like Bob Pease....Don Lancaster....

If an amp doesn't sound good, I recommend finding the little dweeb who designed it, and kicking his ass. No, wait, better yet, find the bastard who hired him, and kick THAT GUY'S ass.

I would be so happy if more audiophiles actually removed their swollen little heads from their buttholes, and started boycotting inferior products. But since most can't seem to agree on what good sound is, I'm not gonna wait around for it.
Prefer to talk to the minority who actually listens carefully.
(sorry, that probably leaves out most of the SE-triode mafia--they often listen to amps that sound worse then cheap table radios...)

I often build amps that switch current to a load connected directly in series with the output device. That's a simple and effective configuration, one that is often used in transformer coupled tube stages. Another one I like is to use an unbypassed emitter (cathode) resistor, and sometimes use the voltage swing across it as the in-phase output. It doesn't give the full gain of the device going that way, but it is a simple configuration that is stable and works well. I can't think of any devices that don't work well in these modes, although they are only appropriate in certain situations.

What are some of your topological favorites? What are some you steer clear of? And what are those horrible feedback implementations you mentioned? I don't necessarily care to know names, but descriptions of ill-configured circuits would be interesting. Particularly if compared with others that were done right.

That may be a bit much for a messageboard post, but it would certainly make a good article. Or maybe a teaser with excerpts on the forum, followed by meat and potatoes in a printed article at a later date. I think everyone would like to know the why's and wherefor's so they'd know what to watch out for in future purchases, kits and homebrew projects.

I believe these articles address the main issues of bandwidth, gain, distortion and stability. The advantrages of NFB include potentially improved frequency response, reduced harmonic distortion, better gain control, increased input impedance, and decreased output impedance. Those are some of the things that can be gained from a good circuit that incorporates negative feedback.

The disadvantages of NFB are that it can cause peaking and other response anomalies bandwidth extremes, sometimes even enough to enter oscillation. It can also increase susceptibility to RF interference and make clipping more abrubt. These disadvantages are usually limited to global feedback implementations, and aren't as likely to occur when feedback is introduced locally, in a gain-stage. But even here, if a device is pushed to its limits, instability can result.

So I guess the thing is that each circuit should be taken on a case-by-case basis. There are likely some components that are more susceptible that others to having problems in certain configurations. Each device has its own set of strengths and weaknesses, and the challenge is to find those configurations that work best. It really isn't appropriate to view negative feedback, in and of itself, as a problem. What is best is to examine each circuit to find and address potential liabilities, and to get the most performance afforded by the components used.