Is it possible to create any active circuit with zero feedback?

[PMA]

Some 12 hours ago I have displayed in another thread my meaning that it is impossible to create any active circuit without any amount of any kind of negative feedback. My post was erased by the moderator, so I am starting the thread on this topic.

[dimitri]

http://www.diyaudio.com/forums/showt...122#post331122

Originally posted by millwood : I tend to think it is hard to design a high-performance amp without any kind of feedback, marketing hypes aside.

Pavel answered: Not only hard, but impossible.

[Nelson Pass]

Yawn. This can only end up being a semantic argument.

The internal mechanism of a gain device creates feedback,
therefore you can't technically have a no-feedback circuit.

But as a practical matter, we are really asking the question
of whether the designer has created a feedback loop for the
purpose of controlling gain or reducing distortion. If not,
these circuits are commonly called "no feedback".

[peranders]

I think most people mean "without global feedback" is the same thing as feedback. What about local feedback?

I don't agree though. What about tube and fet amps?

[Claude Abraham]

PMA,

Greetings from the USA. The answer to your question is as follows. In an absolute sense no, but in a practical sense, yes. Let me clarify. "Active devices" include op-amps, SCR's, transistors, comparators etc. An op-amp operating open loop (no feedback at all, or a comparator), could be considered as an active circuit without feedback if you view the op-amp as a single device. Internally, however, the op-amp consists of gain stages, each of which has inherent local feedback. Capacitance and resistance inherent to the active devices result in a portion of the output of that stage being coupled to the stage input, even to a small degree. This is "local" feedback, as opposed to "global" feedback, where the op-amp output is fed clear back to the op-amp input. The end result may be barely detectable, so that considering the circuit as having "no feedback" may be accurate enough for all practical purposes.

Where this issue becomes controversial is when I, or others suggest that the inherent emitter resistance or emitter-to-base inherent capacitance constitutes "internal feedback". Some will not accept "internal" or "inherent" feedback. The following example should demonstrate that internal feedback is indeed present with semiconductor junctions.

Have you ever done work with "SCR" devices (silicon controlled rectifier)? An SCR is a four-layer "p-n-p-n" device. A drive voltage is applied to the gate-cathode junction, and current flows from anode to cathode. If the gate to cathode drive voltage is then removed, the current will continue to flow from anode to cathode until it is externally set to zero. What keeps the current flowing without gate drive is internal feedback. The p-n-p-n junctions form an npn-pnp bipolar junction transistor pair. The emitters and bases are interconnected in such a way, that one transistor's emitter current drives the other transistor's base, and vice-versa. This is "regenerative" feedback, aka positive feedback. The mechanism responsible for this feedback is entirely internal. In control system theory, a system with positive feedback is unstable, because it is uncontrollable. An SCR gets turned on by applying gate drive, but the same gate drive signal cannot turn it off (I know, gate-turn-off SCR devices were developed in the '70s, but that's beside the point).

A common emitter, or "CE" amp stage without an emitter resistor, or with an emitter resistor bypassed by a large capacitor can be considered an active circuit without feedback to a large degree of accuracy. If you're splitting hairs, and you wish to consider the inherent emitter resistance present in the transistor, then there is a very small amount of negative feedback. The effect of such is quite minimal. To all reading this, let's please not argue over this. Also, there is stray capacitance across all junctions as well as circuit board traces. There is an ac path from the collector, which is the output of the CE stage, back to the base, which is input to the same. This is feedback, but not very much.

To summarize, active circuits intentionally designed without feedback still posess a small degree of feedback due to inherent R and C, but in most cases can be neglected. My stereo amplifier at home, as well as my CD player in the cellar claim "zero feedback" designs. As far as I'm concerned, this claim is legitimate. By the same token, circuits which claim to be "balanced", are legitimate as well, even though no circuit is perfectly balanced. I hope I've answered your question. Best regards to all.

[Eva]

Since everything in this world suffers from feedback [not to talk about the usually forgoten thermal, mechanical, capacitive and inductive coupling phenomena present in every circuit], I think it would be more sensible to talk about 'low feedback' [feedback is itentionally avoided] and 'high feedback' [feedback is intentionally added] circuits. 'Zero feedback' is just a purely theoretical concept

[fdegrove]

Hi,

I don't agree though. What about tube and fet amps?

What about them?

What tube type or FET for that matter?

I don't agree though.

What with exactly?

Cheers,

[Claude Abraham]

A common emitter, or "CE" amp stage without an emitter resistor, or with an emitter resistor bypassed by a large capacitor can be considered an active circuit without feedback to a large degree of accuracy.

What I just said in that preceding post is correct at ac, but not at dc. With a large enough emitter resistor in a CE stage, there is dc feedback, but no (actually very little) ac feedback. With zero ohm emitter resistance in a CE stage, ac and dc feedback are practically zero. I should have made that clear. Interesting responses. It looks like universal concensus. Best regards.

[PMA]

Thanks to all for the valuable inputs. I am glad that they make sense.

[Pabo]

I have allways considered a emitter degeneration resistor to be a linearising component, not feedback. What happens when a emitter degeneration resistor is added is that the nonlinear voltage variation over base emitter with collector emitter current is given less influence. You do not actually feed a current from the emitter to the base with this resistor and you do not measure the collector voltage with this resistor. Whait a second, it does. The collector current will pass through the emitter down through this degeneration resistor thereby generating a negative feedback signal on the base emitter voltage.

I surrender.